RV-7 Aerobatics Centre of Gravity Problem

I have an RV-7 with an O-360 and Sensenich prop. The Weight & Balance details are:

Metric US
Empty weight 498.8 kg 1,099.9lb
Empty C of G 206.3 cm 81.2 in

I weight 80kg / 176lb so solo aerobatics aren't a problem. However as soon as I take a pax weighing more than about 65kg / 143lb, the centre of gravity moves beyond the aft limit for aerobatics. If I load more fuel, the C of G moves slightly forward but I am then over the 1600lb max aerobatics limit.

I did see the email from Van himself saying that fuel isn't included in aerobatics weight but it seems quite controversial.

I am not planning high G aerobatics and will be able to stay well within the "Utility" limits of +4.4 / -1.75. If I were to take say a 100kg / 220lb passenger, which option would be safest:

Weight C of G
1/3 tanks 1577lb 85.2 in (aerobatic max = 84.5in)
Full tanks 1745lb 84.7in

My guess is 1/3 tanks but how problematic is being 0.7in behind the aerobatic C of G limit?

I am sure I'm not the first to have this problem. Any ideas?

Stuart Aberdein

Aerobatic zero fuel weight?

Stuart,
I am not familiar with the letter from Van you are refering to. Where can I find it?

Scott A. Jordan
N733JJ

Hi Stuart!

I don't know that I would want to fly acro with any RV that is out the back of the CG box - the controls tend to get VERY light, and that is an easy way to get yourself into an overstress condition. Obviously, if you are very skilled and familiar with this feel, then my thoughts may not apply to you, so please don't take offense - but personally, I don't think I'd want to operate outside either the weight or CG limits without flight testing.

The interesting thing, of course, is that these are experimental airplane,s and we are allowed (and for phase 1, encouraged) to do flight testing, so you can always sneak up on the CG thing. You may not really want to go hunting for ultimate structural loads by testing at heavy weights....

Paul

My lovely and thoughtful wife gave me Stein?s lightweight tail wheel for Christmas. Being one pound lighter than the stock tailwheel my CG moved forward approx 2/10?. While not a silver bullet, every little bit helps and there is no easier way to gain that much forward CG on a completed plane. Good luck.

Hi Stuart...

The eMail(s)/discussion re fuel weight not counting re Aeros Weight, are, as you say, "controversial" It is based partly on where the wing would fail under excess 'g'. In simple terms, if it is at the root, then you could discount fuel, since the fuel is outboard and relieving the bending moment. If the wing fails outboard of the fuel, it should count. Given a tapered spar, and the early RV-8 accident saw the wings separate outboard of the tanks, I would not rush to discount fuel

However, you can reasonably limit 'g' in proportion to excess weight e.g. 6g at 1600lb equates to 5.3g at 1800lb. Not legal, but fairly sensible.

None of the above translates to aeros CG limits I might have flown our RV-8 near the aft limit, maybe beyond At that point, it is a "delight" to fly, but not for someone inexperienced at aerobatics... you can need to "push" to prevent 'g' building up, and as above, very easy to overstress, potentially to the point of structural failure. From teaching aerobatics, this should not be discounted - you don't want to say "I can manage a loop in ideal conditions" at that CG - you need to say "am I good enough that when I scr*w up a manouevrve, with a poor horizon, am screaming downwards approaching VNE and maybe cloud, I will recover promptly, correctly, and able to avoid overstressing".

Aircraft are designed to need a good "pull" to overstress in the above situation, with adrenalin adding to the pull. By flying near, let alone beyond the aft limit, you remove that i.e. I am not saying the world will end 0.1" aft of the limit - even 0.1" fwd of it is more hazardous than 1" fwd of it.

My personal viewpoint would be that if you want to fly aerobatics, and are not ex-military / competition aerobatics / extensive aerobatics trained, I would seriously look at finding a means of moving the CG. Battery? I am not familiar with the RV-7 layout / options/

Andy
RV-8 G-HILZ

7A better?

Is the 7A less prone to this? I'm planning on building a 7A. I weigh 210 lbs and would like to be able to do light aerobatics with a passenger. I imagine with the little wheel up-front that it would make a difference.

T.

NO!

Av8torTom said:

Is the 7A less prone to this? I'm planning on building a 7A. I weigh 210 lbs and would like to be able to do light aerobatics with a passenger. I imagine with the little wheel up-front that it would make a difference.
T.

Click to expand...

Andy's comments are common to all aircraft with an aft CG condition.

Yes, but...

My question was - is someone who builds a 7A less likely to end-up with an airplane that is easy to load to an aft CG condition?

