Inline vs. Radial and superchargers

Inline vs. Radial and superchargers

Last update - 21 March 1998

bjh posted 03-12-99 01:03 PM

I've got a few questions for the more technically informed amongst us.

I'm curious as to why in normally aspirated engines, Radial engines typically suffer more at altitude as compared with Inline engines? From what I've been read this is nearly universal.

Also, I was reading about the p47 and one of the things that was stressed in the artical was the advantages of a Turbo-supercharger. Now versus a normally aspirated engine, and at altitude the benefits are clear. The implied arguement was that they were also superior to mechanical superchargers. Unfortunately they didn't support this with any reasons. For the life of me, I can't see any reason to think there's an advantage for the turbo, in fact I would have expected the opposite, that a mechanically driven charger would be able to work in a broader range. I'd be curious as to what the difference is.

-bjh--

Squad Leader

VMF-58 Wildcards Rogue Squadron

Windle posted 03-12-99 01:11 PM

I would suppose the turbo supercharger is more efficient due to the fact that it draws it energy from spent gasses which has more of a passive effect on the engine where the mechanical blower draws its power from the kinetic action of the engine itself thus decreasing efficiency. You dont realize how much of an effect this has until you compare it to, say, turning on your car's air conditioner while you are trying to drive uphill. Small toll - big effect. Whereas the turbo would be more effectively using spent exhaust gasses to drive the process and actually draw less power from the engine.

I've experemented with both systems on cars and the turbo in inherently more efficient due to its design IMHO.

-Windle-

Bino posted 03-12-99 01:13 PM

The explanation I've heard (my Dad recently retired from teaching A&P mechanics) is that a Turbo harnesses some of the energy of the expanding exhaust gasses. A geared Supercharger is just another load on the crankshaft, ultimately.

Also, in aircraft, operation across a range of engine speeds is far less important than in a car, for instance. Most of the time, a plane's throttle is not moving all that much.

Brian posted 03-12-99 02:32 PM

I also wondered about why the radial-engined a/c did worse at higher alts. I think I remember reading somewhere that in the pacific, Corsairs were used primarily at low and medium alts. while 38's or 40's flew top cover (I guess this was earlier on in the theatre). This despite the turbo in the Corsair's 18cylinder radial. Another thing is the FW190. I don't think the 14-cylinder radial it had was turbocharged was it? I don't have enough references to know for sure. Thus the 190 didn't do well higher up, and this lead to the 190D and finally the Ta152. Both of these had inverted V engines, liquid-cooled. But I don't remember off hand, were they turbo'ed? If not, then we're back to the original question, why radials suffer at high alts. vs. liquid cooled. I hope some more of you can comment on this subject.

miko 2dashes posted 03-12-99 02:32 PM

Guys,

Your explanations have a big hole in it - the law of energy conservation.

The exhaust gasses are expelled from the cylinder. The piston acts as a pump. The pressure of the gas opposes the piston movement. That is why efficient exhaust system is important (ever saw a muffler on an aircraft?).

Any restriction put in the way of escaping gasses causes the pressure in the chamber to decrease slower and piston to work harder expelling gasses (since amount of work it has to perform expelling them is directly proportional to pressure). The energy of exhaust gasses is in no way free. Even using the heat causes longer pipes and greater resistance.

Now a neat simple mechanical system connected to the crankshaft should be more efficient efficient than the same mechanical system connected to the turbine (extra step, hence inefficiency) working in the worst environment for any mechanical part - inside the hot exhaust pipe.

It would be more responsive, since as engine speeds up, the charger rotates faster while the turbine would need time to accelerate. During that time the engine would not get enough air, so it would not accelerate as fast as it could.

My friend's Mazda Miata has mechanical charger installed with its custom engine . I think it rates around 500hp, more then half of my T72's. No way wil I let that maniac give me a ride!

On the other hand, aircraft and helicopter pilots try to keep RPM constant at the optimal value while changing the propeller blade pitch. Same as vehicle drivers shift gears to keep RPM constant. Also when a car is going uphill, it works harder while RPM stays the same or even drops. But the engine definitely needs more air. So while the pressure of exhaust gas increases with more power, rpm stays the same.

May be that is why chargers are often porwered though the exhaust rather then gear.

miko--

Kodiak posted 03-12-99 03:14 PM

Miko--,

One thing you may not have considered is the necessity of maintaining some exhaust backpressure. For an internal combustion engine to operate most efficiently (volumetric efficiency?) at least a short exhaust pipe is required. That is the reason raceing automobiles have short carefully sized pipes. You never see large zero restriction pipes/chambers used to direct the gasses away from the engine. On a bracket car I used to run the optimum length was about 24 inches.

