Warbird Information Exchange

We all know--and I have even heard it, having been about 20 during the airplane's heyday--that the B-36 had an unusual sound when flying overhead, even high overhead. A low, chest-pulsing drone. What created it? Was it the pusher props working in the wake of the wing? Certainly it wasn't the exhaust sound, since there were plenty of R-4360-powered airplanes that didn't sound anything like the B-36. One explanation that I have heard, from a former B-36 crewman, was that it was caused by "the low-frequency hum of the prop tips."

Anybody care to theorize? (I'm preparing a B-36 article for Aviation History Magazine.)

An obvious guess from a non-engineer...
Propwash hitting the fuselage.

To me, many pushers have a unique sound: Cessna Skymasters, Lakes, and Avantis come to mind.
I don't have enough experience with Long EZs or Starships to give an opinion on them...but the may sound different still because their props are aft of the fuselage.

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Im pretty sure its the way the exhaust is piped through the propeller. It throws the sound around in a different way.

I would love to see a computer model that better shows how/why this happens. It would make a good thesis for someone in an engineering course.

Sean

A lot of pushers have unique sounds and I think a big part of it is the exhaust being cut by the prop blades as it passes through.
Listen to a turboprop like a DASH-8 start. Although not a pusher. the inlet air gets cut by the prop as it enters the inlet. The sound changes as the speed increases.
https://www.youtube.com/watch?v=GDdOGqSMK7M
Listen to a Piaggio Avanti. They have a "buzz" that is for sure caused by the exhaust stream being cut by the prop.
https://www.youtube.com/watch?v=NhuZ-YoityQ

Skip to 1:50

The prop blades are in dirty air. On each rotation, each blade passes the trailing edge of the wing twice - once going down, once going up. Multiply by 18 blades and you get a characteristic sound. Passing through the exhaust may have some effect too.

Go to your local EAA fly-in and listen to the canard types as they come and go. They have a unique sound too because of the same factors, but instead of 20,000 hp and 18 blades, their noise is from 200 horsepower and 3 blades.

The Tu-95 and An-22 sound similar: must be prop rather than engine-related.

Stephan,

Perhaps either this 1952 NACA study or the 1990 NASA study will offer some insight.

https://apps.dtic.mil/dtic/tr/fulltext/u2/a377112.pdf

https://ntrs.nasa.gov/archive/nasa/casi ... 012515.pdf

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The B-36 unique sound was the way the props were heated - the propellers were hollow allowing heated air to pass from the shaft and vented out the tips.

Pusher aircraft also have a unique noise, I can identify a Piaggio P.180 by it's unique sound.

https://youtu.be/QS9ZwqjyonY

Not a unique sound though: lots of props more to do with it I think.

In speaking to several C-133 crew I think it has more to do with the propeller. The C-133 had a similar propeller to the B-36, also a Curtis type, and was also known for a similar, distinctive drone and high vibration. The forces and speed of this propeller were really pushing the limits of blade limits with massive power, torque and speeds. Even with reduction gear the blades tips were travelling very fast and moving a tremendous amount of air.

Thank you for all your theories. I do agree that the sound probably was created by the props, not the exhausts, since typically any prop-driven airplanes propeller[s] produce far more noise than the exhaust[s].

It quite possibly is due to the exhaust flow out of the nacelles and through the prop discs, or maybe better stated as the exhaust being ejected into the prop disc. In any case it is a unique sound.

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http://www.angelfire.com/dc/jinxx1/Desrt_Wings.html

Blow into a turning fan, I would hazard it is exhaust hitting turning prop.