Prop Noise

[jetstuff]

Hello Gents... i've just got my turboprop powered 'Turbo Raven' into the air at last and already suspect that the neighbours won't be happy with the prop noise.... I'm running a 24X10 Menz S wooden 2 blade, that's giving quite a 'bark' at times.... noticeable because the engine is VERY quiet.
I would appreciate your suggestions on the science involved here and the best way of reducing the noise....larger/smaller/more pitch/less pitch/carbon/three blades etc.... I want to maximise the efficiency ( it climbs vertically but eventually runs out of puff at the top)....
Thanks in advance
John
http://www.jetstuff.co.uk

[vintage1]

Less RPM and greater separation between blades and fixed surfaces.

[jetstuff]

I assume that tips are going supersonic as blades unload in the air.... (sounds OK on the ground)
Q.. what RPM does a 24" prop go supersonic?
I might be able to limit RPM with the power turbine controller.

[JMP_blackfoot]

I assume that tips are going supersonic as blades unload in the air.... (sounds OK on the ground)
Q.. what RPM does a 24" prop go supersonic?
I might be able to limit RPM with the power turbine controller.

24 inch diameter at 10,650 rpm. Better keep the rpm somewhat lower, like 9400.

That's an awful lot of power though.

[vintage1]

Blimey. It IS that low isn't it. I had to check the maths, and yes, 10k RPM on a 24" prop...and yes, it IS a huge amount of power.

Sounds like those turbies need a deeper ratio gearbox.

[jetstuff]

Thanks for that gents... is there a formula? I drew a circle on the bench and measured around it...(maths not my strong subject)! and came up with 77" circumference and 10,285RPM....on the ground it's only reading around 6500RPM.... so do you think that it really is rotating that fast?..... as it is coupled to a free power turbine and has no direct connection to the engine (gas generator) except for the hot gases...then it can in theory rotate at whatever speed it wants to...although I can limit the maximum speed of the input to the gearbox (currently set at 55000RPM).... the main noise is when unloaded (in level flight or dive).....
what thoughts on 3 bladed props..efficiency/noise etc....
thanks again

[Bill Glover]

Blade tip shape (and thickness) affect noise significantly, that's one of the reasons why APC props became so popular for normal IC use.

To get the same blade area & therefore power absorbtion a 3 blader needs to be smaller in diameter. So at the same RPM as a 2-blader it will have a lower tip speed, and generally be quieter. However a 2-blader is normally a bit more efficient.

[JMP_blackfoot]

Thanks for that gents... is there a formula
A formula is :

If diameter D is measured in inches : rpm = 255650/D

If D is measured in millimeters : rpm = 6493520/D

[Bill Glover]

Thanks for that gents... is there a formula?

Sorry, missed that!

Tip speed in mph is (near enough): diameter inches x RPM / 336

So 24 x 10,650 / 336 = 760.7 mph

(the speed of sound at sea level is 761 mph).

Therefore a 20" 3-blade prop at the same rpm would have a tip speed of only 634 mph.

[gkamysz]

Forward speed of the aircraft does come into play. The helical distance traveled by the tips should be considered if you need complete accuracy. The faster the aircraft flies the more this correction is needed. Text books suggest M .72 or less for quiet prop operation.

Greg

[Bill Glover]

Good info. Greg. - thanks.

[jetstuff]

thanks for all that.... I can't believe that prop is gaining 4000 rpm in the air and wonder about blade shape and the proximity of the engine air intake...about 10mm away from the mid point of the prop.... I hope to try a 22x12" carbon prop to see what effect that has....

[vintage1]

You don;t have to actually have the blade tips excceed teh speed of sound to get a lot of noise - locally teh air is accelerated over the blade tipps much faster thah teh general speed of the tip would indicate - hence the 0.72 factor greg quotes. Which I didn't know - just that it was some figure less.

On FS aircraft of WWII they ran into this problem too - hence multiblade props to absorb the power. Not so efficient at low RPM, but more effcient than a 2 blader going transonic

[raptor22]

That's not an exact number though; it varies with prop design how much the air is accelerated.

--Alex