I did look in the "Real World RPM" thread, but could not find an answer. I know it will differ from engine to engine, prop and fuel used, but I am just looking for a general idea of how much a motor / prop combination will unload in the air. If you get 7,000 rpm on the ground, what will the max rpm in flight be? Other size engines will also be of interest.

Roughly 800 - 1200 RPM, sometimes more depending on propeller and airframe combination.Slippery airframes will allow the engine to unload more than say a Yak or Cub.

Stock G62, 8 inch long open header, APC USRA race prop, 7500 on the ground, 9000 in the air..Been doing this for 15 YEARS, the information IS correct....
Not sure of the prop, I think 19-14...

DLE 55 with Vess 23B. 7100 on the ground, 8100 in the air.

So a guy saying his 50cc Extra with a standard DLE55 and 22x8 prop is doing 100mph in level flight is dreaming.

Don't know, it could be. I had a 50cc Yak that was radar clocked at 86 MPH in level flight with no preceding dive, it was later confirmed with GPS. It used a 22x8 prop also.

If it did 86mph it means the rpm must be 11,400 just for pitch speed, then prop slip must be accounted for as well.

To do 100mph the engine must do 13,300 rpm plus provision for prop slip. I am not convinced that a standard 50cc +- size will do that.

You need to go study how an airfoil (propeller blade) works. Aircraft over ground speed is not solely a product of pitch speed.

Could have had a tail wind. Better chance he is blowing smoke wherer the sun don't shine.

You need to go study how an airfoil (propeller blade) works. Aircraft over ground speed is not solely a product of pitch speed.

Where can I get more information about it?

Could have had a tail wind.

He claimed it was early morning without any wind so to speak of.

http://www.lmgtfy.com/?q=How+does+a+propeller+work

http://www.lmgtfy.com/?q=Propeller+p...s+ground+speed

http://www.lmgtfy.com/?q=How+does+pr...o+ground+speed

Our T 6 race planes clock over 125 on radar and Eagle tree. 9000 rpm x 14 pitch...
GT 80s @ 11500 clock about 190 oN the course....19 x 18 prop.

Interesting that while I am busy studying for a degree in aerodynamics which is mostly about full size planes, I do a lot of searching and reading on the internet and other study material and did not find where it is explained how a propeller can produce an aircraft speed of 46% more than its calculated airspeed using geometric pitch and rpm. (Say 9000 rpm on 8” pitch = 68mph calculated to reach 100mph) I thought that it might be the model engines that unload much more and reach higher rpm in the air, therefore the question in this thread in the first place, but it appears not to be the case. I do not have a gas engine plane to test myself. That is also why I asked you where to read about what you said a few posts back; maybe you have a direct link. Not a lesson on how to search.

Antique, that is all within reasonable calculated speeds, so there is no reason to question that. What manufacturers do is to measure the pitch of the prop in different ways and call it whatever suits them. For the same given pitch number, if they measure the face side and ignore the chord line, the effective pitch will be more than when the chord line is measured and less when the camber side is measured. Take two model props of same size and given pitch but from different manufacturers and measure it to see. Then there is the question if the prop is true helical pitch or not, but in all cases the difference will not be 45% unless they are really trying to fool us.

So, I think that Ericb is correct and that some modellers “is blowing smoke where the sun don't shine”.

Always wondered about that myself....We know the radar speeds are right, after about 19 years of competing...Started out with 22-10s and 90-95 mph..Most all the improvement came from the APC props developed for us..Smaller diameter, more pitch...The airframe designers came up with some slipplier designs that helped some but mostly the props did it..
We had some very special props made by Stuart Sherlock for our big 289s that worked better than all the other props we tried...They unloaded about 1800 in the air....
We're now working with Mejzlik for some new race props, gotta use CF for our 40 HP race engines...
Stuarts web site is supecoolrprops.com..His prop theory book is over my head about halfway down the first page..
Pe Reivers has the best prop website for normal users...

I have a 40% sbach with a da150 on re3's and a 3bld 28.5 12s that I had clocked at 126 mph. Staight and level with no wind. It tacked 6100 on the ground not sure in the air but can tell it is considerably more.