Kodiak III./JG54

Bino posted 03-12-99 03:23 PM

Miko,

I disagree. A Turbo is more efficient precisely because it harnesses the energy of the exhaust gasses, which continue to expand as and after they exit the cylinder. While a Supe is indeed better at acceleration across the mid-range because of the constant ratio of compressor speed to crank speed (i.e. no "turbo lag"), a Turbo is better at the top end due to its superior efficiency. For Le Mans, run a Turbo. For the Monte Carlo Rallye, a Supe.

Y'see, the piston does not have to push on those hot exhaust gasses very much at all to coax them out of the cylinder. Hot as they are, they expand out of the cylinder very quickly once the valve is opened. Besides, ever seen a "tuned" exhaust system? It is far more efficient, yet far more "restrictive" than straight pipes.

bino-- <II./JG54> bino.warbirds.org

Lugnut posted 03-12-99 03:57 PM

Preamble: this is a half assed semi informed reply. I was recently reading one of my multitude of WWII related magazines that I subscribe to (which of course I can't find now)that covered the P47 and its predecessors (the Seversky P35/36 models) in a fair amount of detail. Seversky knew early on in the 1930s that the next airwar was going to be fought above 25,000 ft with big fast planes, not small TnB fighters. He had a lot of teething pains with the concept, but ultimately, a turbo supercharger was the only real combo that gave him the high alt performance he wanted out of the R-2800. Part of it was based upon space considerations. As big as the P47 is, a huge amount of its internal space is dedicated to the turbo supercharger and its elaborate plumbing system, which resides in the back of the plane, to the engine up front. I gather that a standard 2 stage supercharger falls short in comparison, and having the turbo 10+ feet away from the radial should impart some intercooling effect on exhaust gases, which would add to its effeciency.

Although not an engineer, I would probably disagree with miko's assertion that a blower is always more effiecent that a turbo for a given application. I can't fathom that the back pressure from a turbo impellor creates greater mechanical drag that a supercharger. Individual designs vary, but I was under the impression that most superchargers wasted 20-30% of the HP increase they provided from driving their internals.

Lugnut

"Hand me that Metric hammer, will ya?"

bjh posted 03-12-99 04:04 PM

Ok, so the idea being that harnessing the unspent energy from the exaust gasses is net more efficient than a mechanical supercharger, despite possible reductions in the exaust system efficiency. Still a little surprised by this, and I had considered the possibility of "tuning" the turbo like exausts but suspected that there would be limits with all of the ducting etc. As usual, with all other things being equal they aren't. So taking this a bit further, a turbo is more efficient, but which weighs more? And what's the physical size difference?

However, no one has yet answered the first part of my question yet though. Why are inlines apparently less altitude sensitive than radials when both are normally aspirated? Or have I been misled?

-bjh--

Squad Leader

VMF-58 Wildcards Rogue Squadron

funked posted 03-12-99 04:23 PM

Here's the deal on mechanical superchargers versus turbosuperchargers. There is no discussion of altitude effects because I don't know jack about that:

In general turbosuperchargers are more efficient. This doesn't mean they make more power. By definition it means they make more power for a given fuel consumption rate.

The reason is that the kinetic and thermal energy of the exhaust flow is discarded on a normally-aspirated or mechanically-supercharged engine, but on a turbo engine this energy is reclaimed and used to compress the intake charge. As bino-- explained, the back pressure is not a big deal. The added power from the compressed intake charge more than compensates for this effect.

Not only does the mechanically supercharged engine throw the exhaust energy away, but it leetches power off the crankshaft!

With both systems, increasing the engine speed will make the compressor run faster and pump more air into the intake ports.

But if we hold at a constant engine speed, the turbosupercharger has the advantage. Exhaust energy increases with power output at a given engine speed. So the more power the engine produces, the more exhaust power there is to drive the compressor, which in turn creates more engine power! It's a feedback loop.

With the mechanical supe there is no feedback loop - if you are at 5000 rpm you get the same compressor output regardless of your throttle setting.

Another issue is control of the supercharger. You could put in some complicated, expensive, heavy, and power-robbing mechanical device to control speed on a mechanical supercharger. On a turbo all you have to do is vary the orifice sizes and/or use a wastegate. Much cheaper, lighter, etc. and costs you no power to operate. You may still have some turbo lag, but the other benefits normally counteract this.

Also a turbo system will be much lighter than a comparable mech. supercharger system. It has far fewer moving parts with lower mechanical loads, so it will also be more reliable and cheaper to produce.

The only difficult issue here is the high-temperature of the exhaust gas, but this has been conquered even on production vehicles with nasty duty cycles.

Look at racing. The most powerful racing cars ever built were the turbo F-1 cars of the late 1980's. With 1.5 liters displacement (91 c.i.) they regularly developed 1200bhp for race conditions, and there are reports of significantly higher figures with "one-lap wonder" qualifying engines. Mechanical superchargers were perfectly legal in F-1 but turbos resulted in a much faster and fuel-efficient vehicle.