Av8torTom said:

My question was - is someone who builds a 7A less likely to end-up with an airplane that is easy to load to an aft CG condition?

Click to expand...

(For the benefit of those still building)

Generally 7s tend to get built with the empty weight a little aft of ideal. I have a 7A (with a heavy constant speed prop) and my CG range is fine. Even with that weight up front, it would be even better if I had put my ELT & strobe pwr supply forward of the baggage bulkhead.

Av8torTom said:

My question was - is someone who builds a 7A less likely to end-up with an airplane that is easy to load to an aft CG condition?

Click to expand...

Sorry, I misunderstood your question. The answer would depend entirely on how you intend to equip you airplane. There are so many factors that contribute to this situation, that only you can decide. If you are considering a wood prop, light weight starter, light weight alternator, etc. you will be looking at a pretty critical aft C/G limit.

StuartAberdein said:

I am not planning high G aerobatics and will be able to stay well within the "Utility" limits of +4.4 / -1.75.

Click to expand...

These are famous last words.
As a builder and pilot of a very potent One Design, there have been a few times that I did not plan any high G aerobatics. Sure glad the plane is stressed for 10gs positive and negative.
Be careful Stuart and allow yourself a WIDE margin for error
Regards
Jack
RV9A
One Design

Thanks

Kevin... sure would hate to end-up with an airplane that couldn't do what I expected it to do.

T.

Thanks for all the advice - much appreciated.

I have very little aerobatics experience (and only in gliders) but I would like to start aerobatics training with a friend of mine. He has a lot of military (jet & prop) aerobatics experience as well as in RVs but he is uncontactable for a while and I haven't been able to run the C or G problem past him.

I appreciate the comments about the dangers of pulling too hard an aft C of G as well as the warnings that "light" aeros can go wrong and end up as high G ones. All very wise words.

Gidday Stuart, I have exactly the same problem - CoG is fine for solo aeros, but when I put anyone of adult size in the seat next to me the CoG goes out the back of the prescribed aeros envelope.

Having said that, in doing my type rating my instructor and I did the usual gentlemanly aeros (loops, rolls, barrel rolls, stall turn/hammer heads, etc) without any problems, and spinning wasn't a problem in either direction. Naturally we didn't let the spin develop past a couple of turns so don't know how long it would take to flatten out and become a problem.

Along with that, spinning and more aggressive manoevres prob'ly aren't the sort of thing we want to do with inexperienced pax....I believe they tend to show us what they had for breakfast

Ditto the other comments re good training, experience and currency, not to mention if we do want to fly the a/c outside the (Vans-) defined envelope, it's our responsibility to thoroughly test the a/c in those scenarios before taking pax - prob'ly not for the inexperienced or faint-hearted....

harmonic balancer

I had similar issues with my 7 with similar setup (0-360 and sensi prop), also impacts how much baggage you can carry with min fuel. I ended up putting a landoll harmonic balancer ring on the front which adds 12+lbs right behind the prop. Works very well (smooth at cruise) and moves the CG forward enough to carry full baggage without going aft of CG limit. I did not do the W&B calcs for aero but assume it would improve the situation.
I can send you my spreadsheet if you like.

7A acro envelope

I have a recently-completed 7A with a fixed pitch composite 3-blade prop. My airplane partner and I are 150 lb each. Figuring 30 gallons of fuel at takeoff, zero baggage and a 15 lb parachute each, we'd be barely within the acro CG limit at takeoff yet well under 1600 lb. Our compact Odyssey battery is on the forward side of the firewall.

I own a 6 and weigh 200 ish.

It seems like a catch 22. There's no replacement for acro training in type, but not possible in my RV 6. So we have to learn in other types and carefully practice in our own airplanes later by ourselves.

I love my 6 but man I wish the wt / bal Acro envelope was bigger. For my own competency, I trained in a rented super D with an acro instuctor and later became proficient in my own 6. Loving acro and teaching, I would love to share this with others but the parachute rules and tight wt/bal don't make this possible unless (like others have eluded to), staying with in the utility envelope which is hard to do with a pax.

Av8torTom said:

My question was - is someone who builds a 7A less likely to end-up with an airplane that is easy to load to an aft CG condition?

Click to expand...

Having read about aft CG issues with the -7/-7A, and my plan to use a light weight Catto prop (12 lbs), I opted to install a 20 lb crush plate in front of the prop. No CG issues with 2 up and a light enough fuel load for acro but I haven't done much yet.

Also planning for an EarthX battery to replace the PC-680 on the fwd firewall to gain back 11 lbs of useful load.