RV-4 and 6 questions...

[Cook11]

Are the RV-4 & 6's landing gear rough field capable? Can limited aerobatics be performed in either with a passenger on board? Thanks!

[Sid Lambert]

Define rough. Can handle about what most certified aircraft like 172 or Bonanza can handle.

Define limited. The -4 and -6 have a lower aerobatic max weight than the 7 or 8. I would never do any slowish type aerobatics in a -4 with a passenger because of CG. Of course it depends on how the plane was built (actual limits) and registered (legal limits).

Some videos on youtube of guys doing pretty much everything in a -4 with 2 people on board. Better pilots than I am.

[rv7charlie]

Definitely suitable for any surface a reasonable pilot would call a runway, but I wouldn't personally land in an unchecked area because the gear isn't designed to take abuse from rocks, holes, etc like you see in bush flying videos.

I do positive G acro in my -4 with passengers all the time, but you do have to observe the lower gross weight limits (or adjust G limits) for acro, and CG can become an issue fairly quickly in a -4. I never do negative stuff, and never do snaps. Even a light -4 will weigh around 1100 lbs with full fuel, and Van's recommended gross weight for 6 G's is 1300 lbs. You can lose fuel, lose cockpit weight, lose G margin (<3 G loops can be done in a -4), or pretend Van was too conservative with his limits. (Please don't do the last without *informed* consent from your passenger.)

[Snowflake]

The CG on the -6 goes back slower than on the -4 when you add a passenger, so you aren't as likely to hit the CG limit (although with reduced fuel to make it under the aerobatic gross, it does catch up).

There has been discussion here on VAF that Van once said that fuel weight isn't considered in Aerobatic gross due to the fact that it's carried in the wings... Nobody has it in writing, of course...

[rbibb]

Originally Posted by Snowflake

There has been discussion here on VAF that Van once said that fuel weight isn't considered in Aerobatic gross due to the fact that it's carried in the wings... Nobody has it in writing, of course...

I know nothing of such discussion but it seems illogical at best. The principal structural item in an RV is the wing spar and the design spec of +6/-3 G (+9 ultimate) is the design spec the spar is designed to (at aerobatic gross weight).

Why would a designed specify such weight and not include the fuel? The fact the fuel is carried in the wings has nothing to do with it. Weight (or more precisely mass) that the aircraft has to produce a lifting force equal to the multiplier of acceleration due to gravity times such mass (primarily by the wings) doesn't care whether you counted it or not.

Now I suppose Van's could spec the strength spec not including fuel, but if he did so he would be highly negligent not to clarify that fact in a manner such that the owner knew that to be true.

Long way of saying you can do two person aerobatics in a RV-4 but you better not have much gas and be two rather small people if you want to pull anywhere near the max specified G rating (intentionally or in recovering from a botched maneuver) and land safely afterwards.

I do "gentle" rolls as a demo but that is about it.

[Ironflight]

If you want to reasearch Van's position on gross weight and fuel for aerobatics, this thread is a good place to start:

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

It doesn't tell the whole story, but might give you more to think about on the topic, and lead to more information.

[Snowflake]

Originally Posted by rbibb

Why would a designed specify such weight and not include the fuel? The fact the fuel is carried in the wings has nothing to do with it. Weight (or more precisely mass) that the aircraft has to produce a lifting force equal to the multiplier of acceleration due to gravity times such mass (primarily by the wings) doesn't care whether you counted it or not.

Simple explanation:

The usual way to think of loading an airplane is that all of the weight goes in the fuselage, and the wings pick it all up in flight (or in high-G manoeuvers). This stresses the joint at the wing root commensurate with gross weight, as you say.

However, when you load weight (in our case, fuel) outboard of the wing root, in g-loading manoeuvers the fuel *unloads* the wing root. So for least stress on the wing, flying aerobatics with more fuel is actually easier on the airplane. Counterintuitive, I know.

The catch is that with only the inboard wing sections carrying fuel, but the outboard sections empty, you get a stress concentration on the spar at the outboard end of the fuel tank. That's the part where it falls down for me.

[pierre smith]

Originally Posted by Snowflake

The catch is that with only the inboard wing sections carrying fuel, but the outboard sections empty, you get a stress concentration on the spar at the outboard end of the fuel tank. That's the part where it falls down for me.

The RV-8 that sheared its wings did so exactly there...the end of the tanks, IIRC.

Best,

[Russ McCutcheon]

Originally Posted by pierre smith

The RV-8 that sheared its wings did so exactly there...the end of the tanks, IIRC.

Best,

To be fair, that wing was analyzed and it was said that the failure was caused by a 9+G pull with two up.

[SportAvServ]

You can break anything if you try hard enough, google Partinavia P68, a certified plane being used in an airshow. There is a reason there are limits.