Warbird questions

[B-17 guy]

I know many of you here have direct access one way or another to real live warbirds so I have a couple of questions of your expertise.
There is a thread over on one of my modeling forums now about the whistling sound of the P-51. The author of the thread is asking what makes the whistle sound and a few people have replied with interesting ideas. Though one of the ideas is that the whistle sounds comes from the passage of air over the gun barrels in the wings, which sounds not true to me. Most other explanations suggest the supercharger, which is much much more logical to me.

So, what does make that beautiful sound that I love so much?

Also, another poster suggested something about how most warbirds are are flying with much less power than was used during combat ops, while I agree to a point as warbirds are not carrying a bomb load and operating under combat conditions. But the airplane still has to get off the ground right? So how much truth would there be to that statement?

Thank you for your time, and also for what you guys do on the ground, in the air and at the bank to keep those beautiful planes flying.

[flightsimer]

I know many of you here have direct access one way or another to real live warbirds so I have a couple of questions of your expertise.
There is a thread over on one of my modeling forums now about the whistling sound of the P-51. The author of the thread is asking what makes the whistle sound and a few people have replied with interesting ideas. Though one of the ideas is that the whistle sounds comes from the passage of air over the gun barrels in the wings, which sounds not true to me. Most other explanations suggest the supercharger, which is much much more logical to me.

So, what does make that beautiful sound that I love so much?

Also, another poster suggested something about how most warbirds are are flying with much less power than was used during combat ops, while I agree to a point as warbirds are not carrying a bomb load and operating under combat conditions. But the airplane still has to get off the ground right? So how much truth would there be to that statement?

Thank you for your time, and also for what you guys do on the ground, in the air and at the bank to keep those beautiful planes flying.

Not really sure about the P-51... My answer would be just because its a P-51, so it sounds like a P-51... my mechanical answer would be the combo of the Allison engine and that large prop. but to answer your other questions...

Power Output of these vintage engines is indeed lower than what they put out during the War. The reason being is the fuel. The only thing available now is 100LL (Low Lead). Back in the War, they were using 100/130 and 115/140 Octane fuels which both gave higher horsepower outputs.

On Our C-123, We have R-2800-99W Engines. The most powerful variant of the R2800, with water injection which was rated at 2500HP each. We removed the water injection system and with the use of 100LL, we have determined that at Take Off Power, we are only producing around 2000-2100 HP.

Another thing is that people baby the engines. By that I mean, they just in general are not run as hard as they were during the war. IIRC, Our engines are supposed to top out at 54in manifold pressure on takeoff at 2800RPM. However, We only take them up to 52in Manifold at 2800 to help preserve the engine. As soon as we have cleared any obstacles, we are powering back to METO power (48In), which is then pulled back to Climb power (which is around 35In). So we are only running the engines at a high power setting for maybe all of 2 minutes of the initial takeoff run and very initial climb out.

Hope that answers at least part of your question.

[Rob Mears]

Personally I believe it's the inboard gun port in each wing where the .50 caliber is recessed inside the wing. I think this only because the howl is far more pronounced when the plane pitches up at high-G. It would have very much the same effect as blowing across the top of a big whiskey jug, only at 300mph!

That's my unscientific presumption anyway. I know the pitch of the plane really lends to the effect, and that's the only physical characteristic I've spotted on the Mustangs airframe that would seem to contribute to such a thing. The Corsair's whistle apparently emanates from the scalloped oil cooler fins in the wing root intakes, which would be a similar effect to what I described above. Makes sense to me I suppose, though I'm more than willing to consider other perspectives.

[JimH]

it's a Merlin

[Enemy Ace]

Air passing through the Radiator tunnel which also produces a small amount of thrust IIRC.

[JimH]

61" in a Mustang is the same as it was 70 years ago...49" on an 1830 is the same as it was 70 years ago...do we run less manifold pressure?, yes, it prolongs the life of the engines. We use 50" for take off in the Mustang and no more than 44" on the 1830, 40" on the 1820, BUT the power is there if needed. The C model has the same scoop as a D and doesn't make the howl...I think its the gun ports.

