SonexBuilders.net

Greetings,

I'm planning a 912UL (80HP) installation on a Onex. What's the largest diameter prop that will typically fit on the plane?

Thanks,
Rusty

Hi Rusty,

I assume you have read "14 CFR 25.925 - Propeller clearance".

If not, here is an excerpt:

§ 25.925 Propeller clearance.
Unless smaller clearances are substantiated, propeller clearances with the airplane at maximum weight, with the most adverse center of gravity, and with the propeller in the most adverse pitch position, may not be less than the following:

(a) Ground clearance. There must be a clearance of at least seven inches (for each airplane with nose wheel landing gear) or nine inches (for each airplane with tail wheel landing gear) between each propeller and the ground with the landing gear statically deflected and in the level takeoff, or taxiing attitude, whichever is most critical. In addition, there must be positive clearance between the propeller and the ground when in the level takeoff attitude with the critical tire(s) completely deflated and the corresponding landing gear strut bottomed.

That should give you a good starting point.

Sonex LLC recommends a 54" prop. That's 27" from spinner to tip. I suspect if you had an AeroVee mounted and you propped the tail up for a level attitude, you would measure approximately 34" from spinner to the ground (with tires inflated). Perhaps one of you guys with a flying Onex could verify this. So, empty would give you about 7" clearance with the plane empty and tires inflated. Really no room left to play with. And as you have evidently discovered, 54" is cutting it pretty thin even with an 80 HP Rotax.

I went through the same exercise a long time ago and convinced myself that I simply couldn't make a Rotax, or for that matter any other geared engine, work with the standard landing gear. I think some folks are trying it with a tri-gear with the nose wheel set up to taxi nose high.

Good Luck,

Wes

Part 25 is for transport category aircraft, we are building experimental aircraft! I am planning on a 6" clearance for my Onex tail dragger. I need as much prop as possible for my turbo charged engine.

Hi Michael,

I only suggested the CFR as a starting point and highlighted that a conventional gear Onex is already well below that "recommendation" while using the Sonex recommended prop. Of course you can do whatever you want as long as it is deemed airworthy. Not trying to steal your freedom.

Questions:

Why do you think you need a larger diameter prop for a turbo revmaster?
What diameter prop do you think will give you 6 inches of ground clearance?

Seems that worst case would be a flat tire that sent the airplane into the boonies. Maybe not with a tail dragger though.

Wes

Thanks for the comments. If someone could measure the distance from the prop shaft to the ground on a complete Onex, that would be good information. I've cut prop clearance pretty short on some previous planes, but I'll try not to do that again.

Longer Prop...?

Something to keep in mind is "tip speed". If you spin it too fast the tip will go "sonic".

I gained some prop clearance with my 5:00x5 upgraded tires...….however I also have the 6" tail wheel so maybe the gain was minimal.

I am using a Prince P tip 54x48 which will turn up 3530 WOT at 3500 ft...IAS..160 mph. I have the Hummel 2400 85 hp engine. I have not measured the spinner to ground clearance (level attitude)….next time I get to the hanger I will measure it. It's still winter here in Canada.

Gordon...Hummel 2400....Onex.

Gordon wrote:Something to keep in mind is "tip speed". If you spin it too fast the tip will go "sonic".

I just found this calculator: https://warpdriveprops.com/propspd2.html

I plugged in some numbers. At the stated max RPM of the Aerovee at 4000 RPM, it appears that 59" diameter is about where you'll encounter the "danger zone" of 92% the speed of sound at 20 degrees C, and 57" diameter at 0 degrees C. The standard 54" prop hits that mark at -30 degrees C.

Granted that we rarely, if ever, actually see 4000RPM, if you did increase your prop diameter you may also want to consider lowering your redline RPM.

Tip speed is no issue, since the 912 is geared. I have a 68" prop on it now, but that's a different plane of course. I could swear I saw someone selling a 60" prop that they said came off a Sonex with a 912, but maybe it wasn't a Onex.

Another way to decrease prop diameter is to add another blade (3-blade), though that comes with its own tradeoffs in terms of cost, efficiency, weight, and experimentation.

Hi Guys,

13brv3 wrote:Tip speed is no issue, since the 912 is geared. I have a 68" prop on it now, but that's a different plane of course. I could swear I saw someone selling a 60" prop that they said came off a Sonex with a 912, but maybe it wasn't a Onex.

Like Rusty points out, tip speed isn't the issue. Rusty's problem is on the other end of the spectrum. The problem is getting the proper pitch (in inches) to achieve the airspeed expected with 80 HP. At the low prop RPM of the Rotax, using a 54" prop, the prop blades will come close (or worse) to exceeding their critical angle of attack during the takeoff roll.

During the early stages of my build I considered every engine I could think of. That included Rotax, AeroMomentum, Viking, ... I proceeded as if one of those engines could be used. And like I said, I couldn't convince myself that a geared engine, especially the higher HP ones, could be used on a Onex with the supplied landing gear.

So ...

A pic of the landing gear. Can't see it in the low res pic but the gear is 4" taller than standard.



And here is a pic of the tip of the spinner with the longerons level.



The aircraft was brought back to the proper attitude with a 6" tail wheel and by raising the hole in the tail wheel bracket enough that the bolt through the tail spring cleared the lower longeron.

I can't recommend this setup, it hasn't flown (or landed :-)). But, if it all works out I should be able to swap to any engine I want in the future without worrying too much about prop clearance. I should also point out that the taller gear is heavier than the standard gear and I haven't performed a W & B yet.

We'll See,

Wes