This maybe a dumb question, but I can't find an answer to it.
Why aren't there any gearboxes for glow engines?
Shouldn't that give way more thrust and efficancy?
I don't see a problem, it should be easy to acomplish!

-Nik

the most power is in the upper end of the power curve, so i don't know why you would want the engine going full speed, but the prop only a fraction of that.

Got ya!
Dumb question! Sorry!

Not a dumb question at all.

Actually, the reason is more of reliability and cost than anything. There actually used to be a few geared .61 engines available. But the power to weight ratios of our engines is such that it's just not needed.

Turning a larger prop at a lower RPM is generally more efficient. And as it was pointed out, many of our engines produce peak power at higher rpms. So it makes a lot of sense to wind up the engine and then gear down the prop. It's just too much hassle. Just think of trying to keep them oiled. And we are talking about an input rpm that could exced 20krpm in the air, so you're going need some quality gears and bearings in there. That's $$$$.

Dubro, Sullivan, byrons and possibly others in the past have made belt reduction units just for sixty size and larger glow engines.

I doubt that any are in current production because the proliferation of large gas engines pretty well killed the market. These units were popular during the early days of the popularzation of giant scale flying before the gas engines were available and worked well for the reasons you outlined and were reliable. I would love to get my hands on an old Dubro unit.

John

i wish gearboxes were standard, common item, i need some quite badly

In Aus i'm pretty sure we can still get them, generaly for the 60-90size engines, there used a little bit buy the WW1 scale type guys to swing a bigger slower prop

I think it would be worth it, to get a .60 size warbird to turn a nice scale looking prop, instead of having a little 2 blade on it. Depends on the application.

electric motors don't replace gas engines, and probably never will. to turn that same prop, an electric motor typically has to have a reduction of something like at least 4:1. a gas motor will turn the same prop at about 4-6 times the rpm. If you get a big enough electric motor t oswing that prop at the same rpm without reduction, it would draw so much current you would need something like a car battery to power it. now, car batteries weigh so much that it would be completely useless.

Where did the electrics come in to the conversation????

Beats me, Woodsy?

John

Brschmid,

The most efficient setup would be the engine running at peak power (higher RPM) and the prop running at peak efficiency (larger diameter and lower RPM).

This is true for full scale as well (turbo props are a great example as they are geared). Recip engine full scales match engine RPM and propeller RPM fairly well (and add adjustable pitch props in many cases).

I'm guessing the reason it isn't done often for models is, as everyone has said, for reasons of cost and reliability. I'd also add that the need for efficiency in models isn't as great. Thrust to weight ratio is typically much higher than for full scale airplanes and also no one really cares (too much) about fuel economy, which is a big factor in full scale (sometimes).

it would weigh more to get an electric motor to spin a 14x6 prop at 9500 rpm giving you 10 mins of flight time (battery weight included) than it would to have a gas engine that would swing the same prop at the same rpm for 10 mins.

Some of the RCV engines use gearing. The output shaft is attached to the cylinder valve which turns at a slower rate (half I think) than the crank shaft. Never used one but the design looks interesting however I've heard mixed reviews.

Nick

Okay Spaceclam,
Once again. WHY are you talking about electrics?? No one else has mentioned them, this is a thread about geared glow engines.
Dennis-

Woodsy,

Where can you buy/order one? How much?
Wouldn't think of building an ultralight without one! An engine that wouldn't taxi an ultralight on D/D will fly one fine with a reduction/larger prop.
As far as "why" everyone so overpowers their moidels these days, the market was lost for these units-just buy a bigger engine for the money you'd spend on a gearbox/belt drive.

Jetts

i am comparing the neds of gas motors tho those of electrics. electrics need them, gas doesn't

Wouldnt it be a nice feature to gear a .60 engine to spin a scale 4 blade prop? Thats where gearing could come in handy.

njl: some of us can't afford RCV engines lol

It really wouldn't be that hard to custom build your own belt drive unit. I'd start with using the belt from a helo kit and maybe borrow some of their gears and such as well. Though they do use a lower ratio than you'd want on a prop. A bit of maching and some bearings from boca, and you'd be in buisness. I think the hardest part would be a suitable prop shaft.