Re: Test Flight SE5

John P. Roll (
Sun, 29 Oct 95 14:37:25 -0600

In message <> writes:
> >
> > > Is there such a thing
> > >a scale rpm?
> >
> > Well, since revolutions is a dimensionless quantity, I would think that
> > say 1000 r.p.m in 1:1 scale would be 1000 r.p.m. in 1:6 or any other scale.
> I feel rather ashamed of not noticing the dimensionlessness of rpm!
> I'm a Math/CSCI double major! I've taken enough physics classes to know
> exactly how hard I hit the ground if I jump out of an airplane without
> a parachute! Oh, the shame of it all 8^)

Well, I just can't resist adding my two cents (well, perhaps a little less...)
worth. There is something <<like>> scale in RPM's when talking about prop
design - even tho' RPM as a measure is 'dimensionless'. It is tip speed. One
cannot get the same efficiency from a 6' diameter prop at 2000 RPM as a 12'
diameter prop at 2000 RPM. In fact, the 12' diameter prop's tips are travelling
twice as fast. This becomes particularly relevant when approaching sonic

Refer to page 150 of "The German Giants" by Grosz & Haddow (Third Edition -
1988) regarding the Linke-Hofmann R.II. It was a scaled-up conventional
biplane. The wingspan was 42.16 m (~138'). The propellor was 23 feet in
diameter but turned at 545 RPM. It says: "Since the designer had chosen to keep
the propellor tip velocity equal to that of smaller diameter, higher RPM
propellors, that of the R.55 turned at only 545 RPM."

My guess is that if you took a 'scale' propellor and ran it at the same speed as
a full-size prototype, you would get a rather low thrust.

Now wouldn't THAT be a helluva scratch-building project!

John Roll