pe reivers

In the 1920ies, there has been a lot of research by NACA and the English aeronautical institute. These years, many props approached, or exceeded Mach 0.9 tip speeds, which in air is the speed of sound.
Tests have been executed to eliminate the blade twist from efficiency calculations. It was very surprising to learn that blade efficiency hardly suffers at those high tip speeds, as long as thin foils were used. Any appearent efficiency loss could be explained by pitch changes.
These tips indeed go supersonic, and they do not need tons of power to do it.

(grammar errors edited)

gkamysz

My Feedback: (19)

In full scale aviation texts it is recommended that prop tip speeds be kept under ~Ma 0.72 to avoid noise issues. You don't need to be at Mach to have noise problems.

pe reivers

That is true. In my calculator sheet I have a redline of mach 0.6 where low noise operation is needed, but not because of efficiency issues. For us, efficiency is no big deal anyway, since we need not consider the delicate payload/cost relationship of air freight.

BOUND_4_HELL

Hi blw ,

I do not want to be controversial or argumentative with a fellow aviator, but I would like to educate the people reading this, as there is a lot of misunderstanding about propeller tip speeds.

In my youth, a life time ago I was also in the aviation business. I held a Helicopter Engineers Licence on Hugh’s 500, Bell 206, Bell 204 & 204B’s and the stretched 205 version which you call the Hueys.

If memory serve me correctly the Bell 204’s and 205’s rotor speed was around 294 RPM and 48 ft in diameter. The cruising speed was 110 Knots max speed specified was 120 Knots, but pilots took it further then that on swallow dives.

This makes the static blade tip speed approx. 492 MPH then add the forward speed of the advancing blade on a shallow dive of approx. 150 Knots or so if the pilot was brave enough to take it that high, which is the equivalent of about 173 MPH and you end up with 665 MPH.

So even a helicopter blade 48 ft in diameter does not break the sound barrier, which is about 770 MPH or so depending on atmospheric conditions.

What you heard is the sound of the air accelerating to transonic speed and then decelerating back to normal over the top of the advancing blade, the blade itself never travel in the transonic or supersonic range. If the blades were to go transonic or supersonic you would get a high speed blade stall and all kind of nasty things would happen, the Bell Helicopter designers made sure this scenerio would not happen.

Geppino

BOUND_4_HELL

These are the formulas that I have been working with to calculate static tip speed. I got them off the net, if the formulas are not correct then I’m out to lunch, so discard everything I posted, but I believe that the formulas are correct.

You will get a higher tip speed when the aircraft is moving forward and that is why full scale aircraft traveling at high velocity ( more than 400 MPH ) have their propeller tips go into the supersonic range.

There is a square root formula that takes into account forward speed. However our 60 to 80 MPH forward speed of our airplane models is almost negligible and will not get the propeller tips into supersonic speeds. I worked with this formula the first time around. At 60 MPH forward speed it only added 9 MPH to the tip speed. I did not want to confuse the issue and left the forward speed out. But feel free to add 10 to 15 MPH to your calculated static tip speed, you will still not get the propeller tips of our model airplanes to go supersonic.

FEET PER SECOND (ft/s) = RPM x diameter in inches x .00426

MPH = DIA x RPM x 0.0029714

Do the math; you will be hard pressed to find today’s model engines turn a prop to a high enough RPM that will let the propeller tips break the speed of sound.

Maybe we are all saying the same thing in different ways. I totally agree that the air traveling across the curved part of the blades is going at transonic or supersonic speeds, that is what creates the noise. The propeller tips themselves however do not break the sound barrier.

Geppino

dirtybird

My Feedback: (5)

You don't know gory Al? (I had two many r’s in gory)
Last year he won the Noble prize for global warming.
A subject he probably knows less about than my cat

gkamysz

My Feedback: (19)

First, we need to reiterate that the tip speed exceeded Mach is not relevant to noise. We already know that Mach .72 tip speed is the Full scale standard for prop selection if noise is a factor.

I'm going to use the above as an example because the numbers were already presented. The forward speed is important. Using a formula out of a text book that above 7100 RPM 0 airspeed is about M 0.75. However at 70MPH this tip speed becomes M0.81. This will probably result in noisy operation depending on atmospheric conditions that day. DMcQuinn can you verify?

http://www.supercoolprops.com/articles/transonic.php

And another interesting topic on noise.

http://www.supercoolprops.com/articl...rsmufflers.php

blw

My Feedback: (3)

You wouldn't be around to tell about it if your rotor blades exceeded Mach 1 on a helicopter. I don't think any blades would survive the twisting forces.

My info came from wind tunnel tests. Electric motors were used.

Swept tips help reduce tip noise and it would be nice if we had props like those for our models.

pilot727

There are some clubs that have some guys that do not like the sound of many gas engines not to think what they would think if i started my ys-160 with a straight pipe, nothing like the sound of that, but when your in a spot that just do not want to hear that loud noise you kind of have to go with there rules if you want to fly there, myself i guess i would find a back road to fly off of, there really is no need of it as there are many sports like at the race track that can also be heard from miles and as one said lawn mowers under your window at 6am in the morning, but that is ok with them, i love harleys, are they going to put baffles in there pipes out of the shop, but there are a few fields like that and if you want to fly there you fly by the rules, we do not have that problem and i am a member of two clubs one very close to homes and everyone loves the noise and also many come out just to watch and get such a kick out of the hobby

pe reivers

That's one hellova sentence.

pilot727

I am sorry that it was a little long but you need not read it. I read much longer getting to this point. and i am not complaining