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Hanaldo Grand Master

Hanaldo

Posted
Posted

Hah, realized where I went wrong. Just asked dad what the deal was and he told me his prop tacho in the glider is from another aircraft which used a different gear ratio, so this one's readings are well off :blush: He knows the equation to work out it's true speed, but really he just uses it to monitor the 'norm' for prop-slip.

So my example is not as relevant as I thought, but I think the main point of the strength of nylon is still valid.

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Hanaldo Grand Master

Hanaldo

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I just calculated the tip speed of a 1m diameter prop at 200,000 rpm at over 600 km/h. So there is definitely something not right there.

Care to calculate the tip-speed of a 1.63 meter diameter prop, alt. of 10,000 feet; engine speed of 5000rpm, prop pitch of 137.6cm and a gear ratio of 1.18? Those are the specs of dad's Stemme. Dad gave me the formula to work it out, but I tried it and got lost lol.

GTSBoy Grand Master

GTSBoy

Posted
Posted

I must be on drugs. I wouldn't trust the 600 km/h speed I reported earlier. I did it in Excel and it looked good and so I posted and then closed the spreadsheet. So to answer your question I had to redo the calc and the numbers came out quite different. I must have divided something when I was supposed to multiply it I think. Anyway, the answer for 80000rpm by 1m prop is more like 15000 km/h, which is of course totally stupid. For your 1.63m propeller at 5000 rpm x 1.18 (=5900 rpm) the tip speed is about 1800 km/h. Which is also silly, because supersonic tip speed are not kosher. Even if I got your gear ratio back to front and the prop speed is only 5000 / 1.18 (=4237 rpm), then the tip speed is still 1300 km/h. I don't like that answer either. The altitude and the prop pitch don't impact on the tip speed. Tip speed is just how many times per second the tip completes each circle, multiplied by the length of the circumference. 1.63m diameter is 5.1m around. 5900 rpm is 98.3 revs per second. Multiply number of turns by distance and you get 503 m/s tip speed, which is 1812km/h. Nasty. Now, if by chance I read your gear ratio really wrong, and there is actually an 18:1 reduction from engine to prop shaft, then the tip speed is 85 km/h, which seems really far too low. I went and googled up a tip speed calculator, http://www.pponk.com/HTML%20PAGES/propcalc.html which seems to suggest that 0.9 mach is the optimum tip speed. I put your dimensions into it and it came back with >900mph (mach 1.5) as the tip speed (at 5900 rpm) and said don't do it. So that agrees with my calcs. I don't know what to say about your tip speed. I don't like the answer, so maybe one of the inputs is wrong.

fastjetjockey Collaborator

fastjetjockey

Posted
Posted

Care to calculate the tip-speed of a 1.63 meter diameter prop, alt. of 10,000 feet; engine speed of 5000rpm, prop pitch of 137.6cm and a gear ratio of 1.18? Those are the specs of dad's Stemme. Dad gave me the formula to work it out, but I tried it and got lost lol.

Vtip = πdn

Where d is the diameter in metres, and n is the angular velocity (RPM).

=3.14 * 1.63 * (5000 * 1.18)

=30212.69m/min

=1812.76km/h

1812.76km/h at the tip on a 1.63m prop at 5000 engine RPM's through a 1.18x gearbox in a standard atmosphere. You sure about those figures?? When a prop is operated so it's tip exceeds mach ~0.88 (934km/h), it's efficiency starts to go downhill due to the loss of laminar airflow over the aerofoil; shockwaves that interrupt the thrust being generated. Back to the drawing board for you Hanaldo! lol

Just for shits'n, I took the liberty of calculating the same prop's tip speed, but at 200000RPM.....

Over Mach 49. :3

Hanaldo Grand Master

Hanaldo

Posted
Posted

I must be on drugs. I wouldn't trust the 600 km/h speed I reported earlier. I did it in Excel and it looked good and so I posted and then closed the spreadsheet. So to answer your question I had to redo the calc and the numbers came out quite different. I must have divided something when I was supposed to multiply it I think. Anyway, the answer for 80000rpm by 1m prop is more like 15000 km/h, which is of course totally stupid. For your 1.63m propeller at 5000 rpm x 1.18 (=5900 rpm) the tip speed is about 1800 km/h. Which is also silly, because supersonic tip speed are not kosher. Even if I got your gear ratio back to front and the prop speed is only 5000 / 1.18 (=4237 rpm), then the tip speed is still 1300 km/h. I don't like that answer either. The altitude and the prop pitch don't impact on the tip speed. Tip speed is just how many times per second the tip completes each circle, multiplied by the length of the circumference. 1.63m diameter is 5.1m around. 5900 rpm is 98.3 revs per second. Multiply number of turns by distance and you get 503 m/s tip speed, which is 1812km/h. Nasty. Now, if by chance I read your gear ratio really wrong, and there is actually an 18:1 reduction from engine to prop shaft, then the tip speed is 85 km/h, which seems really far too low. I went and googled up a tip speed calculator, http://www.pponk.com...S/propcalc.html which seems to suggest that 0.9 mach is the optimum tip speed. I put your dimensions into it and it came back with >900mph (mach 1.5) as the tip speed (at 5900 rpm) and said don't do it. So that agrees with my calcs. I don't know what to say about your tip speed. I don't like the answer, so maybe one of the inputs is wrong.

You're correct, altitude only comes into when trying to work out tip mach speed.

I just googled the Stemme 10 specs, and the manual gives the same specs as I gave you. Maybe there is another gear set somewhere along the line which gives another reduction that isn't mentioned? I have no idea, I'm confused now :/ I know for a fact the engine is revved to 5000rpm on take-off, that was drilled into me when I was learning to fly it. Cruising rpm is more around 2400.

Hanaldo Grand Master

Hanaldo

Posted
Posted

the rotax engine itself has an integral reduction gear too...i mentioned it earlier was 1:2.2 something

so the prop will be running through 2 gearboxes, 1 at the motor and 1 in the prop hub

The Rotax is out of the S10 VT, dad has an S10 VH-GTS which uses a Limbach L2400 motor. The only specs I can find on that motor are from a Stemme S10-V, but that's an L-2400 EB1.AD motor which is different to dad's. I can guarantee that at 3400rpm, there is no way dad's glider would get off the ground.

Rolls Grand Master

Rolls

Posted
Posted

In any case the RPM is quite irrelevant to the earlier point; would everyone here agree that 90% of mach 1 is pretty stressful operating conditions?

Because I haven't seen it in real life I am going to call you a wanker and not believe you.

The Mafia Grand Master

The Mafia

Posted
Posted

not really - I still ahven't seen a plastic compressor, even though I have visually inspected a few GTT and GTS-T turbos.

If you want to keep crapping on about shit thinking your winning, good for you. Whatever makes you sleep better at night.

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