Prop tip speed of sound

455 Bravo Uniform

455 Bravo Uniform

Final Approach
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455 Bravo Uniform
So playing with math, at -27 deg C my prop tip speed is at Mach 1. What’s it gonna sound like? What happens other than reduced efficiency. Not that I’d fly when it’s that cold.
 
Probably sound like a smaller version of a T-6 Texan. You don’t have to hit Mach 1 to lose efficiency though. Compressibility will start in the transonic regime.
 
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It's cool that you did the math and that -27C isn't that uncommon up north. I wonder how they handle it.
 
Many of the larger two-bladed props can get supersonic at max RPMs even at more moderate temperatures.
 
Mount your prop backwards so the flat side is on the front to make it look more like this:
whitcomb.gif

You may have to tweek the trailing edge a bit.
 
455 Bravo Uniform said:
So playing with math, at -27 deg C my prop tip speed is at Mach 1. What’s it gonna sound like?
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If it is like the prop on a 206 and 207 it will be loud as heck. It really annoys anyone close by. Try living next to a lake that seaplanes use regularly.
 
it'd be loud but the idea that slower RPM is better is incorrect. Full RPMs works as well in the cold as any other time. Noisy? Oh hell yes!
 
The tip doesn't have to reach the speed of sound. The propeller blade is an airfoil, and the air passing over the top is going faster than the blade at that point so that a shock wave is formed before the tip reaches supersonic speed.
 
Dan Thomas said:
The tip doesn't have to reach the speed of sound. The propeller blade is an airfoil, and the air passing over the top is going faster than the blade at that point so that a shock wave is formed before the tip reaches supersonic speed.
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Duh. The other way to think about it is the stuff on the bottom of the airfoil is going SLOWER than the stuff on top. We're not talking about the very "tip" of the blade, just the stuff closer to the tips than to the hub. The stuff going over the top of the airfoil nearer the tips will go supersonic before the stuff nearer the hub.
 
A 185's IO-520 redlines at 2850 RPM. With the 86" seaplane prop turning at redline just after takeoff with the airplane doing 70 MPH, say, those tips are doing 732 MPH, still under the 766 MPH speed of sound in standard conditions; they're at Mach .96. In cold air it would be noisier, as the speed of sound is related solely to temperature, and the prop tips would be even closer to Mach 1.
 
If you like noise? Take off with an 86” prop spinning 2850 at 15’ ASL from a creek bed in winter when DA is -7000’. Short run, tall trees, max power required. The neighbors will know you’re leaving.
 
If you’ve ever heard a Huey from miles away and on a cold day, that loud clap is from the tip of the advancing blade going transonic. For most helos, the compressibility area becomes a Vne.
 
The 185s I flew in Alaska make a terrible racket at full power on takeoff.

I had a prop overspeed once and it sounded like an air raid siren, but louder!
 
In Alaska, if I could I took off at 25 squared to try to keep the prop noise down. And for landing I would throw the prop forward on very short final to help keep the noise down. All about being nice to the neighbors.
 
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