RV-8 Static Longitudinal Stability

[Kevin Horton]

Originally Posted by tinman

I am getting ready to start shifting my cg aft during my phase 1 test flying and had a question about ballast and arm. My plan was to start securing sand bags to the rear seat, but I was unsure of what the actual "arm" of the rear seat would be. My thinking is that when you place a passenger in the rear seat, some of his weight is actually forward of the seat pan since his legs are going forward quite a bit. Am I over-thinking this?

Your observation with the location of the CG of the pax is correct. I haven't measured the location of the rear seat to see how it compares to the arm that Van has used for the passenger. The only way to really sort this out, I think, would be when doing the weighing. With the aircraft leveled and on the scales, weigh it twice - once with both seats empty, and once with a typically shaped human in the rear seat. Then a bit of math can tell you were the CG of that human was. This will give you a reasonable arm to use for CG calculations with rear seat pax.

For your ballast, simply measure the location of the centre of the ballast, figure out how far aft that is from the datum, and use that as the arm in your CG calculation.

[aeropunk]

Originally Posted by Bill Wightman

A simplified explanation: The lighter stick forces are because there's less trim load on the horizontal tail to keep the airplane in a balanced flight condition. At forward CG, the tail must provide more down-force to balance the airplane, and that requires more elevator deflection. More deflection requires more control force from you, the pilot. Said another way, when nose heavy, the tail must work harder to maneuver the airplane in pitch, requiring more force on the stick.

Thanks, Bill. That makes more sense.

[scsmith]

This is indeed a great discussion, with many good comments from everyone. I thought I would resonate with just a couple of them.

One of the main reasons that stick force gets light when you are slow is that there is just less dynamic pressure, so it takes less physical forces for everything. Of course this is true of certified airplanes too. So various things are often done to augment the stick forces for feel. In my sailplane, the stick forces are completely synthetic, with springs. The elevator is so small, I doubt you would feel much of anything without the springs. I know the Cirrus SR-22 is full of springs too, perhaps other GA airplanes are too.

The RV's have a pretty low aspect ratio wing, which puts lots of downwash on the tail. The change in downwash with angle of attack reduces the stability contribution from the tail, but not the control effectiveness. I have not thought all the way through how this would effect trim forces, but it may partly explain why RV's are more sensitive to c.g. change than certified airplanes usually are.

Rob Byers hit it perfectly with the distinction of stick position vs stick forces. If you give me the stick position for trim vs speed, I can do a set of companion aero analyses (vortex-lattice) that, in combination with the data will tell you the static margin. I've occasionally thought about trying to set up a quick-and-dirty system for measuring stick position.

AndyRR brought up a question about servo tab. Yes, that is one of the reasons that tabs are used for is to modify the stick feel. I wouldn't suggest doing that to an RV - its a somewhat subtle business to get the tab gearing right, etc. to do all good things and not bad.

AndyRR brought up another thing that is worth discussing, and relates to everyone's comments about just being ready for the light forces and/or force reversal. Up-trimming on downwind, so that it takes some fwd force to stay at approach speed, and then during flare, the fwd force is reduced. This is a worthwhile human-factors question - does it help the human to have already biased toward forward pressure moderation, rather than moderating aft pressure and suddenly adapting to force reversal? Based on my experience with gliders, when you are on tow, you usually have fwd pressure. You adapt to it fairly quickly. Once adapted, that might be a better condition to start with as a human in the loop controlling the flare.
The trade-off would be the increased risk of getting too slow on approach when lots of things are happening and you might loose track of speed. So this is definitely a pilot judgement/skill/workload issue, but worth considering.

I hope everyone takes to heart Kevin's point about exploring this progressively with sand bags instead of loved ones in the back. I certainly will. And I guess I better learn how to wheel land better too

Thanks everyone, this has been very timely for me and, sounds like a few others, that are just approaching this step in phase I, and nice to know what to look out for.

[Kevin Horton]

I've spent the last few flights progressively moving the CG aft with ballast, doing some testing and circuits at each loading. Yesterday I finished the testing at the aerobatic aft CG limit.

