Aerobatic Gross Weight

aviationgeek84 · #1 09-02-2009, 08:17 AM

Hi All,

I'm sure this has been discussed, but a search of the forums didn't bring up what I was looking for. I think I remember hearing of an article that Van mentioned aerobatic gross weight did not include fuel on board.. can anyone confirm/deny that or point me in the right direction?

I don't care either way whether it is or isn't, but was just running some numbers.

Thanks!

Bill Wightman · #2 09-02-2009, 08:35 AM

Hello Casey,

I haven't heard of the article you mentioned, but I can explain why fuel would be omitted when running structural design analysis. The short explanation is that fuel, when carried in the wings, relieves bending and shear loads on the spar when under load. This is the case for symmetric maneuvers only, but those constitute the bulk of the analysis.

So, fuel on board is one variable that gets set to "worst case" when designing primary structure and since we carry fuel in the wings, that would be zero fuel for the RV aircraft. Other factors such as gross weight, CG location, dynamic pressure (airspeed), load factor, rolling loads, gust loads, etc are also sampled across a range chosen to include worst case conditions.

kevinh · #3 09-02-2009, 08:40 AM

I think this should answer your question:

http://www.vansairforce.com/communit...ead.php?t=4595

http://www.vansairforce.com/communit...4684#post84684

aviationgeek84 · #4 09-02-2009, 08:46 AM

Bill & Kevin,

Thanks for the responses. I will do some reading there. Again, not that I need to do aerobatics with full fuel.. I was just curious.

Thanks!

Bill Wightman · #5 09-02-2009, 08:54 AM

Casey - I just read through the linked threads.

One thing I forgot to say in my first reply to you is indeed important: fuel in the wings only relieves stress on that portion of wing *inboard* of where the fuel is carried. Since we carry fuel in wing root tanks, our fuel does increase structural stress on the rest of the wing outboard of the tank.

A detailed analysis of the fuel's mass would include integrating across the tank's span (spreadsheet strip integration) to find exactly how the load gets distributed. In my first reply, remember the designer uses the unrealistic combination of gross weight AND zero fuel to figure the structural needs of the airplane.

Bottom line: we need to include fuel weight in our gross weight for all operations of our aircraft, aerobatics (especially) included.

aviationgeek84 · #6 09-02-2009, 03:53 PM

Hi Bill,

Yes - for sure include the fuel weight in with the aerobatic gross weight... I was just curious.

Andy Hill · #7 09-02-2009, 04:06 PM

As Bill says above, it is not a "simple answer" (what ever is!).

If the spar was constant X-Section, then ignoring fuel weight for Aeros would likely hold true. But that is not the case... and I believe the Van's RV-8 fatal accident, assessed to be an overstress, resulted in the wings failing outboard of the tanks. So in that case, clearly the Fuel Weight was equivalent to Fuselage Weight in terms of what failed i.e. Fuel Weight needs to be included.

RV-3 is somewhat different, since it was (originally) designed with a fuselage tank. But then, of course, with the RV-3B the wing has been re-designed... so complicating the question again

Andy