rpm scale

[noobflyer92]

Another noobish question for the smart ones out there! Say a full scale extra 300 or whatever operates at 2700 rpm max. Why do rc planes, similar in scale, have to operate on such higher rpm levels?

[DT56]

The main reason for the lower speed on the full scale prop is to keep the tips below the speed of sound.

[lincoln]

If you multiply the radius times 2pi times the rpm divided by 60, you'll get the tip speed in units per second. I think that you want to stay well below the speed of sound (about 1100fps) to avoid local supersonic flow. At 2700 rpm, anything over 7 or 8 feet in diameter will be very loud. (like, above Mach 0.7, which, depending on the critical mach of the particular airfoil, could really be a problem). At one time, a turboprop with supersonic rotational speed was tried on a fighter, which was then nicknamed "Thunderscreech".

On the other hand, if you've got a 10 inch prop at 15,000, that's only Mach 0.6 or so. A little loud, of course. If you're using a 4 stroke or electric, you can get lots of torque and less rpm and things will be quieter.

I heard a very loud airplane overhead today. I don't know what it was, but it had a high aspect ratio rectangular wing, the fuse shrank down right behind the wing, and it was white all over. Lanceair? (Not sure what AR they use, but I figured one might end up overpowered.) Kind of looked like a Katana, but I don't think they'd put that much power in one.

[macboffin]

A big slow prop is more efficient than a small quick one. Add to this that the revs may in part be dictated by the type of powerplant. Note that some electric motors are geared so they can turn a bigger and more efficient prop.
Glow motors tend to idle at 1000 rpm or a tad more, produce most torque around 8/10k, so props on scale planes are smaller than scale.

[JetPlaneFlyer]

There are a couple of main reasons why model props run faster..

Macboffin hit the first one on the head insomuch that most model powerplants produce their peak power and torque at much higher RPM than full size engines so props are smaller and faster turning to suit.
However there is a second reason.. a prop blade works just like a wing. Like a wing the lift (aka thrust) is proportional to velocity squared. The prop speed relationship between model and full size needs to be to the root of the scale factor in order to provide 'scale' thrust. So a 1/4 scale model with a scale prop would have to have a blade speed of 1/4^0.5 = 0.5.
To get a prop speed of half the full size on a 1/4 scale prop means having twice the RPM.. So even if you had a geared prop drive on your Extra 300 driving an exact 1/4 scale prop it would still need to spin at twice the RPM as the full size to provide 'scale' performance.