Reduction Drive Design

[cymaz]

A fellow club member is building a 1/3 scale SE5a. We want to put an Evolution 33gx in it with a reduction drive. This will keep all the engine within the fuselage.

The Evo 33gx drives an 18x8 at about 7500/8000. The 1/3 scale prop diameter of the model should be 32”.

How do I work out the reduction ratio of the drive. I still there a rule of thumb...say 2:1 or do I need some more involved maths.

Need help if anyone is good at formula

[2W]

Cymaz, I don't know if this will help, but it may provide a starting point. There are various threads on other forums with power calculators for electric powered planes, but I couldn't get any of them to download or work, probably because of my security settings.

William

[bert baker]

32x18 at 2.8-1 on my Zenoah reduction drive.

[Don Fry]

My take, twice the diameter, is four times the swept area of the prop, so you need a 4 to 1 reduction unit, but a 32 by 34 prop. The pitch increase is needed to get the same theoretical forward speed per revolution.

But, a third scale light loaded biplane just does not need that much pitch. Not a racing machine.

So in the absence of a guru, you need to get a 32 by say 10 prop on an old mains motor, washing machine type, and a suitable belt drive, and measure the thrust.

You should be  something like at about 2000 rpm to lift  that airplane.

Be careful.

Edited By Don Fry on 28/10/2018 15:26:51

Edited By Don Fry on 28/10/2018 15:30:02

[Don Fry]

Bert, is that the 38 cc Zenoah. If so very similar to Cymas proposed engine.

[Peter Beeney]

cymaz, Just thinking in very very general terms here and I may be way of the beam anyway; and I’m also assuming the 7,500/8,000rpm is on direct drive; so if you were to use a 2 : 1 reduction unit this might give you a prop shaft speed of around 4,000rpm.

It seem to me that the SE5a’s one third scale air speed would be in the order of 46mph, but if you were using a 32 x 8 inch prop at 4k it would only give you an air speed of about 30mph, perhaps not quite fast enough really, in action I guess the pilots were at times trying to muster every last ounce of urge they could!

So to reach an air speed of around 46mph you would need a minimum 12 inch pitch; or alternatively a 1.7 : 1 gearbox would give you 4,700rpm which results in around 44mph on a 10 inch pitch prop.

All of this stuff is back of the fag packet of course, and I tend to think the reduction gear makes the revolutions more obtainable, but by the the same token the gearbox inertia might tend to soak up any benefits…

Hope this might at least be some sort of starting point…

Good luck!

PB

[cymaz]

Thanks Gents just the stuff I was after. Reading between the lines a 2:1 reduction and a 32x12 might be the answer. Is there any way , apart from what 2W gave me, to work out the thrust maths etc??

[Don Fry]

No, we are messing about with a REALY complicated subject without the maths" and an expert will ask which prop are you using, i.e., make of prop.

But, Bert Baker has an older type mother, and I suspect a bit bigger, so it's going to be about what you need. Ask him what it lifts. As I recall, these come from Toni Clarke Practical Scale. Last time I looked they used these in a humongous Tiger Moth.

Edited By Don Fry on 28/10/2018 15:58:11

Edited By Don Fry on 28/10/2018 15:58:38

[cymaz]

Umm, it’s this kit....Bert will hopefully come back with some numbers. The Toni Clark reduction is 2.8:1.

[kc]

I suppose it's worth noting that Mick Reeves sells reduction units and his site gives some data which might help.

(Don what an amusing typographical error! Maybe Bert will see the funny side too-- if you are lucky! )

[cymaz]

Found this !

[Don Fry]

Yes Cymas, but that does the geometry. It does not do inefficiency, all the complex bits that the idiots solve by a bit of rule of thumb.

You need to get Burt over a coal fire, and find out what he knows.

[reg shaw]

Hi Cymaz, what is the SE5a, , which plan, kit etc. There are a couple of SE5 designs out there at this size and they vary massively in terms of accuracy, structure and therefore final weight. The lighter ones come in at about 17kg, the heavier ones at 25kg plus. The worst design (just my personal opinion obviously) is the newish offering from Vailly which is available from the States or Belair in the UK. This ends up being heavy and has a massive thick wing and looks and is very chunky in its design. The best design (again, just my humble....) is the old Dennis Bryant plan, available at 1/4 but blowuppable to 1/3 with no alterations whatsoever. Quite a fine design, no missing info and very accurate. These end up around the 19 / 20kg mark. The lightest are a blow up of the DB Sport and Scale 1/4 scaler. With these differences in weights and sections etc, some need greatly differing power to fly properly. Up to 20 kg and the geared Zenoah 38 Toni Clarke unit flies them well, using a 32" x 18" prop. The heavier ones obviously wouldn't fare as well on the same power set up, but presuming the 33GT kicks out a good chunk more grunt than a Zenoah 38 then a similar gearing with a tad more pitch would do well. You could be really posy and go for a four blade 32 x 18 if your mate is building the geared Wolesley SE5 version.

