Yes. The limit comes from compressibility effect when the speed of the propeller blades approaches speed of sound in which case drag increases dramatically and efficiency falls off rapidly. I haven't seen any experimental data for thrust (Ct) and power (Cp) coefficients for propellers operating at supersonic speeds. That would be really interesting.
Of course, mechanical strength may also be a consideration.

/Red B. (back on-line tomorrow)

oye vey

dick Hanson wrote: Azoy? Enschultig meir!

/Red B

Ok it sounds reasonable, but we may also think in terms of prop efficiency, which becomes zero when the forward speed is zero (static condition).

Efficiency = (thrust * velocity) / input power

Also propellers use to have their highest efficiency at a moderate rpm.

Better late than never.

I just want to agree with most of what has been said. Yes there is a point that extra rpm really doesn't get you much. Both have been talked about. One, the prop tip speed. I try to keep mine down around mach .5 for the noise and safety reasons. Remeber if it is loud it is draggy. As was stated before around mach .8 you really start to pay a big price in terms of power for any additional thrust. Two, the plane/prop relationship.

The first issue is pretty easy to address, how much do you value your safety and ears. Since your limited to an 8 inch prop, this is 2/3 of a foot and the curcumfrence would be 2/3*pi or 2/3*3.14 or a little over 2 feet. The speed of sound is about 1100 fps. So if we want to keep it below mach .8 we have 880/2 or 440 rps or 26400 rpm. Say we didn't want to push it quit this high so we limited our prop to mach .6....660/2=330 rps or almost 20000 rpm.

I've attached two graphs, both using 8 inch diameter, 3.5 pound wieght plane with a wing area of 5.5sqft (the wing area wasn't stated). I also assumed a somewhat draggy airframe, gliders have a lot of frontal area and surface area.

The first graph is of an 8x4 and 8x8 both at 15000 rpm. By my calculations the 8x8 would produce around 50oz thrust static and the 8x4 would produce about 38oz of thrust static at 15000. The big story is that the 8x8 prop would require something around 620 Watts of input power and the 8x4 would only require 220 Watts. The 8x8 would have a max climb rate of about 25 fps and the 8x4 about 12 fps. Notice you will never reach the speed needed for max power output for the 8x8 but with the 8x4 you will. Your top speed would be about 80fps (56mph) for the 8x8 and 60fps (42mph) for the 8x4.

The second graph is of and 8x4 at 20000 rpm and the 8x8 still at 15000. The 8x4 at 20000 rpm would need about the same input power as an 8x8 at 15,000 but the 8x4 would produce about 64 oz of static thrust. The max climbrate for the 8x4 is 35fps and again the top speed is the same for both props..80fps.

Now any relationship to actual reality is only an accident, and I don't fly electric so draw your own conclusion just thought a graph might clear up what everyone is talking about.

Power in the graph must be above the min power line (pink), this line is the minimum power for level flight. Climb is substainable climb.

Happy Flying.

interesting topic. i am just a hack at rc planes just started less than a year ago. so i dont really know what im talking about. but after reading his original question. i cant help but wonder if he is talking about as an example:
you took his 8x8 prop and put it on a motor. the motor was just vice gripped to a saw horse and you were measuring thrust at different rpms

would there be a point where the thrust would decrease or stayed the same the faster the motor spun?

fake made up numbers here

15000rpms = lets say 61 grams of thrust
15500 rpms = 65 grams of thrust
15001 rpms = 64 grams
16000 rpms = 61 etc.
so the peak ammount of thrust would be 15500 with that prop
so i guess if he had 3 different motors to choose from 10000rpm motor, 15000rpm motor, 16000rpm motor. if the maximum thrust for that prop was at arround 15500 would it be worth it to buy a 16000+ rpm motor? or will the thrust decrease at all?
i know the whole thing is more complicated than this but (i hope) i translated his question geared more to the answer he was looking for.
crankypants

