RV7/9(A) Design

I have a history question harking back to the initial design of the 7 and 9.

If these airframes have a tendency to become tail heavy as fuel burns, was there ever an airframe built with a more rearward wing, or longer snout to circumvent this issue?

If so, what were the results?

Cheers,

Pat.

What issue?

The CG still remains in the envelope as long as you pay attention to loading.

The CG's move aft in a Cub, Tcraft, Champ, and probably some others as the fuel burns off. That is because the fuel tanks in those airplanes are between the instrument panel and the firewall.

However, with those same planes with wing tanks, the CG's move forward as the fuel drains from the wing tanks.

N941WR said:

The CG's move forward in a Cub, Tcraft, Champ, and probably some others as the fuel burns off. That is because the fuel tanks in those airplanes are between the instrument panel and the firewall.

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Hunh? Doesn't progressively less weight forward move the CG aft?

I concur with airguy. Even after a 4 hour leg following a gross weight takeoff in my -7A, I'll land well within the aft CG limit.

alcladrv said:

Hunh? Doesn't progressively less weight forward move the CG aft?

I concur with airguy. Even after a 4 hour leg following a gross weight takeoff in my -7A, I'll land well within the aft CG limit.

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Sorry, I was thinking backwards and I left off one important line, which I have added above.

I figure if the tendancy is for the CoG to drift rearward there might have been scope to begin with a more forward point from MTOW.

This is not a criticism of the design, I'm simply curious if anyone has any information regarding the initial design rationale.

Our flying club has recently taken ownership of an RV9A for hire. Our members and frankly the greater flying public in our area aren't all RV savy therefore should I be asked why "this aircraft has been designed differently to the XYZ aircraft I usually fly" I'd like to be able to provide some intelligent and interesting context beyond pointing out the fuel tank locations etc.

PM me your email address and I'll send you a W&B Excel sheet I made for a friend's -9A. You will just have to change the weights for each wheel.

You can play with different loadings to see who the CG moves.

OZCleco said:

I have a history question harking back to the initial design of the 7 and 9.

If these airframes have a tendency to become tail heavy as fuel burns, was there ever an airframe built with a more rearward wing, or longer snout to circumvent this issue?

If so, what were the results?

Cheers,

Pat.

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Don't know answer to you question on if anyone ever tried, but as a aircraft configuration designer, I see no need to move the wing as all the typical loadings keep CG inside the allowable box assuming the airplane maintains the max gross weight that Vans has published. RVs are not intended to be a truck to load up the back. Here are my RV-6A typical loadings



To minimize trim drag you want the CG as far aft as possible and still allow manageable stability. It also makes the airplane more maneuverable.

Moving the wing aft has consequences in it also is moving the CG with it as it moves aft. Also, control power of the elevator and rudder will get less and they would potentially need to got larger (and heavier moving the CG further aft). Potential to make the wing bigger but that also increases wetted area and thus drag. Moving the engine forward could help but it is also adding the weight of additional fuselage length and therefore loose performance.

Its a balance and Van has it right if you stay with in the weight limits set by the designer for the aircraft. If you want to carry more baggage then, your right something may want to change, but there is a penalty to do so.

The OP is assuming that the wing location was fixed from the beginning and has never changed - I'll hazard a guess that during the original design process for this airplane, a lot of things were moved a lot of different places to get it "right" and resulted in the current design. That wing spar location may have been moved fore/aft in the design software a dozen times as the rest of the design was fleshed out and operating ranges were explored in the software. There is compromise in an all things aerodynamic - the compromises made by the designer resulted in the current final product.

It would be a wise practice to:

- Determine Passenger & Luggage loading to bring aircraft to Max Aft CofG at Minimum Fuel, always be sure to load less.
- Than add fuel to not exceed Max Gross TO Weight, or less.

No getting in trouble if this is followed.



To the OP's original questions, I can only think of one instance where a Vans model offered differences.
RV-9(A) at some point offered 2 choices of engine mounts, one moved the engine forward about 1.25".

Others- RV-4 kits converted to Harmon Rockets, repositioned fuselage aft with different structure forward to accommodate heavier engines. F-1 Rocket repositioned spar attach (& a lot of other things) for aerodynamic considerations. F-1 Rocket spar position again for a different (EVO) wing.
A number of RV-6, RV-7, & RV-8 builds with bigger engines, fuselage stretches and wing relocates as required for their intended purposes.
I've had the opportunity to fly in 2 different Super 6's and found them superb flying machines.

Thanks all. That's the kind of thought provoking stuff I was after.

I especially like the point made regarding trim drag.

No doubt designing the aircraft to operate at a higher cruise speed was an attribute that Van and co were keen to have over the certified types. It's certainly the main selling point we're able to use to encourage new membership. Time is money after all.

Being new to the airframe I appreciate the point made regarding W&B technique which we can include in our inductions.

I appreciate the input!

Pat.

The answer is RV7/9 (totally different wings) has only one configuration/design. There are no wing and gear location variations, one design. Almost every airplanes from small to large will change CG slightly with fuel burn, unless the tank is right on the CG and has a symmetric shape. Fuel burn CG change on RV's is a small change. Because it is on leading edge and some of the mass is forward CG it will change CG slightly.

Payload (crew, passenger, fuel) changes CG, so it is really a moot point if the fuel tank is right on CG. Fuel burn and CG is a factor on larger planes with swept wings and fuselage or tail fuel.

RV-4 flew like a different beast from solo to passenger and aft baggage

gmcjetpilot said:

Payload (crew, passenger, fuel) changes CG, so it is really a moot point if the fuel tank is right on CG. Fuel burn and CG is a factor on larger planes with swept wings and fuselage or tail fuel.

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Not just big airplanes. A bonanza burns aft quite a bit.

In my Phase 1 testing with the most aft CG, I was still just inside the aft-most CG range of 86.82. The aircraft trimmed up and flew just fine.

Most Aft CG (Pilot, Pax, 5 gals Fuel, Full Bags)
Item---------Weight-----Arm--------Moment
Aircraft-------1,198------------------96,427.75
Fuel (5 gal)------30------80----------2,400
Pilot-------------170------97.48-----16,571.6
Passenger-------170------97.48-----16,571.6
Baggage---------100----126.78-----12,678
Total------------1,668--------------144,648.95
CG: 86.72

I can't be sure, but I suspect that once or twice I might possibly have flown back from Oshkosh with one too many cases of Spotted Cow in the baggage area, possibly moving the CG back to (past?) the limit. Can't be sure: I was precision-estimating the weights and CG to the best of my ability. Still, KELLI GIRL trimmed up and flew beautifully.

I contend there are no problems with the RV-7A as designed.

scrollF4 said:

I can't be sure, but I suspect that once or twice I might possibly have flown back from Oshkosh with one too many cases of Spotted Cow in the baggage area, possibly moving the CG back to (past?) the limit. Can't be sure: I was precision-estimating the weights and CG to the best of my ability. Still, KELLI GIRL trimmed up and flew beautifully.

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Rumor has it that someone I know may, or may not, have committed the same sin on the way TO Oshkosh, and it flew just fine.

Darn pesky rumors...