I guess I left drag racing out of the above paragraph. Those are 8+ litre engines, and don't put out nearly as much power as the F-1 engines per unit displacement. They do use mechanical superchargers, but I think this has more to do with frozen technical rules than what provides optimal performance. The superchargers in Top Fuel dragsters are based on old truck blowers, and are "spec" units - no design changes allowed, you use that supercharger or none at all. Also efficiency is not an issue in this class - these guys just want full power for 5 seconds and that's it. BTW the fuel pumps on those suckers are 60 gallons per minute GOD DAMN!

Ok I gotta get some work done sheeeesh.

rsns posted 03-12-99 04:49 PM

Sheesh, funked.

But I guess this is what you get when you load a squad with engineers... The only thing I'd like to underscore from funked's thesis is that turbo matching is all about tradeoffs. There are indeed penalties in adding a turbo to the system (decreased exhaust flow efficiency and the added pumping loss that miko described) but with a properly matched turbo these effects are far surpassed by the benefits of the turbo in creating higher intake flow density (burn more air, get more power out).

A poorly matched turbo on the other hand can result in performance that is worse than a naturally aspirated engine. And there will always be "turbo lag" at lower rpms before the exhaust pressure builds up enough for the turbo to kick in.

As for radial vs inline at high alt? Could it be that it's more a matter of the efficiency of the overall airframe (pointier noses) than just the engine that leads to that impression?

-rsns-

925 CABS

bjh posted 03-12-99 05:04 PM

Thanks funked, that's the kind of answer and explaination I was looking for. Make's me remember why I *almost* picked Mechanical Engineering instead of Electrical. Then I remember how bad fluid mechanics and dynamics really was. :-)

rsns: Understanding is that this was a relative power loading type of issue but a true % of HP available by altitude. So the radial may be down to 70% HP whereas the inline is still at 75% HP at a given altitude.

Now comes the next question: Which aircraft had turbo? I know the p47 and I believe the p38 had turbo's, but what others? And which aircraft had mechanical supercharging? Which did without? It probably shows up on many of the speed charts, but I be interested to know. Seems like most source neglect to mention these details.

-bjh--

Squad Leader

VMF-58 Wildcards Rogue Squadron

Lugnut posted 03-12-99 06:04 PM

Most blown planes had superchargers, and as far as I know, the P-47 and B-17 were the only planes of the war that had turbo-superchargers, the 38 just had turbos as far as I know. I read an excerpt on the P-47 evolution to the J/M/N series that described the testing of the "new" C series R-2800 by Republic, they tried to find out at what manifold pressure detonation would occur. They boosted the motor to an MP that produced 3,600 hp! Not satisfied, they ran it at 3,600 hp for 250 hours, without any failure!

Kids don't try this at home with a Merlin...

Lugnut

rsns posted 03-12-99 07:35 PM

Okay, back to the inline vs radial thing. Here's my thought.

The radial engines were generally air cooled. So we would expect the thermal loss through the cylinder walls during combustion and expansion to be greater for a radial engine than an inline engine which cools mainly by drawing heat away in coolant flowing through the cylinder head.

-rsns-

925 CABS

Pyro posted 03-12-99 08:01 PM

bjh wrote:

I'm curious as to why in normally aspirated engines, Radial engines typically suffer more at altitude as compared with Inline engines? From what I've been read this is nearly universal.

Not sure exactly what you mean here, but if those observations are based on WB, they're incorrect. None of the planes in WB are normally aspirated, they all have at least a single-stage supercharger.

bjh wrote:

A mechanical supercharger had no means to adjust its speed, except in the case of two-speed superchargers. Even then, they only had a low and high gear. The speed at which the turbine spun and thus created pressure was a direct relationship to the speed of the engine. Since it does take a lot of power to drive the supercharger, engineers had to make sure that the increase in power was greater than the power consumed in running it. It also would do you no good to create pressures greater than what the engine could safely use.

Because the supercharger turned at a constant rate, it's ability to generate a given pressure would decrease as altitude increased. At a given altitude, the supercharger would then kick in the second stage or switch to high speed and performance would be boosted but continue to drop as altitude increased.

Now with the turbosupercharger, one of the stages is driven by the exhaust pressure. A nice benefit of this is that the speed at which the turbine spins can be manipulated by a valve controlling the exhaust outflow. This means that a constant manifold pressure can be maintained as altitude increases.

The exhaust pressure in the turbosupercharged engine stays constant as altitude increases. This has a good and bad effect. The good effect is that it as the outside pressure decreases, the exhaust pressure can be made to do more work. The bad news is that compared to a mechanically supercharged engine, there is a lot of energy lost because of the higher pressure in the exhaust manifold. More energy has to be spent pushing the exhaust from the cylinder. With the mechanically driven supercharger, the pressure decreases as altitude increases. This lower pressure outside the cylinder is like having more pressure within the cylinder and thus more power.