I found the following stick free static longitudinal stability results:

With full flap, at idle power, trimmed for my current final approach speed (70 kt IAS), the stick free static longitudinal stability is almost neutral for speed reductions. The stick force to stabilize at 60 kt was an estimated 0.1 lb pull force. The longitudinal stability for a speed increase was slightly stronger, but still quite low. A 0.5 lb estimated push was required to stabilize at 85 kt.

With full flap, with power set for a -3 deg flight path angle (e.g., a standard ILS approach), the stick free static longitudinal stability is essentially neutral, with zero stick force required to stabilize at 60 kt after having trimmed at 70 kt. The stick force required to stabilize at 85 kt was an estimated 0.1 lb push.

At typical cruise speeds, and at high speed, the stick free static longitudinal stability was quite positive at both idle and full power.

At Vy with flaps UP, and full power, the stick free static longitudinal stability was noticeably negative for speed reductions. The aircraft was trimmed at 95 kt, and an estimated 1 lb push was required to stabilize at 80 kt. If the stick was released, the speed would continue to decrease to the stall. If the speed was increased to 115 kt, with the aircraft trimmed at 95 kt, an estimated 0.5 lb push force was required, indicating light positive stability for the speed increase.

At Vx with 1/3 flap, stick free static longitudinal stability was quite negative for both speed decrease and speed increase.

Bottom line - at aft CG at low speed, and especially with high power, pilots must pay very, very close attention to airspeed. You can't simply trim the aircraft for a speed and expect it to stay there. Climbs at Vy in IMC require particular attention.

Notes - as expected, these tests showed the classical destabilizing effect of power on a propeller aircraft with an engine on the front of the fuselage. Aircraft with lower powered engines (mine is 200 hp) and/or a fixed pitch prop would probably show slightly better stability at high power. But, I suspect they would still exhibit negative stick free static longitudinal stability at low speed with maximum power.

Things will be even worse at the full aft CG limit. I haven't decided yet whether I will go to the trouble to find some more dense ballast to allow me to ballast to even further aft, as it looks like the CG range I have tested will cover all the loadings that I will ever need.

[Andy_RR]

Originally Posted by Kevin Horton

At Vy with flaps UP, and full power, the stick free static longitudinal stability was quite negative for speed reductions...


... Climbs at Vy in IMC require particular attention.

Would it be too alarmist to suggest that this is verging on dangerous?

I cannot imagine you want negative stability in a potentially high-stress, high-workload inadvertent IMC scenario where climbing away from known(?) terrain might be a sound solution (in a valley, for example)

[Kevin Horton]

Originally Posted by Andy_RR

Would it be too alarmist to suggest that this is verging on dangerous?

I cannot imagine you want negative stability in a potentially high-stress, high-workload inadvertent IMC scenario where climbing away from known(?) terrain might be a sound solution (in a valley, for example)

I do agree that this is not a good characteristic, and you can certainly imagine situations where this could lead to an accident. But, the accident history so far suggests that pilots are either able to deal with this characteristic, or perhaps there are not many climbs at low altitude in IMC.

There are a few things working in our favour here:

1. The pitch attitude must get ridiculously high before the aircaft will stall with full power.
2. The airspeed must get very low before the aircraft stalls (I saw stall speeds with flaps UP and full power of about 48 kt IAS, at about 1780 lb weight).
3. The stall is quite noticeable (sharp left wing drop and nose down pitch on full power stalls, on my aircraft), and the aircraft recovers quickly from the stall if the pilot applies conventional inputs.

Safety will be improved if pilots are aware of this characteristic. Hopefully all RV owners are doing a full set of flight tests that cover the whole CG envelope, rather than simply "flying off the hours". Note that I use the word "owners", rather than "builders", as these tests are just as important for the guy who bought an already flying RV as they are for the guy who built his.

[Ironflight]

Outstanding work Kevin - I knew we'd all be benefiting when you got back in to Phase 1!

Your quantified data correlates very well to the qualitative handling qualities evals I did.

Paul

[DanH]

Originally Posted by Ironflight

Outstanding work Kevin - I knew we'd all be benefiting when you got back in to Phase 1!

.....and Kevin, might I add that we are all very thankful to have someone with your brains and particular job skill set here on VAF.