# prop question

• posted

OTOH going to an 11x4 will result in slower top speed and increased rate of climb...wheras going to a 9x9 will give greater top speed and less rate of climb, but going to an 8x8, IF the engine can breathe well at the higher RPM will give hugely increased engine power, and greater top speed AND rate of climb..on a small model.

• posted

I'm running a GMS .47 in a Tower Kaos on an 11x6 Pro Zinger. Another guy has a Tower Kaos with a (Chinese) ST 45 ABC on a 10x6. His is faster, mine has better vertical, IMO.

BTW, that Chinese ST .45 ABC looks like a lotta bang for the buck.

CR

• posted

Why not just sit down with your hand calculator and figure it out for yourself? The formula for a prop is:

Power absorbed at a given rpm = diameter**4 X pitch X a constant to take care of units.

But for what you want to know the constant is irrelevant.

The answer is it takes 1.7 times as much power to spin an 11x7 at a given rpm as it takes to spin a 10x6 at the same rpm. As the engine can only put out a finite amount of power it will not turn the 11x7 near as fast as it turns the 10x6. If the engine is cranking the 10x6 at an rpm close to the engines max efficiency the 11x7 will kill your performance badly. On the other hand if the 11x7 is causing the engine to run at an rpm close to its max efficiency then then 10x6 will underload the engine and it will spin faster but with less power out put.

Without knowing the rpm the engine is spinning one of those props at in the air everyone who has answered so far is correct. But I would say it is either a VERY unusual 46 or a very unusual airplane that 46 is mounted on to make an 11x7 a better choice. But if you think an 11x7 would be equal why not try a 12x8 or a 13x9 or a 14x10?

• posted

--------------------------------------------------------- My guess is that you would get poorer climb and better top speed. But so much depends on the engine's horsepower vs rpm curve and the drag of the aircraft.

A plot of propeller load versus rpm for a specific engine is the most useful data that you can have. A table containing values of propeller load can be used to compare propellers of varying diameter and pitch, as to the load that they impose on the engine. Rpm data taken with three or more propellers over a wide range of propeller load can be plotted against propeller load. The resulting chart can be used to predict engine rpm for a proposed new propeller. The predicted rpm can then be used to calculate airspeed (approximate at best), static thrust and horsepower. The shape of the curve will tell you what load range the engine is designed for.

Use the same brand of propeller for all testing.

All this is illustrated on my web page at:

Brian

• posted

Well... You take a motor that is rated for 3S Lipo direct drive using

10X6 and run an 11X7 WOT on a test stand.... and I'll take an identica motor, esc and battery and run it WOT with the 10X6. My motor wil survive at least twice as many cycles of the battery as yours (possibly yours will not even survive just run one and mine would kee going like the Energizer Bunny)

Overloading an electric motor causes it to run slower putting out les USEFUL power, but it draws MORE amps (and by power[watts] = amps volts... its drawing more power). The difference between the usefu power and the power it draws from the battery has to go somewhere... That extra power is going to HEAT in the motor coils. When the hea produced is more than the cooling available, you get smoke.

Over-propping is GUARANTEED to be very bad for the motor, and ver likely to be bad for the ESC and battery

-- fhhuber50677

----------------------------------------------------------------------- fhhuber506771's Profile:

• posted

I suppose you know about these things Paul, but I would have thought that to get Max acceleration the max power curve would need to be somewhere less than the top end of the next lower gear - eg at the revs where you 'land' 5th on changing up from 4th in a 5 speed box. That way you are generating max power to move you to your desired speed.

David - who might be mistaking the power curve for the torque curve?

• posted

Er, I did say if the efficiency was relativly similar, whih for an IC engine, it is. I also made the pint that the IC engine lost power as its RPM was dragged dowmn. Electric motors tend to suck MORE power as they slow down.

So at least read what is written before coming in from a totally irrelevant angle.

We are talking IC engines here.

>
• posted

Say, Charle - is your Kaos from the ARF dimension? If so, I'd like to know what you think of it. I'm lookin' at the Tower ARF Kaos for a spare plane.

As to props: experiment, experiment, experiment! I'm still amazed that the best performing prop for my long since retired Carden Gambler

40/K&B .61 combo turned out to be the APC 11x10 pattern prop, which lugged the engine badly but flew the plane like a motha, way better than the more logical recommended sizes that ran in the engine's power band. I'm no scientist so I can't explain it but that's the way it was.

My ofb's sometimes ridicule my admittedly large prop collection but there's a very good reason for it, reason being you just never know until you try. The results can surprise. Or not. I'm not the slightest surprised that the APC 11x5 is the best overall performing prop for my GP ARF Super Sportster/TT Pro .46 (w/Tower .46 muffler) but I wouldn't have known it for a fact had I not experimented with all of the other applicable sizes, along with some not so applicable.

Texas Pete

• posted

Sometimes it is difficult to isolate the sound of the engine and the performance of the plane.

Many years ago (1972) I was flying a Sterling Lancer (40 sized low wing, constant chord wing) with a relatively new K&B .40. I ran out of props (10x6) and went to the hobbyshop. They were out of 10" props period. I bought an 11x7 and for a hoot, I put it on the engine and ran it without modifying it (cutting it down). After readjusting the carb and trying to get used to the sound, which was driving me nuts, I flew the model and was surprised at how well it flew. Unfortunately, it was a bit too much for the K&B .40, so I cut it down to a 10" prop (wood). That was the ticket. It let the engine rev up a bit more, got rid of the sagging, and handled the model very well, for the era. No, I went back to 10x6 props when I could, but I did learn that the 10x6 wasn't the only prop in the world that could provide good performance.

Like Pete, I do experiment some now and then. Each model/engine combo has its own requirements. And yes, you can get some strange looks from ofb's.

Ed Cregger

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