One big benefit of the mechanically driven supercharger is that the exhaust can be used to augment thrust. Trust me, this is a much bigger deal than it sounds. It doesn't give much of a performance boost to low end acceleration or climb, but it does have a big effect on top speed.

The turbosupercharger is also pretty complex and bulky WRT installation.

-Pyro

Starre posted 03-12-99 08:03 PM

The one difference that "I" know about with turbo's ..the exhaust turbo has created diesel engines that are developing 500-600hp's in semi's w/o any larger engines The same size engine used too create only 240hp w/o turbo. Another thing that has come as a result are cleaner emissions AND higher fuel economy.

In the trucks the rpm is critical for the turbo.

if you say start at a hill with 1500 rpm's you will not have any turbo boost and you will be grabbing gears all the way and going 15-20 miles an hr up a medium grade.

Now if you hit that same grade at 1850-1900 rpm's you will have 30lbs of manifold boost. and you can top the same hill at between 45 and 55 mph

(this will sound familiar..you had better be watching the pyro for the heat buildup. If you dont..your gonna have a BAD hair day)

many of you may have noticed trucks and truck parking avail at the top of a hill. Its not just for hot engines, but more for HOT turbo's.

Now a geared supercharger would actually drop fuel milage and increase emisions.

Starre

chisel posted 03-12-99 08:39 PM

-Whoo Hoo a question I know the answer too

Radial versus inline at altitude

Question should be Air versus water cooling?

At higher altitudes the air is thinner so less direct cooling for the heads/cylinders higher chance of detonation, cant have as much Manifold pressure.

With water cooling the heat is carried away from the critical areas atleast.

Turbo charging versus mechanical supercharging

Exhaust driven turbochargers act as altitude compensators. Forget all the wastegate crap for the moment, real engines dont use wastgates (diesel forever!). As you go up in altitude the atmospheric pressure is less so the exaust can flow more freely hence (pressure differential across the turbine)the turbo can be driven faster moving more air. (up to a certain point.)

Starre check out a CAT or Cummins engine most of the newer ones have a Altitude rating for HP

BTW its not RPM that increases boost its LOAD!

oh and dont forget that little computer stuck on the side. That helped the HP ratings too

Starre ur my new arch enemy, Damn truck drivers Salute!

Im a Heavy Equipment Technician (fancy title Eh!)

Hold still while I kill you

Chisel,

JG5 'Eismeer'

worr posted 03-12-99 09:25 PM

I'm curious as to why in normally aspirated engines, Radial engines typically suffer more at altitude as compared with Inline engines? From what I've been read this is nearly universal.

I believe some of the confusion above is because the basic premise of the question has not been challenged.

There were radials that did better at alt than inline engines and others that did not! The P-47 was a radial with very big lungs. There is more here than radial verses inline, or even water verses air cooled.

Worr, out

chisel posted 03-12-99 11:02 PM

Sure Worr complicate things

But he's correct it depends not so much on the engine itself but on what it was designed for. What size of blower, Blower gear ratios/turbo charger used, fuel octane used. Take for example the db605a and 605as same longblock but larger supercharger on the 605as

db605a 1,475Hp at 2800rpm 1.42ata Takeoff/emg.

1,355HP at 2800rpm 1.42ata at 18,700ft

db605as 1,435 at 2800rpm 1.42ata Takeoff/emg.

1,200HP at 2800rpm 1.42ata 26,200ft

(i'm assuming they rated them at the altitude they could produce 1.42ata of boost pressure)

So just by changing the blower they turned a Med altitude engine into a hi altitude engine.

Don't quote these numbers for fact they are from Janes 45/46 edition.But you get the idea

Still stand by my first post tho.

Oh ya imol can we get a MAP guage on the p47 atleast.When we luftwaffles start pokin holes in all those intake pipes I want it to lose some performance

Hold still while I kill you

Chisel,

JG5 'Eismeer'

rsns posted 03-13-99 12:02 AM

chisel wrote:

Radial versus inline at altitude

Question should be Air versus water cooling?

At higher altitudes the air is thinner so less direct cooling for the heads/cylinders higher chance of detonation, cant have as much Manifold pressure.

With water cooling the heat is carried away from the critical areas at least.

We must also consider that the radiator of the water cooled engine would be affected by the less dense air at alt in the same way. Furthermore, the lower air temperatures at alt would tend to aid cooling in both cases. So it depends on which trend outpaces the other for which engine type.

My engines class was full of these sort of opposing trends. I loved and hated it.

