bearcat envy

In general concept you are correct regarding relative thrust between an early jet and a late model prop fighter, but there are some incorrect details:

The big distinction is between thrust and power.

A jet doesn't gain more thrust as it goes faster. The thrust remains pretty much the same.
A prop driven aircraft actually has quite a lot of thrust available when taking off. The problem is that as it goes faster, the horsepower remains the same, but the thrust goes down. It becomes even worse because the efficiency of a propeller is seldom much over 80-85% at best and usually it is well below that.

Thrust is Force.

Power is Force X Velocity
which means that for the same horse power, as the velocity increases, the force (thrust) goes down.

In addition, as the aircraft moves faster, a propeller is able to convert less and less of its power to thrust because as the blade angle becomes more coarse, it moves more air sideways than along the axis of travel. There are also mach effects which make things even less efficient as speed increases.

- Ivan.

This is one thing ive always been confused about, in the same sentence it says thrust is constant but the faster AC goes the more power it developes, which is why jets have poor low speed acceleration and climb.

I agree also about prop efficiency, infact at the potential top speed of 568 mph prop are only 50/60 percent efficient.

Dont meen to stray too far from topic, but i new a pilot who i met at an airshow 20 years ago, he flew p51s during the end of the war, but never left the states, he eventually flew the p80a, we talked allot about that since he knew i had an interest in both, one thing i really remember is when he talked about the difference in climbing ability between piston engine and jet fighters, lets say at sea level at max speed in a prop fighter(340/370 mph) when you pull up into a climb, he said that your speed dropped rapidly until you approached your ideal climbing speed, compared to a jet at sea level (520/540 mph) when you pulled up into a climb, he said you dropped speed so damn slow it was amazing.

some people might find this interesting, i was reading about the watson wizzers, one of the the pilots p47 pilot dove on a 262 from an higher altitude but even though the me 262 was flying straight, once he saw the p47, he pulled into a climb and left the p47 standing , the p47 pilot not only said he was out of gun range but was out of sight in a very short time, any way there are a million stories like that about the 262, the interesting part wast the next Paragraph when talked about test flying a fw 190d9 (aircraft he had flown, spitfire, p47, p51, p38) he stated that the 190d9 was the best vertical fighter of the war(piston engine) i dont disagree with this, from what i know, it had a very good acceleration in the dive, a fantastic zoom climb, a high peak roll rate and powerful centerline firepower, this all made for an ideal energy fighter, something should be pointed out though, what fighter is missing from the fighter he has flown list, thats right, the tempest V, this is in my oppinion the best allied energy fighter of the war so i wonder which is the best between these two awesome fighters, and to keep on topic was the Bearcat an energy fighter or an angle fighter if you had to place it into a category.

from that perspective the prop looks darn close to the ground to me. a little lift of the tail and ping goes the prop.

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Steve Hinton recently told me that, like many powerful fighters, you take off in a Bearcat in a tail-low attitude. It's not a matter of getting the tail up, then rolling forward some more, then feeding in some back stick...an airplane like this simply flies off the runway on its own, and then you're already in a climb attitude anyway.

I don't believe the Fw 190D or Ta 152 were the equal of the Bearcat at all. They were and are two completely different approaches to a piston fighter, one of which is a land plane and one of which is a carrier plane.

I think the F8F was the better aircraft by a considerable margin, except in firepower, below about 20,000 feet while the Fw 190D and Ta-152 were better aircraft above about 20,000 feet. The service ceiling of the F8F-2, which didn't see WWII service, was 40,000 feet while the Ta 152 could get to 48,000 feet ... but the Ta 152H was better above about 20,000 feet relative to the F8F-1, while the F8F-2 could probably hold it's own up to about 30,000 feet where it would be down on excess power compared with the Ta 152H ... none of which were in service when the F8F-2 was in service.

At almost any atitude until up pretty high the F8F could handily outclimb the Fw 190D or Ta 152, if that happened to matter at the time. Rate of climb by itself, while a good advantage if you have it, doesn't win too many dogfights. It's what you do with the advantage that makes the difference.

In Europe, considering these two aircraft only, during WWII, I'd have chosen the Fw / Ta at high altitudes and would have wished for the F8F-2 at lower altitudes ... if I had known it was in the pipeline ... which nobody in WWII who was a USAAF pilot in the ETO probably knew about anyway. In 1945, there were several advanced planes that would have been good choices. The Tempest was definitely one of them, as were late-model Spitfires. I discount the single story of the Tempest / Ta 152 duel because we do NOT know the relative pilot skills and whether or not both sides saw each other with time to react and make a difference. We also don't know the time in type for any of the pilots. An ambush victory doesn't mean one plane is better than the other, on either side ... it means somebody got killed while they weren't looking.

