turnnburn

My Feedback: (49)

Im no math whizz but I do know that prop tip speeds near or above supersoinic are a no no. My question is, with a 7" dia prop at what RPM do the tips hit supersonic. Same question for an 8" dia prop ?

Ed Smith

A step by step procedure.

In one revolution the tip of the prop will travel 3.1416 x prop dia in inches

Multiply the answer to the above by the RPM. This will give the distance in inches travelled by the prop tip in one minute.

Multiply the answer to the above by 60. This will give the distance in inches travelled by the prop tip in one hour.

Divide the answer to the above by 63,360 (No of inches in a mile) This will give the MPH of the prop tip.

Now this could be written by a fairly simple formula. However following the above procedure gives a greater understanding of how to arrive at that simple formula.

3.1416 x dia x rpm x 60
_______________________
63,360

Ed S

turnnburn

My Feedback: (49)

Thanks, this has been very helpfull. Had I given it some thought I should have been able to figure this out, its all pretty obvious and logical.

HighPlains

My Feedback: (1)

Too bad that it is also incorrect!

skull1971

up in the extreme prop planes forum they had all the formulas for finding speed and such, at one time.

alpinestar

A simple formula for prop tip speed:

multiply prop dia (in inches) by the engine RPM.
multiply this number by 0.002975, OR divide it by 297.4905.

This will give you the prop tip speed in MPH.

(dia x rpm x pi x 60 minutes / 12 inches / 5280 feet)


The speed of sound varies with environmental factors, but at sea level & normal conditions it is 769 MPH.
So, a 10 inch prop would have to spin around 23,500 rpm to be at Mach 1 (speed of sound).

HighPlains

My Feedback: (1)

Closer, but still not correct.

skull1971

Watch out fella's this guy know's EVERYTHING, but won't share, and when he does it's sometimes incorrect as well.

djlyon

Yup, highplanes is right, still incorrect. Try again. Hey, you don't have to go to school like I did, the enternet has it all. Just look.

Denis

MaxQ

Is there a radius among us?

dwbebens

Highplains;

I suspect that the true tip speed you are hinting at is the vector sum of the calculation that 'alpinestar' gave, and the forward velocity of the plane itself.


tip speed ={ [(pi x d x 60 x RPM)/(12 x 5280)]^2 + [aircraft velocity]^2 }^0.5


Example:

d = 8"

RPM = 20,000 (use the actual flying, unloaded RPM)

aircraft velocity = 150 mph


therefore: tip speed = { [(3.1416 x 8 x 60 x 20,000)/(12 x 5280)]^2 + [150]^2}^0.5 = 499.07 mph


The tip speed calculated without factoring in the forward velocity of the plane is 475.99 mph!

Doug Bebensee

HighPlains

My Feedback: (1)

Now we have an answer. As suggested, you can find this stuff on the internet, but just as often it is wrong. However, this site pops up tonight as the 6th site and explains helical tip velocity fairly well. The first 4 were wrong, and the fifth didn't explain.

http://www.epi-eng.com/Prop-Selection.htm

bl10

What about this tip speed calc prog. http://www.pponk.com/HTML%20PAGES/propcalc.html

If its correct our LS pylon engines around .83 mach.

Barry

Ed Smith

Forward movement was not mentioned nor even considered. The explanation I offered for something whirling round and round was correct.


As usual High Plains Climbed on to his "High Horse" and tried to over complicate a simple question/answer. One day, maybe, just maybe, High Plains will build a Boogie Board and actually fly something! Having written that I am sure we will now be bored to death with High Plains aeromodelling achievements from a different era.

Ed S

HighPlains

My Feedback: (1)

Your answer is correct Ed, as long as you keep the engine on the test stand.

Super Splatter

Arguing with an engineer is like wrestling with a pig, everyone gets dirty and the pig likes it !


So big guys, with our 7.4 inch diameter meat cutters, how close are we to the edge of destruction?

fizzwater2

My Feedback: (61)

Arguing with an engineer is like wrestling with a pig, everyone gets dirty and the pig likes it !


I love it...

dwbebens

Concerning the statement, "Arguing with an engineer is like wrestling with a pig, everyone gets dirty and the pig likes it !", I haven't really had the opportunity to wrestle with a pig yet. But one thing I do know is that the engineer is the one that really has to make things work, and not just talk about them.

In answer to the person who asked about a Q40 7.4 inch prop:

Let's say it is turning about 28,000 RPM in the air and the plane is going about 180 mph.

The tip speed works out to about 642 mph or about 0.86 Mach. It appears that this is well below any serious mach problems, doesn't it. There is another wrinkle I'd like to throw in at this point. It is the fact that over the front (cambered) surface near the tips, the air velocity is even greater than the calculation above would indicate because of the airfoil shape. You could still have mach problems near the tips even though the simple calculations show things could be OK. I'm wondering what diameter the FAI pylon guys are using, since they run even more RPM than Q40 does.

A real life example is the Cessna 180 on take-off. This aircraft has a very large prop and powerful engine. It is known for its somewhat STOL performance and ability to take-off with a large load from minimal fields. That is one reason for the large diameter prop. It is one of the noisiest light planes on take-off. The tip speed on this aircraft works out to about 677 mph or 0.9 mach. Some portion of the tip is probably at or near mach 1 in order to make so much noise. Normal operation of this aircraft is such that take-off rpm is well above cruise rpm, which explaines why they aren't so noisy at cruise.

Doug Bebensee

turnnburn

My Feedback: (49)

Delete

IflyF3D

Hello Doug,

Just a quick reply to your question on FAI engine RPMs etc.

I run the MB profi and turn rpm's of 31-32,000 on the ground and my eagle tree reads around 34-34500 in the air... assuming it's right.
Prop dia's vary with Humidity/Temp/Pressure/Altitude but we generally run around 6.75"

I shape my props to have an airfoil that resembles something like an RG14 as a little u/camber for the outer half of the blade seems to work best with the blade shape I run.

I would be interested on your thoughts here.

Cheers, Chris Graham.

dwbebens

Chris;

Wow, those are really impressive RPM readings, and we used to run Q15 with props near that diameter.

First off, I'd like to say that I don't know much about F3D pylon planes.

Having said that, here's some numbers I've calculated for your situation.

I'm assuming that your planes can get up to 200 mph at times. With that speed and the RPM and diameter you give, the prop tip speed is about 721 mph - - perilously close to the speed of sound. I suspect that portions of the tips are actually supersonic. I'm sure you pay very close attention to the tip shape and airfoil for your props.

Doug Bebensee

proptop

My Feedback: (8)

Check out this thread...there are links to videos and you can hear them "singing"

http://www.rcuniverse.com/forum/m_3740991/tm.htm

HighPlains

My Feedback: (1)

Doug, the quote for this subject should be the following:

"Make everything as simple as possible, but not simpler." - Albert Einstein