-rsns-

925 CABS

Tomb posted 03-13-99 08:46 AM

a quick conversation with an engine basher

its more simple

Air cooled engines need air to cool them

the higher you go the thinner it gets

the faster you need to fly to move the air through it, the more power the engine needs the hotter it gets etc etc

the hotter the engine runs he less efficant the engine can be

water cooled on the other hand maintains the engine at a constant temperture and more importantly allows the use of intercoolers

the merlin 60 series was TWO speed but also TWO staged with intercoolers keeping the air cooler and therfore thicker

compare the frontal area of a radial with the comparatively small intake area of the spit/109/mustang etc radiator to see how much more efficant this method of cooling is

it took something as massive as the P47 to get anywhere near a powerfull turbo charger in a fighter

the axis never had the metalurical finnese to get the high compression superchargers the merlin used or the high temperatures the turbo superchargers used...the me 262 engines suffered as a consequence and the normal piston fighters used fuel additives or charge coolers to gain more power

the Japanese always had problems getting a turbo charger to work well for the above reasons and getting the right metals needed to Japan

at the end of the day it was down to materials and engineeing...then the supercharged inline ruled the day now i would suspect it would be a turbo as meturlurgy has moved on and the jet engine has been around a while

Tomb

Tahoe posted 03-13-99 09:01 AM

Brian asked:

"Another thing is the FW190. I don't think the 14-cylinder radial it had was turbocharged was it? I don't have enough references to know for sure. Thus the 190 didn't do well higher up, and this lead to the 190D and finally the Ta152. Both of these had inverted V engines, liquid-cooled. But I don't remember off hand, were they turbo'ed?"

Answer:

The FW190D-9 used the Junkers Jumo 213A and the Ta152-H used the Jumo 213E, both with superchargers. Hence the large air scoop (and even larger on the Ta152-H) on the starboard side of the engine nacelle. The Jumo 213E had a two-stage supercharger.

funked posted 03-13-99 09:55 AM

Tahoe we talking mechanical superchargers (aka blower) or turbosuperchargers (aka turbo)?

Brian posted 03-13-99 05:00 PM

Thanks for the info Tahoe! I've read through all the posts so far and this is great! The discussion on turbos vs. blowers and the other part about radials vs. inlines was good stuff. I'm an engineer too, though I don't work full-time yet... just finishing up my master's. I'm glad I found agw due to the great people here. If I asked something like this in rec.aviation.military, there would be a good chance for a large flame war to develop after a while. Thanks for asking the question too, bjh.

Brian

Tahoe posted 03-13-99 05:17 PM

funked, my references say "supercharger" and nothing is shown to be hooked up to the exhaust stacks. But then, one reference says the FW190 D-11 (ground attack machine) used the Jumo 213F engine with a three-stage "turbo-supercharger." All my references seem to agree that Kurt Tank preferred the Daimler Benz DB 603 engine.

Brian, NP. I've learned some from this discussion too.

chiselposted 03-13-99 06:18 PM

Ya! what tomb said. Whos the engine basher?

I believe thats why the germans went with nitrous oxide systems to boost hi alt performance?

There was a version of the BMW801 with turbosupercharging but I dont know if it was ever used in the FW190 even on a limited basis. ik/kats you around?

Starre

I work mostly with older stuff/off road equipment and I prefer green too.

The 2cycle kind, there still in production since 1938! Nothin like the sound of a Screamin Jimmy!

Hold still while I kill you

Chisel,

JG5 'Eismeer'

Tomb posted 03-14-99 02:09 PM

found something interesting

mitsubishi MK4R-c engine

turbo charged rated at 1820hp

gave the test aircraft a speed of 362 mph at 30000ft

misubishi MK4U-4 engine

three stage supercharger rated at 1820hp also

gave the test aircraft a speed of 382mph ( 20 mph faster) at 22310ft (7700ft lower)

the test engine is a radial (MK4)

the turbo was also unrelable and dropped

the supercharged version went into production

interesting the same engine with the same HP rating give totaly different results between turbo and supercharger versions

who was the engine basher...well some day i will tell the WB guys about this guy

he is very old now but has taken apart many engines such as the fw190/me109 series not to mention many usa and japenese types

as he puts it he was "something at rolls royce" i took a look he certainly was

most people know who designed what plane..how many know who designed what engine(s)....

Tomb

bjh posted 03-15-99 01:38 PM

Not sure exactly what you mean here, but if those observations are based on WB, they're incorrect. None of the planes in WB are normally aspirated, they all have at least a single-stage supercharger.

Pyro -- Not from observations in Warbirds. More from general reading.

Pyro, this had to be something designed into an airframe / engine combo right? What planes took advantage of this characteristic? Here's the kicker, could this outweigh the other efficiencies described above, of using a turbo instead? Ie despite less HP overall, more speed?

Gentlemen, thank you for all the clear, well explained, well thought out answers. Nice to round out all that knowledge I thought I had.

