G Load Limits

[Scott Hersha]

Here's a question for the group about G load limits on our airplanes. I currently have an RV4 with a max gross weight of 1500#. The aerobatic gross weight is 1375. I believe that at 1375# my design limits are +6/-3G's. My question is - are G load limits v.s. gross weight linear? If so, the design load limits at the design gross weight (1500 pounds) would be +5.5/-2.75. What I would like to do is increase my gross weight to 1600# to allow me to take a human for a ride in the back seat (v.s. a stuffed animal). If I could raise my gross weight to 1600, that would fill my requirements, and the linear calculation on G-limits would be +5.15625/-2.578. I don't know if this is correct, or if the structural G limits are linear. I understand that testing needs to be done at the increased gross weight (1600#), to determine controllability, stall characteristics, and performance calculations - entering phase I again to accomplish this. I also know that this has been done before, but I don't know the limits that were declared. I would be happy to declare Utility Category limits (+4.4/-1.79 G's) anytime I potentially exceed my design aerobatic gross weight limits if I knew that increasing my gross weight (while lowering the G limits) doesn't violate my limitations, and any insurance coverage requirements. Like I said, I know this has been done before - thoughts?

[jrs14855]

You should research the flights made in the RV4 by Jon Johanson. Since Australia did not allow modification of the airplane without approval from Vans, Jon initially got approval for flights at 136% of approved gross weight.
I think that Jon's later flights were at even higher weights.

[sblack]

Yes it is linear.

[Ed_Wischmeyer]

G loads are indeed linear with weight (primary structure, that is), but that's only half the story. The rest of the story is c.g. envelope. Years ago, there were a number of folks who discovered the hard way in RV-4s that an aft c.g. could drastically change the handling qualities for aerobatics. In my RV-4, the same stick force that would give 2Gs solo would give 5Gs with somebody in the back seat. IIRC, the aft c.g. limit was 31% MAC, and I thought that limit was too generous -- I never liked flying with the c.g. back there.

The RV-4 philosophy widely accepted back then was that the RV-4 was a single seater that could carry a second person on occasion, and the handling was favorably compared to the RV-3. Part of the motivation for the RV-8 was to have a more usable two seater.

There are also some airplanes where the aft c.g. limit changes with gross weight, but I don't recall the details.

Bottom line -- don't ask too much of the RV-4. It's great for what it was designed for, but if you go beyond that, you might be applying for a Darwin award.

[Deweyclawson]

The 1973 Cessna 172 is certified in the Utility Cat at 2000lbs and 4.4Gs. In the Normal Cat at 2300lbs and 3.8Gs. This is a linear relationship. The FAA has, on at least one occasion, approved a similar gross weight increase for a Cessna 140 that was only certified in the Utility Cat. In that case the C-140 had a bigger engine installed, 125HP vs. the original 85HP.

Your Max Utility Cat gross weight would be 1875lbs. 6/4.4x1375=1875

In addition to just the structural weight, consider the t/o performance. Can you get airborne at the higher weight?
Since all that weight is in the back, is the CG within limits?
How about the landing gear? Will it take the higher taxi, t/o and landing smashes?

[ChiefPilot]

I won't disagree with the group's logic and instead, I'll add something to consider.

If the weight addition is in fact linear, then you've done the math correctly by starting with the aerobatic G loading and extrapolating from there. But the weight addition won't be linear - it will be concentrated in the fuselage...in the passenger seat, to be exact.

What effect would this have on the bending moment at the wing root? The extra mass is no longer linearly distributed along the span of the wing (or along the length of the fuselage), which would imply that the bending moment at the wing root, if linear, has some coeffcient on it which probably results in a G-loading less than achieved by linear extrapolation.

[Snowflake]

I was going to add what Brad said. The G loading is linear, but only if you add the additional load uniformly to the airframe. Since most passengers won't permit themselves to be blended into a paste that can be so uniformly applied, they have to concentrate their mass into the back seat.

If you want to put a 200lb passenger in the back seat, that'll be a 1200lb load at 6g, vs. an 800lb load at 4g down in utility category. Can the rear seat take the additional 400lb?

[n82rb]

i have no intention of arguing against the limits that van put on the aircraft. he knows what he is doing and there are valid reasons for it.

upping gross weight has been discussed here in depth. that said, my 4 was licensed with an higher gross weight. only to keep a fed from tagging me for being a few pounds over.

I NEVER exceed the aerobatic weight that van has set for the aircraft. flying over gross straight and level is one thing, pulling g is another thing.

cg limits are also something that should NEVER be exceeded. they are set where they are for a reason and flying outside of limits can be fatal.

as we all know, the 4 flies completely different with a person in the back, let alone a big person.

bob burns
RV-4 N82RB

[sblack]

Originally Posted by Ed_Wischmeyer

G loads are indeed linear with weight (primary structure, that is), but that's only half the story. The rest of the story is c.g. envelope. Years ago, there were a number of folks who discovered the hard way in RV-4s that an aft c.g. could drastically change the handling qualities for aerobatics. In my RV-4, the same stick force that would give 2Gs solo would give 5Gs with somebody in the back seat. IIRC, the aft c.g. limit was 31% MAC, and I thought that limit was too generous -- I never liked flying with the c.g. back there.

.

this is all true, but this is not the question that was asked. I don't think that the effect of CG would be any different at 1600 lbs vs 1500 lbs. It is certainly true that the elevator per G is much less at aft cg. But I think your post might imply that this is related to upping the gross weight and it is not.

The stall characteristics and handling are not so much a function of weight, but are obviously very much a function of CG as mentioned above. And the 4, because it is tandem, tends to get very light on the stick with a passenger of any significant girth in the back seat. This is worse if you have a wood prop and a light, simple airplane (bigger engines, fancier avionics tend to move the CG forward - your wallet must be aft of the CG! There can be reversals in the static longitudinal stability at low speed i.e. the stick force required to slow down can go to zero, which is very un-nerving. The thing that will be affected most at the heavier weight is takeoff performance and climb, although the RVs tend to have great performance so you would still be way ahead of your average spam can although you would need more runway than a lighter 4. Top speed has very little to do with weight - it is dominated by drag - gaps, stuff sticking out in the breeze, prop pitch, cooling drag etc.

I believe that the resulting G limits at 1600 lbs are quite acceptable for sport flying. I would certainly never get near them. But that is a decision that everyone has to make for themselves and they have to respect those limits. But you would still have more margin than your average certified airplane (C172 etc)

[Kevin Horton]

Also keep in mind that the loads on the landing gear and associated structure will increase with gross weight. As I understand it, RV-4s are known for buckled firewalls around where the engine mount lower attachment points are, likely due to loads from the landing gear. If you increase the gross weight, it would be highly recommended to only operate from smooth, hard surface strips when heavier than Van's recommended gross weight, and to not operate at night or in high crosswinds. Inspect the engine mount, landing gear legs and firewall regularly.