RV-8 Static Longitudinal Stability

[aeropunk]

Disclaimer: I'm probably ten years away from flight testing anything other than my recliner, so please bear with me on this long-winded post.

In a (vain) attempt to educate myself, I've been poring over the articles on the CAFE Foundation website over the past few days -- WOW is that place an online goldmine! -- and I came across the Aircraft Performance Report for the RV-8A (PDF). Actually, I think I had read it at some point before, but today I was trying to compare numbers with some of the other popular kitplanes that had been profiled.

The thing that caught my eye in particular was the Static Longitudinal Stability graph (reproduced below), which compares stick forces at various airspeeds and CG positions.

According to the CAFE APR Intro/User's Guide (PDF):

The static longitudinal stability graph shows the character of the pitch stability as follows:

A line running 'downhill' to the right indicates positive stability, i.e., the aircraft wants to stay at trim speed and requires the conventional pull on the elevator stick to slow down or push to speed up from trim speed. A steep downhill line means the positive stability is strong and thus that aircraft would be more suitable for cross-country flying, while a gradual downhill line would indicate an aircraft more suitable for aerobatics.

A line running horizontally would indicate neutral stability, i.e., the aircraft requires zero elevator stick force to change speed -- very undesirable.

You'll notice from the graph that in the aft-CG condition, at 140 mph IAS (purple line), the stick force curve actually reverses! I remembered reading on this forum in places that aft-CG flight testing was very important with RVs, and that the procedure should be approached with caution, and well before flying around with passengers.

Here's what C.J. Stephens, the test pilot, had to say on the subject:

Static longitudinal stability was measured by trimming to Va (140 mph indicated) and measuring stick force required to hold speeds in ten-mph increments from 70-190 mph, while maintaining altitude by adjusting power. Note that for the forward cg condition, the result was a very substantial positive force gradient as speed varies in either direction from the trim condition.

In the aft-cg test, however, a reversal of the force gradient was encountered as speed was reduced from 140 to 70 mph indicated, with the maximum force occurring at about 110 mph. It is generally desirable that some positive force gradient exists as speed deviates more and more from the trim point, and imperative that no actual force reversals occur.

I would recommend that pilots explore aft-cg stalls with some care to familiarize themselves with stick force behavior in this region. We did not conduct tests with loadings further aft, but the trend would indicate that the gradient reversal observed would become more pronounced, and stick force during stalls at the full aft limit could be near zero when trimmed for a normal approach. (Emphasis mine.)

As you might imagine, this really caught my attention. My questions are:

(1) What "interesting" flying qualities have -8 drivers noticed while testing in this region?

(2) Did you make substantial deviations from Van's aft CG limit recommendations (29% chord/16.8" aft leading edge/86.82" aft datum) when writing your POH?

(3) If a guy was building an RV-8 and intending to fly around with two Bubbas (200+ lbs.) onboard every once and a while, what modifications (i.e., battery in back or front) might he consider?

Thanks in advance!

[scsmith]

This will probably start a hail storm. It is so often stated in books and flight teting, but it is not true. Static longitudinal stability is the STICK FIXED pitching moment gradient ( d_CM/d_CL). It relates changes in pitching moment to changes in angle of attack with the stick fixed at a trim condition.

To some degree, stick force gradient should correlate with static stability, but there are other important factors that affect the stick force gradient. The hinge moment characteristics of the elevator have a dominant effect - and things like the amount of aerodynamic balance of the elevator, the elevator camber, trim tab position, all affect the hinge moment, and feed through to the stick force.

But none of those factors which influence the elevator hinge moment influence the STICK FIXED stability characteristics.

The plot shown here was for a condition where the airplane was trimmed at 140 kt, and then stick force measured over a broad speed range. So there is a lot of elevator deflection at speeds other than the trim speed. The shape of the curve far away from the trimmed speed point is not very meaningful. IF the test were repeated with the airplane trimmed at 90 kt, the curve would be shaped differently.

I have not flown my -8 at aft c.g. yet, so I can not say what the stick force gradients will be. I will report as soon as I do. I am aware of a glider that has negative stick force gradients because of negative camber in the elevator and a large aerodynamic balance tab. It is unpleasant to fly, but it is not statically unstable.