You know, I just realized that we just went an entire 25+ post without serious flaming or whining. I hope argo doesn't kick me out for starting it. <grin, duck and run>

-bjh--

Squad Leader

VMF-58 Wildcards Rogue Squadron

Pyro posted 03-15-99 03:22 PM

Pyro -- Not from observations in Warbirds. More from general reading.

I see a lot of explanations regarding why this happens, but I think the real question should be whether it is really the case. I can find no evidence to suggest that inlines are better at producing more power at altitude than radials. I think this is completely tied to their methods of supercharging. You can see this just by looking at different power curves of inline and radial engines and comparing their superchargers.

The design of the exhaust stacks has a big impact on jet thrust from exhaust. With the A6M5, they achieved a sizeable increase in speed by redesigning the exhaust stacks despite using the same engine and having a higher weight. The exhaust system was often tinkered with in model to model changes of a design as this was a "free" way to squeeze out better performance.

Here's an example to give you an idea about how much difference exhaust thrust can make. At 300 MPH and with an assumed propeller efficiency of .8, every pound of exhaust thrust is equal to 1 BHP. That means that if you can get 100 pounds of exhaust thrust at 300mph, it is the same as your engine generating an extra 100 HP. At higher speeds, the equivilent HP bonus is even higher. The 190 could produce up to 300lbs of exhaust thrust to give you an idea of how significant it could be to top speed. Your total increase in thrust from exhaust will be equal to the cosine of the angle between the airplane's thrust angle and the angle of the exhaust force.

-Pyro

chisel posted 03-16-99 12:47 AM

Question Pyro

I understand a little about exhaust augmentors and get the basics of how it works but why would an increase in airspeed increase the thrust from the exhaust? The exhaust would still exit at the same speed for a given load OR the airflow would help extract it.Just cant see it. Teach me oh flammable one or atleast point me to some books on this subject.

Hold still while I kill you

Chisel,

JG5 'Eismeer'

Pyro posted 03-16-99 01:37 AM

I understand a little about exhaust augmentors and get the basics of how it works but why would an increase in airspeed increase the thrust from the exhaust?

An increase in airspeed does not increase the exhaust thrust. It increases the equivilent horsepower of that exhaust thrust. The reason for that is that the thrust created per engine HP decreases as speed increases. Jet thrust does not work that way, it stays pretty much constant. That's why a P-51 will easily out-accelerate a 262 at low speeds, yet the 262 will go a helluva lot faster on the top end. The P-51 can generate a lot more thrust at low speeds than the 262, but the jet can produce a lot more thrust than the P-51 at high speeds.

-Pyro

Tomb posted 03-16-99 04:11 AM

i think its more to do with "mass air flow"

a propellor moves a lot of air slowley

so lots of acceleration but less speed

a jet moves a smaller mount of air much faster so less aceeleration but more speed

the ultimate in piston technolgy was the turbo compound engine in which the exaust was expelled into a jet like turbine rear end

the next step up was the turbo prop which was a full jet engine turning a prop

in all cases the prop limits the speed attainable

the most remakable plane is the russian bear bomber with its massive slow turning turbo props...

Tomb

Bino posted 03-16-99 01:12 PM

Tomb: ...the ultimate in piston technolgy was the turbo compound engine...

My impression was that a Turbo-Compound recip engine has both an exhaust-driven intake charge compressor (Turbo-charger) and a second exhaust-driven turbine that is geared to help spin the crankshaft. Here's the page about the P&W R-4360 over at the USAF Museum:

www.wpafb.af.mil/museum/engines/eng34a.htm

bino-- <II./JG54> bino.warbirds.org

chisel posted 03-17-99 01:04 AM

Ok you've made me dig deeper into my A/C powerplants manual.Heres the section on converting thrust into HP.

"Because power is determined by using force and distance,it is not possible to make a direct comparison of thrust and HP in a jet engine. When the engine is driving an airplane through the air,however,we can compute the equivalent horsepower being developed. When we convert the ft/lbs per minute of 1HP to miles-pounds per hour we obtain the figure of 375. That is 1hp is equal to 33,000ft/lb/min or 375mi/lb/hr thrust horsepower(thp) is then obtained by using the following formula:

thp=(thrust{lbs}*airspeed {mph})/375

now using this formula with an 80% efficient prop does drop it down to 300mi/lb/hr at the prop.

now to me this has nothing to do with brake HP as it does not take into account the weight or drag of a certain airframe it compares more to indicated HP but not really either. And using this formula to judge the exhaust HP of a recip engine cant really apply either since the exhaust thrust isnt pushing the plane to that speed by itself.What would the speed gain be with and without exhaust thrust?

Now on the subject of turbojets the reason for the slow acceleration does have to do with 'mass air flow' The faster the airspeed the more effect Ram air pressure has on a turbojet, tries to jam more air into the front, and the more you can jam in the front the more you can fire out the back. Turbojets are very inefficient at low airspeeds.Plus the fact that turbojets spooled up relatively slowly in those days.

