Aerobatic Gross Weight

I'm a pretty big guy, I go around 290. I'd like to get some spin training, upset recovery, and basic aerobatic instruction. I'd also really like to do it in my plane. The problem is if I put any fuel in it, and a normal guy in the back seat, with chutes, we blow by the aerobatic gross weight pretty quickly.

While I know there will be a lot of folks that will say "are you crazy? Do that and you're sure to die!" or somethings along those lines, I'd like to know all ya'all's thoughts on this.

The aerobatic gross on the 8 is 1600 lbs. and that's good for 6 Gs or 9600 lbs.
So if we're at 1750 lbs and limited the aerobatics to maneuvers that would not exceed 5.486 Gs (9600 lbs again), are we good? Seems to make sense to me, and keeping the maneuvers mild I would think would pretty easily stay within that limit. Obviously we'd have to be concerned with CG and keep that as far forward as possible.

Is my thinking all messed up? Am I missing something?

Veetail88 said:

Am I missing something?

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That you cannot necessarily expect to limit the G to a certain number in a training environment. Once you have some skills, you can do all the basic maneuvers at no more than 3.5G all day long, but newbies receiving training are very good at screwing up things and utilizing the full 6G limit of an aerobatic airplane. You'd be cutting your margins in a situation where you need maximum margin. Choose your training aircraft and instructor carefully.

+1
You almost certainly will fall out of a maneuver, and recovery depends on how good your instructor is, and whether one of you panics (could be the instructor ).

Not saying it can't be done, but make sure both you & the instructor go in with eyes wide open & fully informed.

(BTW, you might want to fix the typo; that would be a really bad mistake if it were the actual calculation...)

Charlie

Good catch. 8 Gs plus, not so good. Corrected thanks.

Can you lighten the plane?....

Hey Jesse,

So, I figure 290 (you) + 180 (instructor) + 30 (chutes) + 100 for fuel (8 per side) = 600 lbs.

That only leaves 1000 for the airplane. A plane can be built at that weight, or even lighter (my 8A was 975), but an already built plane is a different thing entirely. Still, if your plane is 1150, you can probably cut some weight. That will at least improve your margin. Here's a post where I listed what I did to keep mine light, you can use a some of the ideas to get you closer to your target weight.

http://www.vansairforce.com/community/showthread.php?t=109916&highlight=975

Lance

Veetail88 said:

.....I'd like to get some spin training, ..... Obviously we'd have to be concerned with CG and keep that as far forward as possible.

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Spin training does not necessarily require an airplane with aerobatic g limits. My initial training in 1974 was in a non aerobatic 150, spins were part of the curriculum and fully approved for the airplane.

I spin my Rocket regularly, and while it is rated for +6, -3, I won't spin it with a passenger. Puts the CG too far back, and as far as I am aware nobody has spun the F1 EVO with a rear loaded CG - some probability of flat spinning and a question of whether it would recover.

Suggest you confirm the -8 will recover from a spin when rear loaded - Like me you probably don't want to be the first to find out if it works or not.

Why not get your initial aerobatic training an another aircraft? I know you said you'd like to do it in the 8, but here it might not be appropriate.

Once you've learned some things, you can practice in your own plane, solo.

Dave

David Paule said:

Why not get your initial aerobatic training an another aircraft? I know you said you'd like to do it in the 8, but here it might not be appropriate.

Once you've learned some things, you can practice in your own plane, solo.

Dave

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+1
Spin or unusual attitude recovery training is no time to be experimenting with aft CG or heavy weight issues. Because of the commercial nature of flight training programs they cannot use experimental aircraft, however the basics of unusual attitude recovery are similar to most all conventional light aircraft. Get some training in a certified Extra, Decathlon, or Pitts then practice the procedures in your RV with confidence.

Go to IAC.org to find an aerobatic instructor near you.

Most aerobatics can be done in the 3-4G range. But that is if you know what you are doing. The next issue is the RV series have very powerful elevators. It would be very easy to exceed the G limit with an RV.

I did my 1st loop in my RV6 yesterday and I was deeply surprised by the G onset at the bottom of the loop. And while I am no expert, I placed 2nd in the Region in IAC Sportsman last year. I could see someone grabbing too much elevator at the bottom and blasting through G limit. It is a natural reaction.

And the instructor is not going to be able to prevent you from over pulling.

I'd second getting instruction in a higher G rated plane with a less sensitive elevator.

ssmdive said:

Most aerobatics can be done in the 3-4G range. But that is if you know what you are doing. The next issue is the RV series have very powerful elevators. It would be very easy to exceed the G limit with an RV.

I did my 1st loop in my RV6 yesterday and I was deeply surprised by the G onset at the bottom of the loop. And while I am no expert, I placed 2nd in the Region in IAC Sportsman last year. I could see someone grabbing too much elevator at the bottom and blasting through G limit. It is a natural reaction.

And the instructor is not going to be able to prevent you from over pulling.

I'd second getting instruction in a higher G rated plane with a less sensitive elevator.

