Quantcast

P-51 Gun Whistle?

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

TXFlyGuy

Well-Known Member
Joined
Apr 25, 2012
Messages
1,984
Location
Republic of Texas
The replica .50's that we made have a very loud whistle when an air hose is help up to them. An ear piercing sound. We will do some flight tests with them, and determine the correct configuration that will produce a whistle.

P1030663.jpg P1030650.jpg P1030651.jpg 45D56212-6658-491B-BFE2-D36CFAB6168B.jpg
 
Last edited:

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
google 'helmholtz resonator' to work out how to get them to tweet at the right frequency. You could analyse the sound from that video to see what it is supposed to be. If you are still at the cleco stage, stay there till they sound right...

BTW, Hurricanes and Spitfires whistle a bit too. With their gun ports covered with doped fabric... That suggests the supercharger is making some noise too. The later P-51s had a license built Merlin.
 

TXFlyGuy

Well-Known Member
Joined
Apr 25, 2012
Messages
1,984
Location
Republic of Texas
google 'helmholtz resonator' to work out how to get them to tweet at the right frequency. You could analyse the sound from that video to see what it is supposed to be. If you are still at the cleco stage, stay there till they sound right...

BTW, Hurricanes and Spitfires whistle a bit too. With their gun ports covered with doped fabric... That suggests the supercharger is making some noise too. The later P-51s had a license built Merlin.
A good friend of mine owns and flies a P-51. He has no guns, as the wing is now smooth. His Mustang (Buzzin' Cuzzin') does not make any whistle sound at all. He claims the iconic whistle is from the gun barrels.
 

fly2kads

Well-Known Member
HBA Supporter
Joined
Jan 2, 2010
Messages
1,757
Location
Justin, TX
What is on the "gun" end of the barrels? To elaborate on pictsidhe's point, you may have an opportunity to tune the whistle to your liking by altering your setup. If the barrel lengths are fixed, then you could, say, change the volume of the chamber behind them (the bottle in your example).

Looks pretty, cool, BTW!
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
Perhaps the p-51 supercharger intake muffles it. Hurricanes and Spitfires have short intakes. I've heard both whistle somewhat while flying directly at me, with either no guns or covered gun ports.
Video clips and software will allow a smart person to replicate the sound, somewhat. I won't have the space or weight to make a Briggs sound like a V12, but I should be able to make it sound a lot bigger. Whistles are easy with helmholtz resonators; basically an empty bottle.
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
I fed the p-51 whistle vieo into my PC and it spat this out:

p-51_inboard_gun.jpg

The peak is at 700Hz. The outboard gubns are 1800Hz, but I sus[pect that the outboard gun contribute little to the noise.
There are several ways to tune the gun ports to these frequencies.
The first is to use a helmholtz resonator, AKA a 'bottle' I use this calculator
taking a wild guess at your gun barrels being 4" long and 3/8", I get 8cc for the chamber volume. I suspect this will need a good blow to work.
Option 2 is to use organ pipe resonance.
You can have a closed end gun port of 5", or link two gun ports together so the mouth to mouth distance is 10". Ideally, use a single length of pipe. Roughness near the mouth will reduce the volume, or prevent resonance. some filler on a join and a dremel should do the job, though.
Sharp edges will help excite the whistle.
Or, you could install a seperate whistle somewhere completely different. Playing around with scraps of pipe and an airline could be informative.

After doing that I analysed a superior airplane ;) and got this:

r-4118_flypast.jpg

Which shows that you have the easier sound engineering job...

That Hurricane has a distinct peak around 700Hz and several strong harmonics when coming towards you. It's not the guns, their ports are fabriced over. It's not the pure tone of P-51, so my money is on the supercharger.
 

TXFlyGuy

Well-Known Member
Joined
Apr 25, 2012
Messages
1,984
Location
Republic of Texas
I will analyze the frequencies from my guns. The barrels are .410, and 1 inch to 3 inches in length.

Why does Buzzin Cuzzin have no whistle?
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
No gun ports, totally different supercharger intake to the Brit planes. The Hurricane just has an elbow onto the supercharger. The p-51 have a long intake open just beind the prop. That long intake will attenuate high frequencies somewhat
I just played around some more with my Hurricane clip and ran some numbers. Supercharger fundamental is around 340Hz, but not very clear on the analysis as there's a slightly lower tone drowning it, the harmonics are visible, though. My memory says that Spitfires have an even more noticeable whistle/whine.

Now, how do I replicate a supercharger sound on a 103 powered by a lawnmower engine?
 

TXFlyGuy

Well-Known Member
Joined
Apr 25, 2012
Messages
1,984
Location
Republic of Texas
The main item to note is that a Mustang does not whistle in straight and level flight. The whistle is only heard when the aircraft is in a pull up, or bank, pulling some G load. This increases the AOA of the air over the gun barrels, much like the Coke bottle reference.

Just did a DB Meter spectrum.

Outboard barrel = 1700 hz
Middle barrel = 2200 hz
Inboard barrel = 2600 hz

https://www.youtube.com/watch?v=Vg3ZUKrMW8M

https://www.youtube.com/watch?v=dTAk38nRodQ

https://www.youtube.com/watch?v=ZyqF3KzKhNk

The tones on the actual P-51 are 1800 hz, and 700 hz. That is a static measurement. Inflight, the tone is 1200-1300 hz.
 
Last edited:

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,463
Location
Saline Michigan
google 'helmholtz resonator' to work out how to get them to tweet at the right frequency. You could analyse the sound from that video to see what it is supposed to be. If you are still at the cleco stage, stay there till they sound right...

