Large Model Association Forum

Hi,
I am currently building a plane bigger than anything I have done before and so need to find a supplier for a big prop. Does anyone have any suggestions?

My plane is 14Kw (18hp) and a scale sized prop would be 38". I have also tried using one of those on-line programs for calculating prop sizes from engine power/speed and it said I needed a 34"x18" or 36"x14". My maximum engine speed is 8000rpm.

The largest I have found so far is 30"x20".

The boys that use the geared petrol motors use some big props. Phil at Fighteraces sells 32x18 and 34x20.

Simon

Andy

8000 rpm is way to much for such a big prop and you will have the prop tips going far to fast. 18hp is in 170 to 200 cc range and those sorts of motors are turning 32 to 34" props with a 12 to 14" pitch at around 6000 rpm (Great variance due to model and make of prop) Try Dave at motors and Rotors for Beila props in this size range.
I would be very wary of using a prop built for a geared motor at the sort of RPM you are talking about as many are lightweight moulded props only deigned for the low RPM of geared motors and could fail under the high loads of higher RPM.

John

Mackay models for large Menz Props. 32 - 34" not a problem.

Steve

Thanks guys, I will try the suppliers you suggest.

John, thanks for speed advice. The 8000rpm is my theoretical maximum as I am using an electric motor. I did not know what a motor of this size would normally do. Depending on the prop loading my elec motor could go to a speed of the voltage x the motor Kv. My voltage is 50v and the Kv 170, so itcould go to 8500rpm. However, with the correct prop diameter and pitch the full power should be able to be extracted at a lower rpm and then the prop will not exceed that speed.

Andy

You won't be needing anywhere near 8,000 rpm on a prop this big for the Spacewalker........I'd image 1/2 that would suffice and give you ample performance. I ran a 39" x 25" carbon prop on a 24HP Moki 400 in the Skyraider......4,400 rpm on the ground and that gave a 180lb model plenty of performance!!! Your model is smaller and likely to weight 1/2 as much!!!!

My 'geared' props are definately NOT lightweight moulded items.....they are traditional 'laminated' wood props. I have the 34" x 20" in both the 'standard' as well as the 'Old Timer' blade shape in stock. Other sizes are available, but beware, props this size are not cheap and if you don't know for sure exactly what you need, it's going to be a cost testing session.

Phil

Andy, my 1/2 scale S/Walker uses a 3W 200, swings a xoar 34/10 at 4800 on the ground, unloads a bit in the air.

Dave.

Dave Collis 2296 wrote:Andy, my 1/2 scale S/Walker uses a 3W 200, swings a xoar 34/10 at 4800 on the ground, unloads a bit in the air.

Dave.

Thanks Dave,
I have just found it very useful to look at the 3w 210 engine http://www.3w-modellmotoren.com/katalog/motoren-alle-motoren-84/3w-210i-b2-f-ts.html as this one is very similar to my electric motor set-up. Power is the same and top speed is 7500rpm. It says use a prop size of 34x12, 34x14 or 36x10.

Have you any idea what sort of current you're going to be pulling at such high rpm's on these sizes of prop?

From my only 'large' electric model experience, we had a Hacker H-200/8 on a 30" 3 blade variable pitch prop set at 16" pitch......we were only going about 4,500 on the ground IIRC and we were pulling over 250A on a 14s set-up........and even at this, prop noise was considerable.

http://www.aircraftinaction.com/Alis_Spitfire_Roger.WMV

Phil

I don't know how good they are but I've been researching large props (24" to 28") and I've found Hobby King a reasonable selection and the prices might be too good to be true . Worth a look though.

http://www.hobbyking.com/hobbyking/stor ... rentCat=58