ETLB Squawk Forums — Archive

Hello everyone, I'm starting my construction
Hi-Max I have a question about the engine, I have a L550  Limbach engine, it has 50 horsepower at 7500 rpm, a 32x25 propeller, weight 18 kg. I'm interested in the opinion of experienced people: is this engine suitable without a gearbox or is a gearbox still needed?

here's the engine

Hi Yuri. It has to have a gearbox or a belt reduction drive. Rotax is 6800 rpm and we run about a 2.5 to 1 ratio. Which means we are lowering the prop rpm by more than half the engine rpm.

its a target drone engine.they are usually used direct drive.it produces 50hp @7500 rpm doesnt mean it can swing the 32x25 prop at that rpm.at that rpm the calculator says this particular prop requires 75 hp.even if the engine gives 6500 rpm on that prop the calc says 49 hp required and the static thrust to be only 120lbs. there are various prop thrust calculators online.one of them says  38hp required to swing a 44x20 prop at 4200rpm.the static thrust being produced as 180lbs which looks a bit plausible.maybe you could contact someone with weedhopper ultralight that used a chotia 460cc 2 stroke engine.that would give some idea about the prop required.the chotia had enough performance to fly the weedhopper.another ultralight that used direct drive engine was Zenair zipper.it used PUL 425 2 stroke engine.Or maybe you can get a performance chart for this engine from Limbach ??wishing you best of luck.

I was thinking about installing a 1/2 or 1/2.5 belt reducer on it

  ..making the choice of right propeller for the direct drive version is difficult and may take a few cut and try efforts.the reduction drive option looks straight forward particularly if you install the rotax b gearbox.looks like there are 4 threaded holes at the front on which an adapter plate can be mounted.you could also install an ACE aviation drive with a custom adapter plate.

A 5 feet diameter prop's tips will go supersonic at about 4200 rpm at sea level.
As the plane climbs higher, tips will go supersonic even slightly earlier, may be at 4100 rpm.

Speed of sound at sea level is 1100 feet / sec.
In one revolution a 5 feet dia prop's tips travel about 15.7 feet.
That is when airplane is not moving forward.
In actual flight the tip travels more than 15.7 feet in 1 rev.
So at about 70 revs per second your prop tips are at speed of sound.

So 70x60 rpm (4200 rpm or slightly less - prop rpm that is, not engine rpm) is limiting rpm for a 5 feet diameter  prop.
Unless of course you have a prop specifically designed for tips to fly supersonic.
Ordinary props will just shatter when tip goes supersonic ( due to shock waves )

That is just the theory. Maybe GA pilots can confirm whether those
cute aerobatic prop planes do actually fly tip-supersonic.

So I feel, a reduction drive for a 6500 rpm engine on a Mini-Max is very much
a necessity, if prop diameter is anything more than about 3 feet.