The turbocompound was the ultimate in recip technology.They used what was/is called a blowdown turbine or power recovery turbine that worked on the velocity of the exhaust rather than the pressure(to keep backpressure to a minimum) The turbine was connected to the crankshaft through a gear reduction and a fluid clutch. Turbocompounds were used on the DC-7 and the Lockheed Super Constellation (sp?)

On the Wright model TC19DA turbocompound the HP was rated at 3,250HP. Normal R3350 engines were rated at 2700HP.

*not official HP ratings, take with a grain of salt. C.Y.A man

If I learned nothing else in Tech school those 3 letters have served me well!

Its amazing how war brings out the best and worst.Human technology ,overall, advanced more rapidly in those 6/7 years than it has before or since!

So bjh have we wandered off topic yet?

Great thread BTW

Hold still while I kill you

Chisel,

JG5 'Eismeer'

Tomb posted 03-17-99 08:01 AM

sorry dude it has everything to do with airflow mass ..a jet chokes if it gets too much and will "dump" it over the side at high speed or suck it in at low speed

thrust is usually stated as "static" thrust

ie its measured from a stationary engine

any way back to ejector exausts my engine friend states that only at high altitude did much difference show because it was preasure differential is what counts and the higher you go the greater the preasure difference between engine exaust and the external preasure...turbos also benifit up to the max allowed boost limit and then must be backed off with altitude

on radail v inline he states that to go fast especially at altitude you need a pointy bit of plane at the front also for the same HP more thrust can be gained from an inline and coolers so that the cooling mechanism adds thrust while a aircooled radial must pass air over the less than streamlined radial engine and this is draggy..the fw190 minimised the effect at much as possible for radials

the biggest problem for radials was to lose heat ..air is not as good a medium as water/gylcol....the Fw190d gained a supercharged inline to gain altitude performance because it could not be obtained from the radial fw190A

got a performance test done by the US marines somewhere for the P51b v f4u-1 i think..the f4u had more horsepower to to be nearly even on performance

it must be rememberd that its more complicated than just bolting xyz engine on it had to be a engine/airframe combination

Tomb

chisel posted 03-18-99 12:01 AM

sorry dude it has everything to do with airflow mass ..a jet chokes if it gets too much and will "dump" it over the side at high speed or suck it in at low speed

Were u refering to me?

I did state that it DOES have to do with mass airflow.

Ram air effect: as the airspeed increases the air entering the inlet gets its velocity turned into pressure as it piles up ahead of the comressor increasing the density. Now the more dense the air is the more mass it has,and the more mass it has the more thrust you can get from the turbojet engine.

Another way to look at it is since the air is precompressed from the "Ram air effect" the less work the engine has to do to compress the air to the same level, letting more energy to be converted to thrust. The turbojets volumetric efficency is very low compared to a recip engine (you just cant compress air as well with a fan compared to a sealed container) At low speeds Alot! of the turbines energy is used just to drive the compessor. The Ram air effect helps out more and more the faster you go {dont know diddely bout supersonic flight and lets not even go there }

Now we started this by comparing the Me262 to the P-51 the Jumo engines just could not create enough thrust at low airspeeds to accelerate like a merlin.

Ok found some numbers to back me up {once again take with a grain of salt.}

** Ok nevermind it doesnt back me up Dont know RPM Or throttle setting but is still pretty cool info.

from Janes 45/46 edition

Junkers Jumo 004b

Height (ft)Speed(mph)Thrust(lbs)Fuel(lb/hr)

0........273.......1605.......2920

0........560.......1890.......3680

8200.....273.......1300.......2290

8200.....560.......1600.......2920

36k......536.......572........1080

36k......560.......715........1275

fuel consumption is just crazy on the deck!

Anyway, trust me it works even with the drop in thrust due to airspeed the ram air effect more than makes up for it i've seen the math!

Tomb

We are in agreement, I believe, as to why recips are better accelerators than turbine engines (turbojets anyway) just looking at it in a different way.

Now about converting thrust into HP: Horsepower is a measure of force and distance.

Thrust is a measure of force only.

I dont believe there is a mathematical way to convert static thrust into horsepower, since we dont have a distance measurement.Thats why the formula is used to give an approximation of HP {we humans like to compare }

Totally agree here! I believe thats what I said in my very first post about turbochargers. Higher pressure differential means higher velocity of expanding gasses=more thrust.

Wont even get into air verses watercooling I know Water is more efficient. Christ even my CPU is watercooled now. But if I say it outloud my stable of aircooled VDubs might disown me

Hold still while I kill you

Chisel,

JG5 'Eismeer'

nrts

Member posted 03-18-99 01:04 AM

Heh just love this thread. Sat here rereading long enough for my modem to disconnect a couple times

Btw, Chisel your VDubs aren't aircooled, they're oil cooled. One of my own german "aircooled" engines holds 13 quarts of oil in an external reservoir and pumps it through the engine and 2 oil coolers with 3 separate oil pumps. Hardly air cooled eh?

