Question on audible engine pitch and RPM

[JozMk.II]

Hi,

 

I've noticed in DCS: Mustang that alteration of the manifold pressure by itself does not change the audible pitch of the engine; the only audible difference in such a case is that the engine volume changes in intensity. Changing the RPM, however, does change the audible pitch of the engine.

 

In other simulation aircraft with constant-speed propellers, such as RealAir's Lancair Legacy for FSX(a very fine payware aircraft if I do say so myself), alteration of the manifold pressure DOES change the audible pitch of the engine, as does alteration of the rpm.

 

My question is, which is correct? In a constant-speed propeller-equipped airplane, does alteration of the manifold pressure with the RPM being constant cause a change in the audible pitch of the engine?

 

Or, are they both correct? IE, are there some constant-speed prop-equipped airplanes whose engine's audible pitch can be altered by change in manifold pressure alone, while there are some who aren't?

[ARM505]

Actually I think the DCS model is ok here, I've found that aircraft with constant speed props just 'sound louder' when you increase the MP. There's often a little 'hunting' as you advance the throttle (especially if you do it quickly), and then you'll notice a pitch change possibly, but if you do it smoothly the exhaust note just gets deeper/louder/more 'oomph' really :)

 

If you think about it, when increasing MP with a constant RPM, there are no mechanical changes in terms of velocities - hence no real change in 'pitch' (ie cycles per unit time), only in pressure (ie sound wave intensity in this case of the exhaust).

 

My 2c.

[Der_Fred]

Thinking through this.. The increase in MP could be perceived as increase in RPM, as the exhaust pressure will be higher, and will travel faster (sound travels at different speeds at different densities) resulting in an increase in audio pitch.

 

You probably want to differentiate between engine noise (RPM) and exhaust noise (MP + RPM).

 

Just a thought.

[Weta43]

as the exhaust pressure will be higher, and will travel faster (sound travels at different speeds at different densities)

 

I don't think so - the air the sound is travelling through is the atmoshere around the aircraft, and that's at a constant pressure regardless of MP.

 

Even if you were right about the shock waves being propogated faster, it wouldn't change the Hz, as the frequency of the sound is governed by is the number of shockwaves per second hitting your ear from the exhaust, and that's constant as long as the engine RPM is constant.

[NoJoe]

Hi,

My question is, which is correct?

 

I am a real-world pilot, and I can verify that in this case DCS is correct and FSX is incorrect (and ARM505's description is right on). With a constant speed propeller, increasing MP will only make the engine sound louder (maybe more "throaty"), but the propeller will keep the engine turning at the same RPM. So the pitch of the sound does not change.

 

One exception to this that we encounter in the real world is when the plane is flying relatively slow at a low power setting, such as coming in to land. In that condition the prop may be at its minimum blade angle setting, so the RPM will begin to decrease. In this condition, adding power will increase the engine RPM up to the governed speed (and make the sound pitch increase). But that's kind of a special case. :)

 

--NoJoe

[sobek]

I've noticed in DCS: Mustang that alteration of the manifold pressure by itself does not change the audible pitch of the engine; the only audible difference in such a case is that the engine volume changes in intensity. Changing the RPM, however, does change the audible pitch of the engine.

 

MSFS has had it wrong since the start and i really don't know why they never fixed this, it was such a blatant error.

 

Anyway, pitch is dependant on RPM, the only noise that might change pitch with manifold pressure is if the plane is turbocharged, then the turbocharger noise pitch will change with manifold pressure as the exhaust pressure increases or decreases.

Edited February 26, 2013 by sobek

[sobek]

Thinking through this.. The increase in MP could be perceived as increase in RPM, as the exhaust pressure will be higher, and will travel faster (sound travels at different speeds at different densities) resulting in an increase in audio pitch.

 

As Weta explained, even if the exhaust sound was to travel faster (which is hardly the case), that would only influence the number of wavefronts being on their way to the observer, not the spacing between the wavefronts, that is solely dependant on the frequency that they are being generated with, which is dependant on motor RPM.

Edited February 26, 2013 by sobek

[JozMk.II]

I am a real-world pilot, and I can verify that in this case DCS is correct and FSX is incorrect (and ARM505's description is right on). With a constant speed propeller, increasing MP will only make the engine sound louder (maybe more "throaty"), but the propeller will keep the engine turning at the same RPM. So the pitch of the sound does not change.

 

One exception to this that we encounter in the real world is when the plane is flying relatively slow at a low power setting, such as coming in to land. In that condition the prop may be at its minimum blade angle setting, so the RPM will begin to decrease. In this condition, adding power will increase the engine RPM up to the governed speed (and make the sound pitch increase). But that's kind of a special case. :)

 

--NoJoe

 

Just the sort of feedback I was looking for; it means much coming from a real-world pilot. Thank you very much for your confirmation.

 

Very cool to hear of another one of the many intricate details that ED got right with this sim.