In any case, both sides lost aircraft in that encounter, so it is almost useless as a data point, all of the victors on both sides were sure their aircraft was better and may gave been right at that instant in time.

The mathematical equation that converts Power to Thrust is
P = F.v

Where
P = Power in Watts
F = Force in Newtons (about 0.1kg)
v = velocity in meters/second.

For instance a 1000kW (1375hp) engine in an aircraft flying at 100 meters second (220mph) would be producing an thrust of 1,000,000/100 = 10,000N (1000kg or 2200lbs)

If the speed is now doubled to 200 meters second (440mph) then the Thrust would be 1,000,000/200 or about 5,000N (500kg or 1100 lbs)

There would probably be well over 100kg/220lbs jet thrust from the engine (depending on supercharger strategy) but as the propeller would only be about 80% efficient the engine would need to produce about 1200kW power.

The Equation could of course be reversed; For instance a Jet engine producing say 1000kg thrust at 200 meters sec (440mph) is producing 2000kW or 2700hp.
A Jumo 004B4 could produce 900kg thrust (1980lbs) at sea level static which reduce to 380kg at 10000m (33000ft)

The jet engine achieved near constant thrust from zero speed to max. There is a slight reduction as the aircraft speeds up due to inlet pressure.

The thrust of a turbojet engine is mass flow x (exhaust velocity - inlet velocity)

The result is that piston engine aircraft have lots of 'thrust' at lower speeds, which of course provides excellent acceleration for take-off and low to medium speed combat.

Don't think so Koopernic! Unless someone changed the laws of aerodynamics, the only thing that produces thrust is the jet engine. The thrust is transferred from the engine to the airframe through the engine mounts ... there is no other thrust produced. The airframe produces only drag and weight. The wings produce lift. That accounts for all the forces in flight.

The exhaust efflux from a jet engine is NOT coming out at the same velocity as the aircraft is moving (either through the air or over the ground). The jet thrust results from movement of air and other gasses produced from combustion through the jet engine and out the exhaust, not from the velocity of the airframe. The velocity of the airframe and the velocity of the jet exhaust are completely unrelated by any equation or physical laws of any sort except mathematical addition / subtraction.

Once you get to Mach 2.5 or better, you tend to get more thrust from suction at the jet air intake than from efflux from the jet exhaust. At Mach 3 the SR-71, for instance, got twice the thrust from intake suction as from jet exhaust. This is still thrust from the jet engine, even if at a different point from the exhaust efflux.

I don't think there is anything wrong with my equations, maybe just the way they were expressed.
In the P =F.v equation the v(velocity) I am referring to is the aircraft speed.
It comes from taking the time derivative of the Work equation: W = F.s (work = force z distance moved) which becomes
dW/dt = F.ds/dt = P = F.v

In the Turbojet Thrust = mass flow x (jet exhaust velocity - intake velocity) I am not referring to aircraft speed though inlet velocity is roughly equal to aircraft speed.

For a rocket the equation reduces to Thrust = mass flow x exhaust velocity.

Essentially F = dm/dt x v (thrust = net mass flow rate x velocity)

An important concept used in defining turboprop performance is eshp (equivalent shaft horsepower) where the equation P = F.v is broadly used to convert jet thrust into power by assuming the aircraft is moving at 100knots. (I think 100 knots is the 'standard') A speed of 100 knots is about 115mph or 51.5 meters second.

The simple rule is to take jet thrust and dived by 2.6 to get eshp.

A Merlin on 100/130PN at 18psig two stage produced about 1700hp and 300lbs thrust which would give the Merlin an official eshp rating of about 1800hp. It would be a little more since one is supposed to account for prop efficiency, say 1820hp).

If the standard was 300knots, closer to Spitfire Max speed and closer to modern turboprop speed, that jet thrust would be equal to about 2000eshp.

There is not much around on the net regarding this.
DatWiki.net - Aviation Dictionary Presented by Aviation Supplies and Academics, Inc.
http://www.aerospacepro.com/product_helpfiles/simple_turboprop.html

The talk of the SR-71 engine 'suction' is the first time I have come across that idea, roughly as I understand it the inlet spikes caused a shock wave that slowed down the air to subsonic the reduced velocity showed up as pressure that was used to create thrust in a sort of ram jet created by 5 tubes that bypassed the turbojet and took the air to the afterburner area.

"An important concept used in defining turboprop performance is eshp (equivalent shaft horsepower) where the equation P = F.v is broadly used to convert jet thrust into power by assuming the aircraft is moving at 100knots. (I think 100 knots is the 'standard') A speed of 100 knots is about 115mph or 51.5 meters second.

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This one is all over the map. Engine manufacturers often quoted it because it makes the engine sound better in advertising, especially the fuel consumption figures. HOWEVER in order for the "thrust" to be doing useful work the jet exhaust has to pointed the opposite way the aircraft is flying, not just dumped out the side.





or top;




what direction is the thrust vector on this aircraft?