Ian.

[Don Fry]

Posted by kc on 28/10/2018 16:39:27:

I suppose it's worth noting that Mick Reeves sells reduction units and his site gives some data which might help.

(Don what an amusing typographical error! Maybe Bert will see the funny side too-- if you are lucky! )

Burt seems to be sulking, and he's the bloke with the knowledge. Selfish I call it.

[Martin Harris - Moderator]

In view of Ian's weight predictions, this project seems to be pushing 20kg even with the lighter versions. This means your friend might need to consider getting the build overseen by the LMA or risk having a completed model which he can't legally use. Should he be caught flying an overweight model he could be in for an expensive criminal prosecution. Worse, I suspect that if he had an accident involving a third party, this is one occasion where BMFA insurance could be invalid.

[cymaz]

Ian et al....it’s the DB sport and scale Giant Mannock. The builder, fellow club member, is and excellent light builder who has the skill of making the airframe strong as well.

I will email DB and ask what their thoughts are on the projected model weight. Thanks to all the contributors. I value your expertise

[reg shaw]

Is that the 96" span Mannock? The weight for that would be nowhere near the weights I quoted for the 1/3 SE5a models. I would imagine the 33GT would more than power it on it's own.

Ian.

[cymaz]

Posted by reg shaw on 28/10/2018 20:41:40:

Is that the 96" span Mannock? The weight for that would be nowhere near the weights I quoted for the 1/3 SE5a models. I would imagine the 33GT would more than power it on it's own.

Ian.

Yep that’s the one , Ian

[Peter Beeney]

cymaz, I’ve just been browsing though the very capable Mick Reeves site as linked by Don F and it throws up some interesting info. and comparisons. His own reduction unit is the Torquemaster and is attached to the Zenoah 62cc engine. It’s reduction scale is 1.75 : 1 and he finds the best prop is a 28 x 14 which turns at 4,400rpm. So this would then indicate that the crankshaft speed is 7,700rpm. Significantly he does remark that the 32 x 12 at only 2700rpm is too big for max power…

He’s installed this assembly in his one third scale Sopwith Camel which has a span of 112 inches and weighs in at 30lbs or 13.6kg, so it would appear to be very light. However, he also says that it will fly straight and level on one quarter throttle and will perform scale aerobatics on half throttle. But he qualifies that by saying that’s it’s nice to have the extra performance in hand for use when you want it.

He also mentions prop slip and the like which I didn’t take into account, and I didn’t quite understand his figures anyway; I’ve read other articles about prop slip etc. and they often offset this to some degree by taking into consideration the fact that the propeller will unload in the air and thus turn faster.

I imagine that today’s all singing and dancing telemetry functioning radios can already now get quite an accurate handle on all these ‘in the air’ situations. Very useful for future reference perhaps.

Incidentally, applying the same logic to Bert’s Zenoah, if the crankshaft speed is 8,000rpm then the forward speed will be about 49mph without any other correcting factors… Mick's 14 inch pitch prop gives the Camel a speed of 58mph flat out...

PB

[bert baker]

Yes it’s a Zenoah g38 on Toni Clark reduction drive in my pic.

Sorry for late reply... got cold been in bed all day

[Dave Cooper 3]

cymaz - I only have very limited theory on this but I did look into it last year for a turbine-powered LMA Westland Wyvern (reduction gearbox with contra-rotation props). Hardly an SE5 I know, but if using a simple 'toothed belt' drive we calculated a power-consumption loss of about 10-15% of quoted 'driveshaft' bhp. Whereas the full 'planetary' gearbox was soaking up some 25% with spur gears (noisy, but efficient !) and anything up to 35% for helical gears.

Prop's also vary enormously in efficiency (we were planning on variable pitch to help with this), so, I would advocate the simple test rig approach - perhaps 2 or 3 props of the same diameter but with sensible pitch spacing. Take thrust measurements at a set rpm (spring extension /Hooke's law). Repeat test with different tooth ratios on the belt set-up (driven over driver gives the gearing ratio....)

You may find you need a different engine or a change to a more efficient prop design. I think I would stay with two-blader's as 3's and 4's add complexity (as well as cost !). BTW those are BIG prop's so take care near the test rig ! Also, have a close look at torque /bmep figures /curves when choosing an engine - these may be more critical than quoted bhp figures especially if going the bigger pitch route.

Good luck

Dave

[cymaz]

Looking at the Mick Reeves figures I might try a 30” prop as well. I have a few big props spare.