Jim Thomerson

This is from an article by Bill Gieskieng in the Aug-Sept 1998 Aeromodeller, pgs 16-17, about an engine for the International power freeflight class, F1C. This is a highly developed, technically sophisticated area of competition. I think the kind of engine set up discussed here is now widely used.

Basically there is a 2.5 cc--15 size engine which unloads to as much as 33,000 RPM in the air. It is turning a planetary gear setup with 4:1 reduction to give prop RPM's around 8,000. The prop is 12.8 x 11.8 inch. Prop has calculated efficiency of 80.5%. The idea being to use the engine HP to turn a large high pich prop slowly to get the most thrust out of the power train. The gears are said to be very precise and there is little friction loss in the gear train.

I thought this interesting, and wondered if anyone knew more about it.

HighPlains

My Feedback: (1)

I think there was a guy in upstate New York that was also developing a setup like that based on the Nelson .15 engine. I remember reading about it in MA 4 or 5 years ago.

buzzingb

My Feedback: (2)

The smaller displacement engines can revup more just look at the small car engines that turn those rpms. I must say that I have no desire to fly such an engine and setup as I am a very practical person and the .40/.60 size engines seem to be the most practical engines for me. The only small engine that tempts me is the little Saito 30.

B.L.E.

I remember seeing a photo of a .61 two stroke belt driving a large prop on someone's pattern plane, back in the days when F3A rules limited two strokes to .61 (10 cc) and four strokes to 1.2 (20 cc) displacement. It is my opinion that unless you are restricted to a certain displacement by competition rules, it's better to just direct drive the prop with a larger engine. (the KISS rule, keep it simple, stupid!)
I even prefer direct drive on my electric planes.

downunder

That's the point that Jim was making. In F1C they're limited to 2.5cc and something on the order of a 10 second engine run to get the model (a glider) as high as possible. Those engines are some of the most highly developed in the world and make huge HP but to translate that HP into the maximum possible thrust they can't use a direct drive because the prop needed would be far too small. Hence the gearing. A similar engine in a CL speed model would use a small (about 3" in blade length) single bladed prop on direct drive because (obviously ) they want the revs for maximum speed.

These engines are different to car engines because they're making max HP at those 30-40K revs where a car engine is way past its peak HP at full revs.

Jim Thomerson

I think F1C is down to either 4 or 5 seconds engine run these days. What kind of HP, more or less, does it take to drive that size prop at 8,000 RPM? There was a letter to the editor in Aeromodeller which showed that the downhill changes in engine run in F1C have fairly closely tracked increases in engine power.

B.L.E.

I would estimate about one horsepower more or less. If I had a 12.8x11.8 prop on hand, I could stick it on my electric plane and measure the watts with a wattmeter.
An APC 14x10E needs about 950 watts to turn that speed. This is probably some custom carbon fiber folding prop if it's on a competition free flight plane.

HighPlains

My Feedback: (1)

Here is a picture of the geared Nelson 15

B.L.E.

Looks to me like a very effective way to make the statement "If I wanted a radio controlled model airplane, I could have bought one!!"

The rubber powered competition planes are also not cheap, especially when you consider that the rubber "motors" are only good for a few flights before strands start breaking.

HighPlains

My Feedback: (1)

The Neslon setup runs in the $800-$900 range. I would guess that the airplane to go behind it would be even more. They gain over 500 feet in 5 seconds and are stuffed with auto-controls to change from a vertical drag racer to glider.

j.duncker

My Feedback: (2)

This is why they use gears on electrics!

HighPlains

My Feedback: (1)

How many know that the WWII fighters used gear drives to reduce prop speed and increase prop diameter?

B.L.E.

Gear/belt reduction is also very popular with homebuilt airplanes, especially when converting automobile engines to aircraft use. The Rotax two and four stroke engines all have prop speed reduction units (PSRUs) on them.

wjvail

My Feedback: (3)

buzzingb... Gear reductions are not restricted to small, high revving engines. This is an OS .61. The gear reduction on this engine was before 4-strokes became common but noise issues were getting to be very restrictive. Folks figured out that the high prop tip speed was producing more noise than the engine and turning the prop slower helped. As B.L.E pointed out, rules in the pattern world at the time also partially drove the development of these engines.

You did know the Satio .30 is out of production?

Bill Vail
www.RCScreenProtectors.com

B.L.E.

Any info on what size prop that geared .61 would turn?

Listening to a high speed electric plane really lets you know just how much noise a prop can make.

[link=http://flyemfast.com/files/BestVen.wmv]Electric Q-40 video[/link]

wjvail

My Feedback: (3)

I'm still looking for that. I went through the documentation last night and it didn't suggest any props. I'll let you know when I find it.

Bill Vail

downunder

I've seen photos of that geared OS 61 before but I just noticed something interesting in the shot of it from the exhaust side. The front housing has been rotated 90 degrees to put the carb over on the side. This alters the crank timing so the engine runs in reverse direction. The gears then have the output shaft running in the normal direction.

lucien

Speaking of that, here's a picture of my gasser (Rotax 503) with the gearbox and prop. The gearbox is equipped with a centrifugal clutch and a 3.47:1 reduction. The max HP of the motor (50 hp at sea level) is reached at 6500 rpm, so a direct drive is basically impossible. But the gearing translates to about 1850 prop rpm and as you can see, it can swing a HUGE one. The prop is a 68" 3-blade.
For sure, it makes lots of thrust and needs LOTS of left rudder on takeoff...

I'd love to see a practical gearbox for r/c motors, as a big one turning slow can really crank out a lot thrust and is much more efficient than a small one really spinning up.

But I'm sure it'd cost out the wazoo. My rear is still sore from buying this gearbox/clutch assembly you see here ....

LS

shakeelsid

Prop dia 12.8
Prop geometric pitch 11.8
Prop RPM 8,000
Prop Efficiency 80%
Gearing 4:1
Engine RPM 32,000
Reduction efficiency 92%
Engine output 1.15 BHP
Power absorbed by prop 1.06 hp
Estimated thrust, sea level 7.60 lbs.
Thrust velocity 90 mph

Now assume it was direct drive, and your engine was still producing 1.15 BHP and you needed 90 mph thrust velocity

Prop dia 6.5
Prop geometric pitch 2.95
Prop RPM 32,000
Prop Efficiency 67%
Gearing 1:1
Engine RPM 32,000
Reduction efficiency 100%
Engine output 1.15 BHP
Power absorbed by prop 1.15 hp
Estimated thrust, sea level 4.74 lbs.
Thrust velocity 90 mph

See the difference in thrust

Now when I see that, I assume the engine is being run rather conservatively, and perhaps would have an active life of more than a few flights - very interesting -- anyone has pictures.