The thrust may help a helicopter in forward flight. It does nothing for take-off or hovering.

Exhaust thrust "power" varies widely with just altitude, let alone speed. The "power"(not thrust)from a Merlin XX in a Hurricane varied from 126.8 to 65hp depending on altitude at full throttle and in high gear. In part due to the back pressure on the engine exhaust affecting the exhaust speed. At 15,000ft for example the charge flow ( weight of air and fuel) was 140.5lbs per minute compared to 144lb/min at 20,000ft. However the velocity at 20,000 ft was 1695 fps instead of the 1395 fps at 15,000 so the "horsepower" was 113 at 20K instead of 86.6 at 15K. Speed of the aircraft was 335mph at 20K vs 325mph at 15K.
That is the problem with converting "thrust" to HP. It varies with the inverse of the speed of the aircraft (or vehicle), technically a vehicle running a jet engine and standing still (brakes on or tied down)is producing thrust but no horsepower. The better the match between the exhaust speed and the vehicle speed the more "power" is produced for the same thrust.

Actually a jet engine makes thrust (force) by moving hot gasses around and through the engine. That force is what is used to move the ariframe. Static thrust is measured by load cell in a test setup. It is the thrust that pushes when the brakes are released (zero airframe velocity) and the airframe starts to move down the runway.

Once the jet aircraft is flying and is moving at constant speed, as in cruise, you have the velocity, with zero acceleration. That gives you ONE of the three variable in your equation: P=F*v. You need one of the other two to solve the equation, and dynamic thrust is not equal to static thrust, so coming up with it isn't easy since there are very few graphs of exhaust velocity and mass flow for current military jet engines with changing airspeed. The designers have those graphs, but finding a real one for the average Joe out there isn't easy, especially as applied to a current mnilitary aircraft. They exist and are classified well past retirement for US types as well as for British types.

You might be able to dig one up for some particular model of, say, the F-86, but good luck finding one for an F-35 or even an accurate one for a current F-18.

With pistons, They usaully back into it by assuming some power level at some rpm and altitude, and say the propeller has "assume 80% efficiency," which makes for a good paper solution but is pure bunk as far as accuracy goes. What they do is successive approximation until their calculation and the experimental reaults match. Then they claim to have it figured out ... but they are usually off whenever they calculate the next program until they again back into the right numbers.

I've seen it happen too often to believe otherwise.

Remeber when you were in college? What was the first rule of Physics lab for the guys who were just going for a grade instead of learning?

For best results and a 2% accuracy, always have your lab report written before you do the actual experiment! Then change the numbers to get the expected error!

Hello Shortround6,

my family and I flew on something darn near that Bombardier turboprop a couple weeks ago when we were on vacation.

Hello GregP,

It seems to me that the Jet thrust problem is a much easier problem to work out than the one for the prop plane. With the prop, you need to make way too many assumptions about the efficiency, prop power coefficients, and drag of a windmilling prop if you just want to calculate airframe drag at a particular velocity. With the jet, you can get a pretty good idea what the static thrust and exhaust velocity are. If you are flying the jet, you can get an idea of airframe drag by timing deceleration with throttle closed.... Or simply get the drag numbers from wind tunnel data.

After that, it's just thrust = drag. Now if the drag numbers are so secret you can't get them, how is that a different problem than if you can't get drag values for an older prop fighter?

Think of all the things that are simplified with a Jet. No prop windmilling drag, cooling drag, blade pitch limitations, intake ram effects, exhaust thrust, cowl interference, vortex effects, etc.

BTW, Isnt this getting pretty far off topic again?

- Ivan.

Don't forget, when talking about turboprops, thrust is produced by the propeller, not by the hot stuff coming out the back. The exception is in big ones like the Allison T-56 and the big Russian ones, where the hot stuff produces thrust. Some of the older ones did the same, but by and large, thrust is produced by the prop. The PW 100 series engines that powers the Dash 8s (ATRS etc), it's all hot air that comes out the back. In the cockpit, the turboprop pilot is watching NP (percent prop rpm), NH and NL (high and low pressure compressor rpm) and Torque. ITT or Inter Turbine Temp is also a very important indicator of engine performance obviously, but equivalent thrust is indicated to the pilot by these gauges. If you look at the pic of the single engined turboprop posted Piper Malibu or something, the exhaust ports are at the front of the engine; this is a PWC PT-6 and is a reverse flow engine; air enters at the back and goes forward. It's a free turbine, meaning the prop has its own power turbine and is driven (through the RGB) by this, rather than the compressor. The PW 100 series engines are the same.

Think of all the things that are simplified with a Jet. No prop windmilling drag, cooling drag, blade pitch limitations, intake ram effects, exhaust thrust, cowl interference, vortex effects, etc.

