Horsepower, why does it do what it does?

[Monocock]

Pardon me for asking what might at first seem like a stupid question...........

Aircraft "X" has a 160 hp engine and fixed pitch propellor "Y" on the front. When it departs it shows 2550 rpm on the dial.

Aircraft "A" has a 180 hp engine and fixed pitch propellor "Y" on the front. When it departs it shows 2550 rpm on the dial.

So, my point is this. The propellors are identical, the a/c is identical and the engine speed is identical. So how come one gets airborne quicker than the other. Where does the HP come into it?

[Mike Cross]

Many reasons, not least the aerofoil. Different aerofoils generate different amounts of lift at different speeds. As a generalisation fatter sections tend to generate their lift at lower speed than thinner ones, however the thinner ones generate less drag at higher speeds, which is why the new generation of fast and slippery glass ships tend to have higher landing speeds than the older ones and require different take-off and landing distances.

Also bear in mind that you are not talking about something that is in a state of equilibrium, the aircraft is actually being accellerated so your horsepower is not only being expended on lifting the thing off the ground (potential energy) it is also building up kinetic energy.

Mike
(Grubbing around in the distant recesses of his memory for the remnants of his O Level Physics):\

[shortstripper]

errrrrr .... maybe the 180 reaches 2550 quicker? :8

Pushbike A does exactly 30 mph in sixth gear when the pedals are turned at 1 rev per second .... so does pushbike B. If A has a stronger rider than B .... it will reach 1 rev per sec and 30 mph quicker than B .... OK?

IM

[IO540]

If both have IDENTICAL props rotating at identical RPM, there can't possibly be any difference in performance.

But a 180hp plane with have a slightly coarser pitch prop (and/or a larger prop) than a 160hp plane, in order to achieve the same RPM with the throttle and mixture both fully forward (the sea level 100% rated hp setting).

[Mike Cross]

If both have IDENTICAL props rotating at identical RPM, there can't possibly be any difference in performance.

So if you took the engine and prop off a Europa and stuck on the front of a Tiger Moth the performance would be the same?:O :O :O :O

More seriously, I suspect you meant that the engine performance was the same.

Even then you're not right. You could fly a 172 at the same RPM with no flap and with 30 degrees of flap. You would use more fuel and fly more slowly with the flap because the drag had gone up.

Although the engine is turning at the same RPM it is actually producing more power, so its performance is not identical.

Mike

[Keef]

If the engines have different horsepower, and the props are identical, they aren't going to rotate at the same RPM. Unless they are variable pitch props...

One prop's coarser pitch, or different profile or something - or the 180HP engine is really a 160HP all along (or vice-versa).

[Obs cop]

Lets make this dead simple.

A prop is merely another form of wing. If you have 2 wings of identical shape travelling through the air at identical speeds, they will generate the same lift.

Relating this to a propellor, if 2 identical props are turning at the same RPM in the same airflow then they will generate the same thrust as they are merely wings themselves.

If you attach the 2 props to seperate engines, one developing 180hp and one 160hp, then if the props are turning at the same speed with the same pitch, the work being done by each engine will be identical. You can therefore say that if the 160hp engine is maxed out at 2550 rpm, the 180hp engine will only use 160 of its finest horses to turn the prop at 2550 rpm.

The 180hp engine still has some power left to give, which can be utilised either by turning the propellor faster (not always very efficient or effective) or by using a coarser pitch on the blades.

For both engines to be maxed out at the same RPM, the props might be the same make and diameter but they will be set up differently to take account of the different power outputs.

Obs cop

[Flyin'Dutch']

I would have thought that torque would come somewhere into the equation. It is not just hp that matters.

Bookworm will be able to explain chapter and verse.

FD

[IO540]

Keef

That's exactly what I meant. Same prop, same RPM, must be same HP being developed. No way around that at all.

But re-reading the original post, I noticed the aircraft are not necessarily identical! They are referred to as "X" and "A".

This of course makes nonsense of the original question, because (obviously) sticking a powerplant (engine+prop) which shifts 180hp's worth of air backwards on a 450kg plane will make that plane go a lot faster than sticking it onto a 1400kg plane.

But surely everybody can see this, so the original question must have presumed identical aircraft... which takes us back to what you and I said: there cannot be any difference in performance if the props are the same and the RPM is the same.

It becomes obvious when one knows how a VP prop works... open the throttle -> more power coming out of the engine, and the governor (seeing the engine trying to increase the RPM) coarsens the pitch to maintain the RPM constant while absorbing the higher torque.

Flyin'Dutch'

Trying to separate torque from HP does not mean anything because HP is simply RPM x torque. The original post says 2550 RPM in both cases, so the torque must be the same is the props are identical.

[FlyingForFun]

Seems to me that the confusion might be because the issue is being over-simplified. There are actually two separate issues here:

First of all, a propellor, turning at a given speed, will create a certain amount of thrust. This is the thrust which we all learn about for the aeroplane technical exam, then probably forget about immediately afterwards - it's what opposes the drag. As Mike points out, we are not talking about equilibrium here, so whatever thrust is left over after counteracting the drag is used to accelerate the aircraft. Less drag means more left-over thrust, hence faster acceleration and less time on the runway. (And, once airbourne, it means a faster rate of climb.)

The second issue is turning the horsepower into thrust. This is a combination of lots of things, the main one being the pitch of the propellor, but also the efficiency of the engine/propellor, and the forward speed of the aircraft (the faster it's going, the more windmill effect there will be on the prop, which will turn the prop/engine faster - and a faster engine generates more horsepower, which turns the prop faster....)