By the way, an easy fix to a negative stick force gradient is to put positive camber in the elevator, or a trim tab that is bent down at the trailing edge. Note that in the plot here, if the airplane were trimmed at 90 kt, that would move the elevator trim tab down. I promise that will give a positive stick force gradient at 90 kt, if you are trimmed at 90 kt.

[scsmith]

I would be much more worried if I had a W10 - I don't know what that is, but it sure has light control forces. That may or may not indicate poor stability, as discussed above.

[RV8N]

Originally Posted by scsmith

I would be much more worried if I had a W10 - I don't know what that is, but it sure has light control forces. That may or may not indicate poor stability, as discussed above.

I believe a W10 is a Witman Tailwind...

Karl

[Mel]

Originally Posted by RV8N

I believe a W10 is a Witman Tailwind...

Karl

That is correct.

[Ironflight]

I think this plot shows very well what many RV-8 pilots have experienced on their first landing with a passenger - the airplane is clearly more sensetive with a full-sized body in the back, but not surprisingly so when they are up and away at speed. Then they come in to land, and as they slow down over the fence, there is this feeling of the bottom falling out and the airplane pitching up - which is exactly what you'd expect if the stick force did what the plot shows.

I think Karl was the first one to tell me about this on my very first RV-8 flight (in the back seat of his -8). Most people end up carrying more speed down final when they have a passenger as a result - and learn to handle pitch very gingerly. An attempt at a three-point in this loading condition is a very good prescription for a HARD tail-first landing, followed by a bone-crushing main gear arrival.

That's a great plot - I've been flying the -8 for almost 1,000 hours, and this is the first time I have got a good, clear way to describe what I know intuitively is happening.

Paul

[N8RV]

You know, Paul, when I studied that plot I thought the very same thing.

[scsmith]

Paul's and Don's comments have me wondering with some concern -- and I'm not far from being ready to go fly at aft c.g. and find out -- but:

Does the control force just get really light, or does it actually get twichy - meaning, is it actually hard to hold a constant pitch attitude, or do the forces just get really light? If I flare to 3-pt attitude and hold it there, will it not stay there?



As a follow-on to the point about stick force not necessarily correlating to static stability, consider a radio control model airplane. It can be perfectly nice and stable, but the stick force gradient is essentially zero on the control box ( unless you add some synthetically). None of this is to devalue the observation that weak stick force gradient is not as nice to fly, and negative stick force gradient is downright spookey.

[Ironflight]

Originally Posted by scsmith

Paul's and Don's comments have me wondering with some concern -- and I'm not far from being ready to go fly at aft c.g. and find out -- but:

Does the control force just get really light, or does it actually get twitchy - meaning, is it actually hard to hold a constant pitch attitude, or do the forces just get really light? If I flare to 3-pt attitude and hold it there, will it not stay there?

No, I wouldn't say it's twitchy - it just gets incredibly light - like close to zero force. If you aren't ready for it in the flair, then you'll over control, the nose goes higher than you want, and if you're close to the ground - bang!

The more I think about it, the more I think this is one strong reason more -8 pilots like to wheel land - it keeps you from getting into this hole when you have a passenger.

Paul

[Kevin Horton]

One more thing to keep in mind when looking at those plots from the CAFE Foundation - they didn't do a pure longitudinal stability test. They did the test in level flight, varying the power to achieve the desired speed change.

There are significant changes in stick force at a constant airspeed if you change the power. The way the CAFE Foundation did their tests makes the result = the sum of stick force changes due to speed + stick force changes due to power. It is interesting to look at, but doesn't necessarily mean anything.

The right way to do the test is to leave the power constant as you go to the off-trim condition. This implies that the altitude will change, but it avoids having the pitching moment from power changes pollute the test results.

I haven't done any testing further aft than mid CG yet. I was just staring a series of tests to walk the CG back over a number of flights when I had my big engine overspeed. Then after putting the engine back on I was back into engine break-in. I should start back at the aft CG envelope expansion this weekend, I hope.