Aircraft props are probably just like boat props. At some RPM figure, the prop will start to cavitate, therefore losing thrust. Cavitation as well as Mach number reached can also physically stress the prop to failure. Very common on controline speed ships before composite props.
What is the RPM? Depends on lots of factors. Prop blade shape, area, airfoil, pitch are probably only a few. Also, with a high pitch prop turning at high speed, there may be problems generating enough thrust to even move the plane. That's why variable pitch props were developed: To tailor the prop performance to the engine/aircraft performance envelope.
Like with the controline speed prop example, just adding power and pitch to a prop is not a guarantee that a given plane will fly better. If you take something like a control line D speed job, roughly a 60 size engine turning about a high pitch 10" prop near 28, 30 thousand RPM, that plane will reach near 200 mph at the end of those 60' lines. Put that same engine and prop on any 60 rated RC job, it may not even be able to taxi in grass.
Another example is a Bipe I had once that could only fit a 40 without extensive rebuilding. Turning a 10-5 or 10-6 at nearly 13000, the plane would barely get off the grass in the length of our field, very low climb rate, and had to be dived do any aerobatics. Changing to an 11-4 at 10 k cut the takeoff in half, doubled the climb rate, and let me do loops and other mild things from level flight.
The couple 2 m gliders I had used a stock Mabuchi turning an 8-4 glow type prop at almost 10k on 7 cells. Don't ask me power or current draw. For the example given based on prop speeds for the 500 to 700 size motors listed in Hobby Lobby catalog, this glider in question could possibly profit by using something like a 600 or 700 sized brushed motor on 12 Volts turning possibly an APC 8-5 or 8-6 props a started. If the questioner wants to try a brushless motor, something could be selected to turn this same size prop in the 10 to 12 thousand range.
Being limited to an 8" prop, options are slightly limited. All the drag characteristics will limit the amount of useable power availble from this prop.RPM/Pitch will have a very narrow window to work in.

Cavitation on a boat prop is caused by a phase change from liquid water to gas. It does not happen on plane props, as far as I know.

Brushless is the only way I go any more. From what readings I can get with my handheld mini tach, I believe I am getting close to 15,000 rpms. I have flown the plane once and had trouble with the throttle settings causing it to try to over throttle causing over amps and heat shutting down the ESC. I now have that solved and am planning another flight as soon as weather permits.

This is an Aero-naut Aerofly. Discontinued kit that I built for a friend. It's over all weight is 3 pounds 4 ounces with 102 inch wing span. The maiden flight was short but, had plenty of power for the plane and would have climbed easily if I hadn't had the cutout problem. I used new Align brushless s400 size that could handle 25 amps and I used two, three cell Kokam 1500 11.1v Li-Po's in parallel.

Thanks again for everyones input.

Dude, on a clean ship, an 8x8 doesn't even start to get happy till it's spinning 20K or better. On the barge like thing that you describe this speed prop would be a waste of good plastic, I wouldn't use any more pitch than 5 or 6 in your application.

After reading all this, I can only wonder....
WHAT is preventing you from using a larger prop?
It's an 8-FOOT airplane!
An 8 inch prop can't make it move.
A 16 to 20 inch prop, a large motor with a gear box, a basket full of batteries.. That's "approriate" for this task.
The configuration police won't be coming by to shoot you down if you go larger, in the prop, the motor, the battery.. anything at all..

Tall, the opening post reveals that this glider is a pusher. The motor is apparently mounted at or below the wing trailing edge, in a position where anything bigger than 8" will hit the backbone of the fuselage. Hobby Lobby used to have a large 500 powered glider of similar size that used an 8" prop, but I don't see it in catalog 44. It would be similar in format to the Tipsy on page 5.

I have had to repeat this several times on this thread. This is a pusher with a special built fuse which would require complete changes in plans to allow a longer prop. CAN"T be done without making new fuse!

Thanks.

combatpigg, This "barge like" plane is a glider. It's not supposed to go fast, just high with authority. The specs of my glider are mentioned at other places in this thread and that has been the entire point of this thread, to find what is the best that can be put on this. The plane called for a 7x6 prop, but, I was able to squeeze a little more out and I am using an 8x6 and it's electric.

Also, it's a kit that I built from planes and stamped balsa. The fuse is "rocket" shaped, but, sets at a downward angle from the wing and tail boom. This makes the rear of the fuse point above the tail boom which is where the motor mounts. So, there is a limit on prop length between fuse rear and tail boom. Maybe this makes sense. I just thought people would accept the fact that I can't put a longer prop on without major reconstruction when I keep telling them that.

Tall Paul, if you had read my post just before yours you would see that the prop does push the plane quit well.

Since no one would believe me, I finally got the 3-d view into the scanner and copied it for all of you. Here it is.

I've done a pusher or 20... There's an amazing device available... a folding prop...
On a test plane, and on the finished item... 16x10 prop.
There are NO 8" props that can fly the plane in question... unless you put in a .25 glow motor.

I FLEW THE PLANE JUST FINE AND I HAVE THE VIDEO TO PROVE IT!

THANK YOU VERY MUCH. And, a folding prop WILL NOT WORK!!!!!!!!!!!!!!!!!!!!!!!!!!!.

Did you see the drawing I uploaded?

Agcatsbest, that 3 view sure comes close to what I was visualizing from post #1.

BTW, you don't do crop dusting, do you? Just a thought.