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A plane that is overly sensitive in pitch control is likely have a center of gravity too far back. It might be a good idea to check your CG position. If it is in the aft third of the aerobatic CG range it may be too sensitive for some pilots. A higher fuel load can often correct an aft CG position and ease up on the pitch sensitivity.

Veetail88 said:

The aerobatic gross on the 8 is 1600 lbs. and that's good for 6 Gs or 9600 lbs.
So if we're at 1750 lbs and limited the aerobatics to maneuvers that would not exceed 5.486 Gs (9600 lbs again), are we good? Seems to make sense to me, and keeping the maneuvers mild I would think would pretty easily stay within that limit.

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Revisiting the load factor question, I think the notion of taking acro gross multiplied by acro load factor the dividing it by proposed weight is misleading and ignores other factors, specifically the bending moment at the wing attach points.

Consider an RV-8 that wants to fly at 1750lbs. Using the above notion, 1600lbs * 6G / 1750lbs would suggest that you'd be safe up to 5.48G. But why are the 1600lbs and 6G figures used while the 1800lbs and 4.4g figures associated with the maximum weight ignored?

If you instead use 1800lbs and 4.4g, you get a different value. 1800lbs * 4.4G / 1750 would suggest you'd be safe up to 4.66G. This is quite a difference from 4.48G but is likely overly conservative, since 1800*4.4/1600 suggests a maximum of 4.95G limit when operating at aerobatic gross.

Consider a formula which accounts for both the utility and acro points on the load factor/gross weight graph. This formula looks like this:

Lmax=min(6, 6+(1.6/(Wgross-Wacro)*(Wacro-Wact)), where:

• Lmax = maximum allowable load factor
• Wgross=Maximum gross weight
• Wacro=Acro gross weight
• Wact=Actual gross weight

Using the RV-8 example from above, we find that:
Lmax = min(6, 6+(1.6/(1800-1600) * (1600-1750))) = 4.8G

This figure is significantly less that the 5.48G figure suggested by the simpler calculation. Moreover, the formula, when graphed, agrees with the factory supplied values for aerobatic and utility weights/load factors.

Of course, the safe answer is that above 1600lbs, no more than 4.4G should be put on the aircraft. But if you are wondering what the actual limit is, I would encourage use of the above formula to approximate it rather than the original approach based off aerobatic weight limits alone.

ChiefPilot said:

But why are the 1600lbs and 6G figures used while the 1800lbs and 4.4g figures associated with the maximum weight ignored?

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For no better reason than It didn't even cross my mind to look for max G at max gross! Thank you!

I'm not entirely sure how to proceed. Still weighing and ferreting out all the facts. Great advice from folks here that I can't ignore, on the other hand, I'll have the same weight problems if I find instruction in a Decathlon. Don't know what an Extra will handle, but I'm getting into some costs that are a bit prohibitive for me at the moment.

Thanks.

RV8JD said:

It would also be interesting to see what max pilot weight Van's used in sizing the seat structure, its attachments, and the supporting fuselage structure.

Scott (rvbuilder2002), can you provide any insight?

Thanks,

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I can't recall the design specifics, but as a standard business policy the weight limit for someone to go on a demo flight is 250 lbs.

If the additional weight added between Aerobatic Gross and Utility Gross were spread uniformly across the entire aircraft structure, from wingtip to wingtip and tail to spinner, then the maximum weight vs. g-loading would probably follow a very linear curve between the two points.

The reality is that when you want to carry more weight with you, it usually goes in discrete locations: Passenger seat. Fuel tank. Baggage compartment. What you need to do the math on, is whether those structures were designed to take the increased weight, at the increased G-loading.

Snowflake said:

If the additional weight added between Aerobatic Gross and Utility Gross were spread uniformly across the entire aircraft structure, from wingtip to wingtip and tail to spinner, then the maximum weight vs. g-loading would probably follow a very linear curve between the two points.

The reality is that when you want to carry more weight with you, it usually goes in discrete locations: Passenger seat. Fuel tank. Baggage compartment. What you need to do the math on, is whether those structures were designed to take the increased weight, at the increased G-loading.

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Absolutely - anything else is just an approximation (which I tried, but perhaps failed, to emphasize). These approximations also fail to account for fatigue life, and I suspect you know it better than most others on here (including myself). Still, I was really interested in this stuff a couple of years ago and dug into the information available in texts such as "Strength of Metal Aircraft Elements" as published by the Department of the Navy (link).

Case in point: a cursory structure analysis of my -6A wing would suggest that it potentially could support >6G @ acro gross without permanent deformation (but other parts of the aircraft might/would not - I considered the wing only). Operating it on a regular basis to that limit would however significantly cut into the fatigue life - as you know aluminum, unlike steel, remembers everything:

Because of this, designers often aim not for the point where permanent deformation occurs but rather for something less - 50% of the ultimate tensile strength is a rule of thumb I've seen referenced in multiple places. As can be seen on the graph below, that would be well below the proportional limit - the point where permanent deformation would occur:

What I learned is that there's no free lunch here - even if something didn't bend, the results of (over)stress are still there and will accumulate over time. Moreover, I gained a new appreciation for the subtleties associated with balancing strength and fatigue life.

I'd certainly welcome input and feedback - I still find this stuff fascinating.