N99JH

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Hello All

I am working on a scale model which requires a 22" diameter scale propeller. I would like to run a 22-18 propeller at around 3000 rpm (propeller speed) to keep the noise level and engine temperature down in a cowled application. The attached pictures show the intended model and the projected weight will be around 20 lbs.
I would appreciate your thoughts on this subject from people with hands-on experience with the SP-120.

Thanks for any help you can offer

Joshua

andi3142001

Hi Joshua,

I remember that the guys from RCV stated somewhere that the rpms shall not be below 4000 at full throttle.
And I am afraid that 20 lbs ist too much weight for the 120 SP. You need a stronger engine for that plane.

regards
Andi

N99JH

My Feedback: (66)

Hello Andi

I just came back from my shop where I ran the engine with a 22-14 Zinger propeller. At 1/2 or less throttle, factory initial setting of 1 3/4 open on the high speed needle and 2 on the idle - the engine fired right up. After it warmed up for awhile, I tached it with my old trusted Ace Tachmaster II tachometer and while still at half throttle, just manipulating the high end needle got it running reliably at approx. 3200 rpm.
I bought this engine used from another fellow on RCGroups 4 years ago so I do not know any of its history. I am going to run it as if it is being broken in for now and will probably run at least a gallon of Cool Power through it.
So far it is running surprisingly smooth, the sound (as you know) is awsome and I am optimistic with what I see so far. Given that the factory recommends a max. practical rpm on the prop of 5500 - at full power I tached the engine wide open with both needles adjust to optimum running condition, at 3900 rpm with no indication of overheating (albeit on a test stand).
I do think that it may eventually drive my 22-18 propeller nicely, possibly at around 3500 rpm. I have been a modeller for over 40 years and this engine is exactly what the scale community has been waiting for! High torque at low rpm with scale diameter propellers. Time will tell though but if you are interested in my experiment I will gladly keep you informed.

Happy New Year

Joshua

N99JH

My Feedback: (66)

Deleted.

Joshua

andi3142001

Hi Joshua,

yes I totally agree with you that the SPs are perfect for Scale Flying. But I am very interested in your results and would be happy if I am wrong.

regards
Andi

N99JH

My Feedback: (66)

Hi Andi

Well I finally got the final results and I am very happy! With the 22-18 propeller the engine ran silky smooth at 3500 rpm without glow heat and 3700 with heat. This is basically what I expected based on preliminary analysis. I used Pe Reivers's very accurate MS Excel spreadsheet to calculate the power at the measured rpm.

To Determine Required HP then use this Equation
engine 1
Propeller Length 22.0 Inches
Propeller Pitch 18.0 Inches
Engine Speed 3,585 RPM
Required HP 1.26 HP
Thrust generated 7.5 Lbs

Although I am not interested in HP but rather Torque, the calculated HP was fed into MotoCalc (which is used to select and optimize electric motors) with an appropriate motor and adjusted until the predicted HP was approx. 945 Watts which is equal to 1.26 HP, and MotoCalc predicted approx. 3500.
Running the engine today totally verified this. The engine ran smooth, sounded somewhat like a Turbo-Prop and was very pleasant to listen to without the obnoxious typical high pitch noise. Also, I used the Genesis silencer from WestonUK.
With glow heat ON idle was smooth and reliable at 1500 rpm. Predicted thrust by MotoCalc is approximately 11.25 Lbs. This should in my opinion fly a scale model like the one I intend on building weighing 15 - 18 Lbs. at scale speed.
I attached some pictures to show the propeller and test stand.

Best Regards

Joshua

N99JH

My Feedback: (66)

Here are some pictures of the propeller and engine test stand.

N99JH

My Feedback: (66)

More Good News!

I just got back from the shop where I ran the engine at Full Power for almost an hour! Two full tanks, 14 Ozs. each. Wide open from start to finish. No Overheating at all, during the second tank the engine died once, but started right away. I do not know why it died because I was away working on my car at the time. With plug heat on I could idle it at 1300 rpm reliably - all day long. In my opinion flying this engine should be with optional plug heat coupled to the throttle so that heat will be applied as soon as the engine goes below 50% power.
I think that the engine did not heat because a prop speed of 3500 rpm is 7000 on the crankshaft. That is a very comfortable speed to run and if ever slightly reach, the engine breathes better and produces close to peak torque.

Best Regards

Joshua

andi3142001

Hi Joshua,

thats an awfully good looking prop on the pics!
Good to hear that the engine runs fine at those rpms.
What I would suggest is to check out the ensemble in the air with an appropriate trainer.

Best regards
Andi