Some thoughts on aircraft sound - comments appreciated

[Sutts]

From what I can tell, CloD aircraft sound is based purely on engine speed (RPM). Try flying a Spit II at +2 boost 2600 RPM. Now open the throttle slowly to the gate. After a few moments the prop will stabilise back to 2600 RPM but now you'll be operating at +6 boost. All that extra power made the prop speed up but the constant speed mechanism increased the pitch (coarseness) of the prop to bite into more air until RPMs returned back to 2600 RPM. i.e. the extra power was absorbed. We're now generating far more power (+4 difference in boost) but the RPMs are still 2600. Has the engine note changed at all? Nope.

Compare this to a car going up an increasingly steep hill. The driver stays in the same gear but tries to keep the RPM (speed) constant. On the flat before the hill the engine note was high and it didn't sound under stress at all. When the car hits the hill the current power setting is insufficient to maintain the required RPM and the driver must increase the throttle to apply more power and maintain the RPM. The RPM is the same but the engine now has a lower note and sounds like it's working harder. As the hill steepens further, more and more throttle is required to maintain the RPM, resulting in a lower and lower engine note. At some point, even with max throttle the power is insufficient and the RPMs drop away, requiring a shift in gears.

What we're missing in CloD right now is that variation in engine note depending on the power output (boost) for a particular RPM setting.


I realise this isn't a high priority item and there are many other things that need attention first BUT.....What I'd like to see long term is a more complex formula for aircraft sound.

I believe the sound of an aircraft is the result of 3 factors:

1. The speed of the engine (RPM). This is currently modelled in CloD.

2. The power being developed by the engine (indicated by boost reading). An RPM setting achieved with a low boost will have a higher pitched engine note than the same RPM achieved with a higher boost. As boost increases (engine working harder), the engine note will become lower.

3. The pitch of the prop blades. Courser pitches will bite more air and create a greater throbbing noise. I live near an airfield and the throb of the props being exercised can be heard for several miles.

So I think the ideal would be a combination of those 3 factors. This would create a wonderful variety of engine sounds and it would be much easier to become in tune with your engine - aware of how hard it is having to work - just like in a car when you can hear the engine load become greater on a hill.

Examples of this in operation:

VARIABLE BOOST AND FIXED RPM SCENARIO
Fixed prop speed of 2650 RPM with a low boost. Increase boost and hear the engine tone lower as more power is produced for the same RPM. Sound will also include a greater throb from the prop as the pitch is now more course and has more contact with the air.

FIXED BOOST AND VARIABLE RPM SCENARIO
Fix boost at +4 with a prop speed of 2200 RPM. Reduce prop pitch while maintaining this boost setting. The engine note will become higher as the RPM increases. Since the pitch has reduced, the prop is biting into less air and the prop noise (throbbing) will reduce.

FIXED BOOST AND FIXED RPM SCENARIO
Fix boost at +5 with a prop speed of 2800 RPM. Start climbing increasingly steeply, keeping the boost and RPM settings constant. As the engine struggles to maintain the set RPM, the constant speed unit will gradually reduce prop pitch, biting less air and thereby enabling the selected RPM to be maintained. The throbbing of the propellor will reduce as pitch reduces. At some point the pitch will likely hit the limits of travel, approaching feathered state. After this the RPM will fall away as no further pitch compensation is possible.


I've said elsewhere that I think CloD sound is fantastic now and I don't want this to be seen as a criticism. However, 1c have always liked to push the boundaries and "lift the bar" and I see this as just one more step towards the perfect simulation.

Would be great to hear your views on this theory.

I would particularly like to hear from Blackdog who used to talk frequently about the original CloD engine sounds and how he could manage his engine accurately simply by the sound it made. Has anything changed with regards to the new sound engine? Do you agree on the additional elements that I think contribute to the overall aircraft sound?

Cheers

[ATAG_Snapper]

Great post!

A couple of things come to mind re sound in COD:

Intake growl: when you increase boost by opening throttle in a carberetted engine, this would become a roar in the pilot's ears. I ride an older carb'd Harley (they're all fuel injected now) that's been bored, stroked, hot(ter) cams, heads ported & polished, and the carb itself replaced by the stock 38 mm bore to the present 44 mm bore. The intake is by my right knee. When I roll on throttle (increase manifold boost) the exhaust note definitely change, as does the (very) audible intake drone -- which DOES become a roar to both my ears and one particular overzealous Ontario Provincial Police officer.

I believe the intake roar emphasizes the deepening exhaust note with increased boost (vs increased rpm's) per your fascinating post above.

The second note is exhaust sound, at least the external Merlin note -- which in CoD it ain't. External start up and idle are great, internal cockpit sound credible....but external cruise (F2 key) or fly by (F3 key) is closer to an 18 wheeler truck diesel. We are all collectively "sighing for a Merlin" (with apologies to Jeffrey Quill!).

[Sutts]

Thanks for the great feedback guys.

I'm no warbird pilot so I'm completely open on this one. However, I have been around Spits and many other old and new aircraft when they exercise their props and taxi past and the throbbing/flapping sound is definitely missing. Not sure if you'd hear that once you're in the air though.

My experience comes from listening to my old wartime jeep struggling up hills so I'm coming from the same angle as you ATAG_Snapper. However, cheesehawk makes some excellent points and obviously has experience with high powered straight stack engines. It hadn't occurred to me that you might not be able to hear the intake growl when sitting behind such an engine.

For me it's just a gut feel that the application of masses of extra power (while maintaining RPM) should be detectable in terms of sound both in the cockpit and externally. At the moment, having set an RPM value I can move the throttle through a huge range of power values and not hear a single change in the engine (unless RPM changes in the process).

I'd love to hear the views of some real pilots.

Would also be great to hear the views of your Dad cheesehawk....is a massive power increase completely undetectable when sitting behind a big engine like this (unless RPM changes as a result)?

Cheers