Jim

[WarbirdRadio]

I vote gun ports.

Matt

[shrike]

Passing along hearsay, but I've heard that the aux air intake grilles (the odd little panels with the holes at the nose) can contribute to the whistle as well.

Given the number of variables on current flying P-51:
the gun ports- faired over, mock stubs, barrels only (plugged and not) or full mocked up guns (plugged or not), taped muzzles on any of those.
the aux intakes; all present and correct, grilles with partial trunking, grilles showing holes, but blanked off, smooth panels.

If there were a single cause, then at least one airframe example flying would be absent the whistle because of the lack of one of those features.

[Xrayist]

I also think it is the gun ports. I had the chance to see up close two P-51's doing flybys. One had the replica guns in it and the one did not. The one with the gun ports still in the wing made a distinctive whistling sound when doing low passes where as the one without did not.

[Scott Rose]

Air passing through the Radiator tunnel which also produces a small amount of thrust IIRC.

AFAIK it does produce a small amount of thrust since it is vaguely a ram jet. My understanding is that the thrust produced helps offset the drag created by the big honking doghouse the radiator is housed in.

[Jack Frost]

It's been a long time since I flew behind piston engine power in DC-3s, Convairs, B-25s etc. but I still remember that the manufacturer's tech reps told us that we should use full power on all take-offs. The reason being that at full power the acceleration is faster and you can reduce to the next lower power setting sooner and reducing the amount of time the engine is at the higher power settings and we weren't flying with new engines either. The engine was wearing just as fast at 44" /2800 Rpm as it would at 48"/2800rpm but it ran a smaller portion of it's time at 2800 rpm which would extend it's service life.

Jack

[KenH]

It's the gun ports.

[bdk]

It's been a long time since I flew behind piston engine power in DC-3s, Convairs, B-25s etc. but I still remember that the manufacturer's tech reps told us that we should use full power on all take-offs. The reason being that at full power the acceleration is faster and you can reduce to the next lower power setting sooner and reducing the amount of time the engine is at the higher power settings and we weren't flying with new engines either.

My understanding is that at full power there is an extra circuit of fuel enrichment. Taking off at lower power settings may allow the engine to run too lean on initial climb when airspeed is low and cooling (on a radial) is less effective.

I think Randy Sohn wrote a good article about this.

[Red Tail]

It's the gun ports. Years ago before the Late Dick James had the gunbays finished out in his airplane "Donnamite" it was quiet during manuvers. After the gunbays were finished with replica guns, it made the whistling sound you speak of. Dick and I spoke at length about it. Our conclusion was, nothing else had changed on the exterior the airplane but the addition of the guns in the ports so it has to be the air across the gun barrels.

Using full rated MP is fine with 100LL if you enrichen the carb jetting 10% and retard the ignition timing. On our Merlin we're running 38 and 42 degrees advance, 10% rich and keep the coolant temp no higher than 95C and the oil around 80C. Failure to make those adjustments and pushing the engine to full MP is going to lead to detonation which will tear your engine apart faster than a a pack of wolfs eating a deer. Reducing MP during take off is a wise hedge to keep your engine together and happy and experience fewer issues. I will tell you a firsthand story that supports the data. Before we made those adjustments we had a pilot fly our airplane who thought 61 in/hg was still fine for take off power. The next 25 hour inspection I removed a cracked headnut and when retorquing the heads/banks I got a lot of movement on each nut. Since the adjustments and informing the pilots that we're limiting takeoff power to 55 inches, I've yet to enounter an issue in almost four full years of inspections. The engine makes a little less power but it's a lot happier on todays 100LL gas. The upshot is, in most cases aircraft performace is still way higher today because the airframes are so much lighter than they were back in the day.

Regards,
John
CC CAF Red Tail

[Jack Frost]

OK redtail, I didn't take into consideration that we were burning 100/130 fuel then, not 100LL. There is a difference, I guess.
Jack