What is the max prop RPM?
 

Good evening all. I was just wondering what the max RPM is for a master air screw or APC prop? Does anyone know?

RE: What is the max prop RPM?
 

Terry, APC claims redline is 190,000 divided by the diameter. I've had their props fail below this, so make sure to always work around this stuff with both eyes closed. [8D].

If you've got a 1/2A engine capable of throwing the blades, then you've got one that is both supercharged and on nitous. ;)
So far I've never seen any engine below a .32 break APC's golden rule.

RE: What is the max prop RPM?
 

cp, The reason I ask is about 25 years ago I bought nitro off road type baja buggy that has a like new motor on it, I think its a HP .21. (I know .21 is correct) It was a great runner and there were 2 motors available, and I got the hotter/faster of the 2 motors. I'm probably wrong but it seems like the max rpm was 28,000. That seems awfully high but its been a long time and I don't have any spec's for it. After physically looking at it, it looks about the size of a regular plan motor. I'm just wondering how it would work on a plane. I don't know what diameter prop I could use either. Any thoughts?

RE: What is the max prop RPM?
 

Might it be the HB 21 Blitz? A very hot motor in its day-competitive with the K&B 21. APC do a number of props suitable for 19-21's: 8x4m 9x4, 9x3, 8.636 x 3.75 etc depending on intended use. For sport R/C you might want to go down in dia a bit and up in pitch to 4.5"-5"-5.5"

ChrisM
'ffkiwi'

RE: What is the max prop RPM?
 

I would think that the max RPM would be when the tip speed is nearly Mach I.

For any of the vintage props, surely they would not be safe at higher speeds.

Not ready to do the math on that this evening.

Robert

RE: What is the max prop RPM?
 

These folks will do the math for us:

http://pprcc.portage.net/fyi/prop_speed.html

RE: What is the max prop RPM?
 

My understanding is that car engines have raised exhaust timing for high rpm, so their rated rpm is only meaningful if turning a little gear. By raising the exhaust port, you sacrifice the amount of time that the piston gets pushed by the expanding gas, so there goes your torque for doing more work [like turning a prop]. These engines are also equipped with big ports and passages that don't breathe right with too much load, not enough rpm. Bolt on any usable sized prop and the car engines turn into very expensive paper weights. If you look at a Picco .21 [the only .21 I've ever owned] it looks about the same size and weight as a .35 [so now you have both a heavy AND guttless paper weight]. If your .21 is going to be well suited for aeroplanes, it's possibly one of those that I was told was developed for airplane racing and speed before I got very far into the hobby. For example, I just finished building a P-51 kit that was turned out just for .19-.21 engines.
If you have any doubts about props and rpm, Steve Wilk at Eliminator Props has good advice and carbon props that wont let you down. I'll bet a hot .21 can turn a 7x6 or 7x7 pretty good. A 7 inch diameter prop will fly a 2-2.5 pound speedster pretty good. Hopefully this .21 of your's works well in airplanes.

RE: What is the max prop RPM?
 

Way back when I had an Airtroncs Jet Fire 20. It was a modified Ugly Stick type of plane. I had an O.S. .21 SE car engine in it. I put a Macs tuned pipe and header on it. I was running 30% nitro and I was able to get an 8.5X6 to turn at just over 23,000RPM measured on my Ace tach. I was running Zinger wood props on it. When that engine got on the pipe, ooohhh man did it take off. It was my first more power then weight plane. It would roll about 5 feet and then straight doing rolls. Had great mid range and idled all day long. The power difference between idle and half throttle was not much. The engine only made tons of power wide open. It was not a good engine when it was out of the power band. It was finiky to tune too. It would eat glow plugs like candy. I ended up loosing it to a ripped of aileron. It was fun while it lasted.


Dru.

RE: What is the max prop RPM?
 

Thanks Larry! Ya just gotta love the internet!

Robert

RE: What is the max prop RPM?
 

That pretty much blows up the myth about 1/2A props going super sonic.
A Nelson FAI .40 turns a 7.2x8.6 prop over 30,000 which puts the prop about 650 mph. What was that again about props going over 75% of mach being no good? There might be diminishing returns, but this is the only way you'll go 200 with a .40 powered engine that we know of.

RE: What is the max prop RPM?
 

Back in the day, I had a host of Norvel AME .061's. I ran them in the 1/2A Shrikes. Used backplate pressure and a couple of tricks. Long story short they turned about 24k static and REALLY unloaded in the air. As long as the prop didn't catch the ground when landing, the prop would last a very long time.

Combatpigg is correct also when he talks about the car engine not being suited for an airplane. The prop will really hold it back.

2Fast4U

RE: What is the max prop RPM?
 

If a .21 car engine is set up to provide max RPM's as this one appears to be then you'll want to run it with a rather small prop. SOmething like 8x4 to 8x5 at most. The hot FAI .15's run with a 7 x 3.5'ish prop to reach the high 20's to low 30's that they run at.

RE: What is the max prop RPM?
 

Hi!
The HP .21 isn't a "hot" engine by todays standards.
Back in the days when we raced Quarter Midget pylon, with 3,5cc engines, over here in Sweden(25 years ago the engine of choice was Rossi .21 and we used APC 7x5 and later 7x5 carbon props.
Both those props rewed 25000rpm with the magic mufflers and long pipes we used then.
An old, not so powerful HP .21 will not reach those rpm figures but still, a 7x5 prop is what I recommend.