[claycookiemonster]

This is way down the road, but I just noted someone's comment that if you use a prop extension, you're unable to do acro? Really?
I read alot of posts that involve prop extensions for looks or cowl fits or CG issues, I guess I'm skeptical that all these builders are going to forgo acro forever.
In this context, what exactly is acro? It can't be the FAA's bank angle restriction. I understand that at some point in extending the prop shaft to foolish lengths, you'd run into problems with instability or gyroscopic issues with sudden G-onset, or sustained high G loads.
Anyone have the real skinny on what is going on here and what the threat is?

 

[N941WR]

I believe I might have been the one who made the comment you are referring to.

With the short Sam James or Van's cowl you need to extend the prop to clear the cowl. With their short cowl or the stock Van's cowl, a builder will need to add an extension to place a fixed pitch prop in the same relative position as if they had installed a constant speed prop.

With the long Sam James cowl the prop needs to be moved further out to clear the longer cowl and this requires a prop extension, even if a constant speed prop is used. If a fixed pitch prop is used with the long cowl, an extra long or second extension is needed.

Acrobatics are not recommended with the long cowl due to the extension. This doesn't mean that people do not perform acrobatics with the long cowl, just that it is not recommended.

 

[deek]

Best to talk to the engine shops that build engines for Lasers, Pitts, Edges, etc. Mattituck used to sponsor and build Leo Loudenslager's engines; perhaps Mahlon will chime in. Lycon would be another good source of empirical data.

The issue is primarily gyroscopic forces imparted on the crankshaft nose bearing from the prop, but there may be other concerns. I have a 9" Sabre extension on one of the EZ's that has seen quite a bit of positive G acro, but the fixed composite prop only weighs 9#

 

[claycookiemonster]

Yea, I've dropped notes to Titan and Whirlwind and Show Planes (it's their RV-8 cowl that I'm salivating over) to see what's what on this. I also understand Van's doesn't approve acro with extensions, could there be a motor mount stress issue here as well?

 

[F1R]

That is why a pitts and extra look like a pug nose bulldog with their props nice and close to the crank flange. If you want to fly unlimited class, hardcore tumbling aerobatics, those machines are purpose made for the job.

RV's not so much... however you can still do plenty of loops, cuban 8's and stay well within +4.5 -1.5

Prop manufactures like Hartzell have a G limits for certified applications.
The air frame has its own limits.

A couple of years ago a pilot of rocket in a sportsman class aero competition practice session made his last radio Tx that his stick broke.

Be smooth and gentle.

 

[sandifer]

IMO, this is a non-issue with lightweight wood/composite props. But even with metal props and extensions, there is a long history of RVs performing lazy loop and roll acro. Most don't do any more than this. Use common sense. The further the prop is mounted from the flange, the longer the lever arm to apply gyroscopic loads to the flange. Gyroscopic stress is proportional to RPM and how fast the prop disc is rotated. Rolls do not rotate the prop disc at all. Loops rotate it very slowly. Spins are done at idle RPM. I would recommend against high power flat spins w/ metal props (though I rather doubt we have anyone doing that in their RV). Hammerheads rotate the prop disc fairly slowly in RVs - especially since most pilots are kicking early, not trying to fly with competition precision. I would definitely recommend against snap rolls with metal props and extensions. Forget about tumbles, torque rolls, and shoulder rolls.

 

[claycookiemonster]

Thanks, so to sum up:

I've gotten several nice replies on and off line to my question. Since someone else may search this topic in the future, I just thought I'd sum up here.
1. First of all, if you're searching for acro limits with a specific engine/prop/extension combination, please do your own due diligence and contact your suppliers directly to get the scoop on your set up. I was only asking generic questions. I haven't even finished my Emp. yet.
2. It seems obvious that extending the prop shaft, and thereby it's moment arm for torque and gyroscopic effects will eventually limit how many total Gs the set up can withstand as well as the speed of G onset, etc. It also seems clear that many guys are doing nice smooth gentlemanly acro in RVs with prop extensions with no ill effect.
3. If you ask the Prop manufacturers or Engine manufacturers or Extension manufacturers about this, they will likely give you a qualified, "yes, but be careful" kind of answer, so, like Sgt. Esterhaus used to say on Hill Street Blues, "Hey, hey, hey! Let's be careful out there."

Thanks to everyone who answered.