BTW, Hurricanes and Spitfires whistle a bit too. With their gun ports covered with doped fabric... That suggests the supercharger is making some noise too. The later P-51s had a license built Merlin.
Superchargers with short inlets do put out a shriek. The longer P-51A and later inlet duct would have a much lower resonant frequency and may tend to attenuate the shriek. Those were Merlin engined.

The original RAF Mustang MKI, USAAF A-36 and P-51 (no suffix) were Allison Powered, while later variants had versions of the Merlin, as did the Mosquito, Hurricanes and Spitfires. Instead of getting into which airplane was a better what, let's remember that the Hurries and Spits were designed to a product spec that was all about protecting the home island. Damned good thing they were producible in enough numbers by summer of 1940 too... The Mustang was a later design intended to fly longer distances and do ground attack, so it was bigger to carry more fuel. Allison ended up developing and building their supercharged engines to the USAAF shortsighted spec for lower cost and lower critical altitude on the supercharger. In the end most Mustangs were built with Merlins, which had a suitable supercharger for bomber escort work.

I have a couple sources showing Merlin peak HP at 2850 rpm, which means the primary exhaust note at full trot is about 285 HZ, and the prop beat is substantially lower than that. I do wonder about the dimensions we are seeing for the pipes. The gun barrels for Browning M2 have a 1/2 inside diameter and are 62" long.

Anyone ever heard the whistles put out by Bob Hoover's Commander during his pull into a barrel rool with both engines caged? Two air cooled flat engines, props stopped, but in the pull, his bird shrieked some too. Makes me wonder if the shriek is not something else, like air whistling through the gaps between control surfaces and the wings and stabilizer...

Billski
 

Tiger Tim

Well-Known Member
Joined
Apr 26, 2013
Messages
3,469
Location
Thunder Bay
Anyone ever heard the whistles put out by Bob Hoover's Commander during his pull into a barrel rool with both engines caged? Two air cooled flat engines, props stopped, but in the pull, his bird shrieked some too. Makes me wonder if the shriek is not something else, like air whistling through the gaps between control surfaces and the wings and stabilizer...
Low-wing Pipers also have a very distinctive whistle on approach. I used to think that it was the gear doors until I heard a fixed gear Cherokee do it too.
 

Voidhawk9

Well-Known Member
HBA Supporter
Joined
Mar 26, 2012
Messages
417
Location
Timaru, NZ
Gliders whistle as they go past as well. All aircraft shed vortices, have small gaps for air to whistle through, etc. so this is expected.
 

Himat

Well-Known Member
Joined
May 5, 2011
Messages
2,868
Location
Norway
google 'helmholtz resonator' to work out how to get them to tweet at the right frequency. You could analyse the sound from that video to see what it is supposed to be. If you are still at the cleco stage, stay there till they sound right...

BTW, Hurricanes and Spitfires whistle a bit too. With their gun ports covered with doped fabric... That suggests the supercharger is making some noise too. The later P-51s had a license built Merlin.
A thick wall pipe closed solid at one end like a gun barrel with chamber closed will be an acoustic resonator both with and without a thin membrane over the “open” end. Having the gun port covered with fabric and dope may change the acoustic behaviour, but if exited it will resonate anyway. If the air can enter the cavity and excite the resonator, so can the air make the membrane vibrate.
 

Himat

Well-Known Member
Joined
May 5, 2011
Messages
2,868
Location
Norway
Gliders whistle as they go past as well. All aircraft shed vortices, have small gaps for air to whistle through, etc. so this is expected.
I did live close to an airport for several years, like a hundred meters from the touch down point. After a 737 had landed the vortices could be heard whistling after the noise of the airplane had faded away.
 

Deuelly

Well-Known Member
Log Member
Joined
Nov 6, 2012
Messages
257
Location
Marshall, MN
The main item to note is that a Mustang does not whistle in straight and level flight. The whistle is only heard when the aircraft is in a pull up, or bank, pulling some G load. This increases the AOA of the air over the gun barrels, much like the Coke bottle reference.

Just did a DB Meter spectrum.

Outboard barrel = 1700 hz
Middle barrel = 2200 hz
Inboard barrel = 2600 hz

https://www.youtube.com/watch?v=Vg3ZUKrMW8M

https://www.youtube.com/watch?v=dTAk38nRodQ

https://www.youtube.com/watch?v=ZyqF3KzKhNk

The tones on the actual P-51 are 1800 hz, and 700 hz. That is a static measurement. Inflight, the tone is 1200-1300 hz.
One of our Mustangs used to whistle when you would pull g's. We tightened up the gear doors, problem solved.
 

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
3,632
I'm OK with no whistle.
 
Last edited by a moderator:

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
8,812
Location
North Carolina
The main item to note is that a Mustang does not whistle in straight and level flight. The whistle is only heard when the aircraft is in a pull up, or bank, pulling some G load. This increases the AOA of the air over the gun barrels, much like the Coke bottle reference.

Just did a DB Meter spectrum.

Outboard barrel = 1700 hz
Middle barrel = 2200 hz
Inboard barrel = 2600 hz

https://www.youtube.com/watch?v=Vg3ZUKrMW8M

https://www.youtube.com/watch?v=dTAk38nRodQ

https://www.youtube.com/watch?v=ZyqF3KzKhNk

The tones on the actual P-51 are 1800 hz, and 700 hz. That is a static measurement. Inflight, the tone is 1200-1300 hz.
That inflight tone suggests something is amiss. It could just be that your analysis is combining both tones into a single peak. Did it look fat? Or it could be that something else is making the whistle.
Are the p-51 guns open or closed bolt?
 
Top