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Actually, all of those things to a degree, with the exception of prop drag also affect pure jet engines. Turboprops are more quirky than pure jets and probably consume more maintenance time. The RGB (reduction gear box) is subject to a considerable degree of wear and tear. You've also got things like prop governing equipment that can cause the odd headache.

Yup, pretty far off topic.

If it were easy, we'd have people all over doing it instead of a few aeronautical engineers with specilaized training in college. But, if it's easy, by all means post the copmplete analysis with references. I wonder if they coincide with mine? Maybe.

To get back on topic, the Bearcat is my favorite US piston-powered fighter. The Zero is my favorite of all, the Fw 190 is my favorite German piston-powered fighter, and I like the radial-powered Tempest and later Sea Fury the best of the British fighters. Perhaps my leaning toward radials is evident here. They are more rugged, and both harder and easier to maintain (some things are easier, some things are harder ... easier overall for a 1 or 2 row; harder for a 4-row). They are not tolerant of overspeed (rpm) but are very tolerant of high manifod prssures. So, you can get very good, reliable power for the displacement. I say relaible and mean that to say the radial is robust for the power produced. Failure rates are very low even if maintenence is high, and it isn't always high ... depends on the radial. The R-2800 is relatively low on the maintenance totem pole and makes VERY good power. The Wright R-1820 was low maintenance and many of the Japanese radials were low-maintenance. I don't believe the BMW 801 was, but the smaller Bramos were. The Soviet radials were reliable, and were reliable in conditions that grounded other Allied planes, radial and inline, due to the cold. They were less reliable in hotter climes. For the Brits, the Centaurus was reliable if you didn't turn it higher than rated rpm. If you did by much, it was nylon letdown time.

GregP i agree with your fav piston engine fighter with the exception of the Zero, my fav American fprop fighter is the Bearcat and my fav German prop is the FW 190 family, the 190A, 190D9 and the TA 152H1, though some think this a completely different AC, the 152 H1 has the benefit of actually seeing combat during the war...though shooting down at most 11 allied AC, i kinda agree about how the Bearcat being better below 20000, but not as good above that, im jut not sure at what altitude the 152 H gains the edge, being able to aout turn a spitfire at medium and high altitudes, its amazing speed 472 at 41010/466 at 29530 ft at 0ver 11000 ft difference thats only 6 mph drop in speed(thats more like a jet), its center line fire power of 1x30 mm with 85-90 rnds and 2x20 mm with 175-220 rnds, (mine shells) the spitfire XIV for comparison carried 120 rnds of 20 mm, ceiling as high as 49500 ft...range of 755 miles on internal fuel, advanced throttle system,etc, it really is a beast with only the me 262 (and he 162 i don't count the me 163, too unconventional) neutering it (of fighters that saw combat) as awesome as it was, 0 to 15000 ft the Bearcat would have probably bested it with is superb handling and climb.

One thing i would like to point out, is that with all of the what i call 4th generation piston engine fighters, there is one that i put above all others when it comes to keeping it in production in the jet age and thats the Do 335, even though its def not my favorite, the reason why is because all prop interceptors/air Superiority fighters were obsolete in the age of the jet, but the DO 335 should be an amazing AC as far as fighter bomber/ ground attack, battle field recon etc...the fact that it was piston engine and had a range of almost 1300 miles on eternal fuel, meant that it had superb loitering time especially at low alt, with its eternal bomb bay(less reduction in speed) and wing hard points meant that it should be able to carry a large payload, 2x20mm and 3x awesome mk 103 30mm cannons gave it fantastic strafing capability, it should also have really good battle field survivability, with its 2x centerline piston engines, the fact that you have 2 and there would be no Asymmetrical problems if one is knocked out, hell, you even have a ejection seat for pilot survivability, its high roll rate meant good target acquisition when changing from one target to another, damn, with its speed, its center thrust of 3600 HP or more, and weight, prob meant it could boom and zoom with some of the early jets..(not the p80 and especially the me 262) basically as a tactical fighter/aircraft it had a allot to offer when compared to early jets. So in the jet age, one piston engine fighter had a place unlike most other piston engine fighters and that was the DO 335, this is of course just my opinion, but i think its pretty sound.

wrong thread

Koopernic said:

The thrust of a turbojet engine is mass flow x (exhaust velocity - inlet velocity)

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Which I think is the same for a propeller driven aircraft.

Propeller Thrust

To get back to the Bearcats wings - in a book I have Stephen Gray is quoted as saying max aileron deflection could only be used up to 4.5 Gs - above that there would be aileron reversal

I would have to say of all the late era prop driven fighters, the Bearcat was one of the best, though I would take the P-51H over the F8F...mainly because it had better high altitude performance...