-nrts-

Pyro posted 03-18-99 01:49 AM

Thrust is thrust, regardless of whether it's produced by a prop or jet and whether you measure it in pounds or newtons. Acceleration is simply derived from Newton's second law, F=ma. Acceleration is the quotient of force/mass.

Thrust is a force, HP is power which is a measure of the rate at which work is done. Power = force x speed. If your force stays constant and your speed increases, your power has increased. Rearranging the equation you get force = power/speed which shows that force decreases for a given power as speed increases. This is the advantage of the jet engine. It produces thrust whereas the reciprocating engine needs to convert its mechanical power to thrust.

The prop plane will create 1/4 the thrust at 400mph that it does at 100mph(disregarding varying prop efficiency). This is why you see the different performance characteristics between props and jets. This is why exhaust thrust becomes more of a performance factor the faster the plane is traveling.

-Pyro

chisel

Member posted 03-18-99 02:55 AM

Ok Ok nrts ya got me Air/oilcooled

But thats just a given You got a Porcshe 911?

Ok Pyro I do understand where your coming from now! I wasnt thinking about prop efficiency losses at high speed. Didnt go thru the propeller section yet LOL.

Now that I got that thru my thick skull that 300lbs of thrust translates into how many MPH increase ?

Oh and I did a few calculations on the Jumo figures above at 0ft there is a gain of 51% in airspeed for a 6.7% increase in fuellbs/hr/lb of thrust generated.geez hope I figured that out correctly.

Man I need a life!

Tomb posted 03-18-99 04:39 AM

enjoying this discussion i think we are all arriving at the similar conclusions at the same and for pyro heres a thought.

what you say about jet/prop is generaly true

but as speed increases you need to increase the blade angle which you can do with a prop

the jet cant do this from this era and even today it takes carefull intake design to keep the intake speed under control

so while i agree with you on principle i disagree on detail

the basic fact is niether prop or jet fan likes supersonic air (which does not like to be compressed)...this is the limit which a prop hits first..as soon as the blade tips go supersonic efficancy is dramatically reduced..the russians got round this by slowing the prop speed down and those huge contra rotating props on the bear bomber allowed it to cruise at 480mph and level speed of 550mph...that performance can embarass a lot of jets

while your formulae is in the right direction it aint that easy

Gotta go for now

Tomb

miko 2dashes posted 03-18-99 12:37 PM

Miko,

I disagree. A Turbo is more efficient precisely because it harnesses the energy of the exhaust gasses, which continue to expand as and after they exit the cylinder.

Have to disagree with Bino. According to the laws of physics, the exhaust causes exactly the same pressure on the piston as on the turbine. Hot gases are not intelligent enough, so they try to expand in all directions, rather then just in the direction away from the piston .

I agree that when piston reaches the top and the valve closes, then the gasses remaining in the exhaust system can push the turbine without affecting that particular piston.

Turbo is convenient because it automatically increases the turbine speed with increase in engine power in wide range of conditions (seed, load, gear, air pressure). For an engine running at constant load (electric power generator) a mechanical system would be more efficient.

miko--

Pyro posted 03-18-99 01:07 PM

Prop efficiency is what you use to convert to thrust horsepower. To convert thp to thrust, you use the formula THP*375/TAS.

This is all about the difference between producing direct thrust and having to convert mechanical power to thrust. You can talk about variations in jet performance and prop efficiency, transonic effects, and turboprops, but none of those are really germane to the fundamental and overwhelming difference between the two.

Anybody wanna talk about sand-filled ping pong balls again?

-Pyro

Bino posted 03-18-99 01:21 PM

MOL should set us up with a testing lab. Y'know, engines, turbo- and super-chargers, dynamometers, wind tunnels, test-bed planes, ping-pong balls, the works. Yeah! That's the ticket!

bino-- <II./JG54> bino.warbirds.org

Pyro posted 03-18-99 01:58 PM

Bino,

Actually you could do that in a spreadsheet. You'll probably learn more setting up the spreadsheet than you will by inputing numbers into it.

-Pyro

rsns posted 03-18-99 05:47 PM

miko,

The efficiency of a turbocharger over a mechanical supercharger was best explained in funked's earlier post, in my opinion. Way back when this thread was only about a dozen messages long!

The basic point is that it is true that the exhaust backpressure will increase the pumping loss on the engine, as you rationally assert. However the mechanical friction of a supercharger also represents a loss on the engine, and an arguably greater one. Furthermore the turbocharger recovers some energy from the exhaust gases to run the turbine while the mechanically supercharged engine discards all of the exhaust energy.

-rsns-

925 CABS