50+, yes, I used to! I grew up in it. I actually retired from the fire dept, but, for a couple years I did my dream of crop dusting! I decided it wasn't something to do part time, tho! Needs your full attention or not at all.

I looked at the picture of it, and I rest my case, it's a barge.

You could say that. What caused all this was just my asking the wrong question in the first place. I knew we were limited in prop size. Period. Doesn't matter what anyone thinks about the prop size. We were going electric, which the plans allows for. However, others who have built and flown this glider with the stock setup complained of it being way under powered. Just barely would fly. With brushless setups you go from reasonable prices to ungodly in a short jump! Motors jump in price, batteries go way high and ESCs skyrocket when you get past the $100 range! We wanted a power setup that would provide what we wanted without going into the "power not needed" range. I had one calculation that said this prop would provide the thrust needed, but, would have to be turning at 15,000 rpm.

Now. I had heard in several places statements like "you need a gear setup with a larger prop because if you just keep speeding the prop you reach a point where it's just beating the air and adding nothing.

So...... I was looking at it wrong and was asking if there was a speed at which point spinning a prop any faster would do nothing extra. I didn't know if this prop would work at that rpm......or, if we were going to have to deal with an underpowered plane or completely rebuild the fuse. The latter was not going to happen.

There. All of this because I wanted to know if that prop would work ok at that rpm.
That is really all I wanted to know and the question I asked was just for my information, about the speed where increasing would do no good.

If the prop would work at that rpm then we had a power plant at a reasonable cost that would do the job.

It flies fine. Bad ESC caused me problems forcing me to cut the maiden flight short, because the nice winds had suddenly gone crazy and I couldn't keep the throttle open where I needed it. It was being carried toward trees. I brought it down quickly, wind gusted forcing the nose up and I failed to respond quick enough. It tip stalled and caught on one wing as it hit. There was no damage at all. Just pride. I am waiting for a different ESC to try it one more time, in a more open space, before I put the trim on the glider and send it to my friend.

There is a couple photos of it in its skeleton form in my gallery.

If you would like to see the short video just click on the link below and when you get to the site select the bottom file "Aerofly Maiden"

[link=http://f1.pg.briefcase.yahoo.com/bc/[email protected]/lst?.dir=/click+on+file+to+play&.view=l]Aerofly Maiden[/link]

If it doesn't load, wait a while and try again. I think only one person can be on at a time.

Hey Desert Tallpaul,
Agcats built the plane for me. It is a Christmas gift from my Brother. Agcats was good enough to build it for me and when discussing what to power her with Agcats started this thread. So the question was started. Some of the posters here were rather technical and went to great efforts to explain their thoughts on the subject. Here you come with the folding prop. DUH. As it turns out Agcats and I guessed right about the motor. It'll turn fast enough to fly the "Barge".
So my thoughts are this. Thanks Agcats for building a plane I never could have.

Desert Paul and combatpigg,

AG started this thread with a simple question that apparently fell on deaf ears. The guy doesn't deserve the rhetoric your giving him.
Paul, take the desert sand out of your ears and Combatpig, remove the mud from yours and L-I-S-T-E-N instead of badgering AG's plane.
We'll all get along better that way.

Thank you,

Ray Ray

Some of the these statements were part of the problem with the thread. People seemed to jump on a misunderstanding of how props work -- maybe a danger of asking a question in this forum.

If you hold a plane on the ground and run your 8x6 prop at 15,000 rpm then the prop blades are stalled and in that case you might say that you are "just beating the air". If you release the plane and let it speed up the portion of the blade that is stalled gets smaller as the velocity picks up. At about 25 mph your prop is not stalled. So whether it is "just beating the air" depends on the velocity also. I still think you would be better off with an 8x4 or 8x5 prop, which will not be stalled on the ground. You will get a lot better pull in the 0 to 25 mph range and I doubt if you care about the extra top speed that the 8x6 prop gives you.

Try the freeware program prop selector. [link]http://www.gylesaero.com/freeware/propcalc.shtml[/link] it gives an indication of when a prop is stalled and how badly it is stalled.

This site has good diagrams showing props and velocity vectors and a little on why they stall
[link]http://www.auf.asn.au/groundschool/propeller.html[/link]



I do note that you had 3 estimates for motor power, 200 W min, 320 W max and a fairly careful calculation stating You went with 277.5 Watts, an 8x6, and seem to be happy. So I guess you did get a some help. I'm glad it worked out well and I'm sure most people appreciate the update and happy outcome on the project .

As for some of the other comments, come on guys, it's a glider. Under the right conditions it doesn't need the prop at all.

Carl