In his original question, Monocock states that the two props are identical, fixed pitch, and turning at the same RPM. In this case, and assuming the same Vr (which wasn't stated), the thrust will be the same in both cases. So the only thing which would explain a difference in take-off runs would be different drag, as Mike suggests.

The next question, though, is why would both props be turning at the same speed? If the props are identical, and the airspeed in both cases is identical, I would expect the prop attached to the more powerful engine to be turning faster, unless that aircraft was travelling significantly slower. In practice, a more realistic situation would be that the bigger engine would have a coarser pitch prop attached to it - and since Monocock explicitly stated that this isn't the case, if this is a real-life scenario then I can't explain it.

FFF
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[Evo]

Trying to separate torque from HP does not mean anything because HP is simply RPM x torque. The original post says 2550 RPM in both cases, so the torque must be the same is the props are identical.

One minor issue - although the torque curve must be a bit odd for the two engines to give the same power at 2550rpm, an 180hp engine would be expected to develop more torque at lower rpm than the 160hp engine and will accelerate the propellor to 2550rpm faster, so I guessthe 180hp engine will be developing 'full' thrust for longer than the 160hp engine.

[bar shaker]

If the aircraft are identical and are being flown with the same flap and AUW, then one won't get in the air quicker than the other. The thrust that pushes you down the runway is directly proportional to the speed of the prop and if both props have the same pitch and turn at the same speed, the thrust will be the same.

In your example, the more powerful engine must be using a coarser prop or the extra power would allow it to rev higher, ie until the drag and load of the extra air pushed matched the extra power.

[IO540]

Evo

That would be true only if both engines were rated to develop their max rated hp at the SAME rpm.

I still think there is something missing from the original question, because with identical planes the answer must be there is no difference.

And with different planes it doesn't make any sense. Then there are so many factors one cannot make a comparison.

Except perhaps to say that, to a very rough first order approximation, if they weigh similar and have same hp, they will have similar Vy climb rates, regardless of aerodynamic properties (because drag is low at Vy and most of the engine power goes into gaining potential energy).

[Mike Cross]

I can of course fly my plane at the same RPM on the front or the back of the drag curve and get very different performance.

Hanging it off the prop at high RPM and a high angle of attack can give me straight and level flight at a very low speed.

Flying straight and level at the same RPM with a low angle of attack I will be going much faster.

In the first case you will need to apply more power to get the RPM than in the second so the suggestion that HP and RPM are directly related to each other is wrong.

Stuff the nose down and watch the RPM increase. Is the engine generating any more power to produce this increase?

Pull the stick back and watch the RPM decrease. Is the engine producing less power?

Mike
Lots of barking - wrong tree!

[shortstripper]

I think you must all be looking too deeply at this .... read my last answer!

The original question asked was why one of the identical aircraft ( with identical props ) gets off the ground quicker than the other. The fact that they both get off the ground with the same rpm is pretty much irrelevent ( at that given moment both will be delivering the same power ). I'd have thought it's obvious that a given rpm gives the same performance if the prop and a/c are the same .... just like my pushbike analogy with gearing and rpm.

What is different is accelleration! A more powerful engine will accellerate the aircraft to the given rpm in a shorter distance than the less powerful one .... so it gets off the ground more quickly.

Surely that answers the question! :ok:

IM

[Sultan Ismail]

And the answer is DRAG also known as useful load

If the indicated parameters are equal, i.e. airframe and rpm, then for any given airspeed the power required for flight is equal. However, as drag in the form of extra pax, extra fuel, extra baggage, are added to the equation then additional POWER is required to maintain rpm and overcome the drag.

Don't forget, the engine requires power to turn the prop at rated rpm, 2550 is given, and also provide thrust to move the inherent drag to a speed sufficient for flight.

In the example given, 30hp is available to bring along additional drag.

Mr. Piper worked this out well with the Cherokee 140, 180 and 235. The 140 can just about lift 4 people and not much fuel, the 235 can lift its own weight. The airframes are similar.

Horses for courses.

Simple.

Sultan Ismail

[bar shaker]

Mike

We are talking about being able to take off earlier and whilst your points about attitude affecting drag are completely correct, they are not relevant to take off roll, as both aircraft will be level on the runway.

SS

This could be the case, but I suspect that both engines will reach max rpm at the end of a 3 second throttle opening period.


I cannot see any other answer than the extra power being used to drive a coarser prop.

[shortstripper]

Ok ... forget the prop just thrashing the air at 2550 rpm. Think of the prop as you should ... an airscrew!

Now, you have to know if both engines are capable of attaining 2550 rpm at a standstill? if they both can, then the prop is way to small for either engine ( either in diameter or pitch) and especially for the bigger engine. If that's the case then the engine will be over revved and it's full power will never be utilised. Max rpm ( assume max rpm for TO) should never be otainable at a standstill ... if it is .... the prop is to small. So in this case having the bigger engine is a waste anyway unless a different prop (airscrew) is fitted. SO if, as it should be, the aircraft needs to move forwards to allow the prop to screw through the air efficiently enough to use the full power available ... the more powerful engine will get there first.

And I'm assuming the same pilot is flying .... because you most certainly can slow an aircraft's ability to get airbourne by pulling the nose up too soon and thus causing drag! ... get off the ground that way and you're straight to the back of the drag curve!!! .... been there, so I know ( same aircraft, same power, same rpm, but completely different performance ) :uhoh:

IM