Speed Limit Explained
I have seen several post either , wondering why the low 200 barrier can not be exceeded ,or reporting speeds of 250 to 300 MPH. I have also seen several trying larger & larger engines , trying to go faster. Most of this is in vain , unless you go much larger , bordering on full size , w/ horse power in or near the thousands w/ props measured in feet , turned at low RPM. Our small engines & props are designed for very high RPM's,thus we run into tip speeds in the transonic to supersonic range . A 7 inch prop turning 25000 RPM at 200 MPH has a tip speed of Mach .96, which is transonic , shock waves are beginning to form . Propeller effiecency begins to suffer tremedously the faster we go . The formation of the shock waves causes drag on the tips , thus more HP is required to turn any faster , but most important , the shock waves ( depending on prop design ) can actually reverse thrust where the prop is supersonic. Giant scale unlimited racers turning 30 props well under 10000 RPM have been a little faster , but the hit the same limit , You can hear it , tip speeds are well into Mach numbers , but the are running 20 HP or more and can power thru some of the tip drag , but can not escape the effiecency issues .
Our off the shelf model props a designed for subsonic flow at the tips therefore 220 or so is about as fast as we can go w/ available resources . Larger engines (.90) require larger props , thus is not the answer . Smaller props does not work well either , they can pospone the formation of the shock waves , but we have to take off ( static thrust suffers & we need thrust to maintain a high speed ). The only solutions are to get rid of the prop , Ducted fans ,turbines , and gliders have all exceeded the 220MPH barrier! There are a few tricks to stretch the barrier but I am keeping that under my hat .
Brooks Brinson
I can do 200 mph standing on my head!!!! SKYDIVE!!!!
above quote really pisses off NASCAR fans on my T-shirt