RV-8 Static Longitudinal Stability

[n5lp]

I have a vivid memory of the first time I flew an RV-6. It was with Mike Seager in "Old Blue." When I did the first stall I was startled enough that I mentioned it to Mike and he did a stall just to check it out.

What got my attention is that as I slowed to a stall the stick force stopped increasing at some point which was something I was not used to at all.

I was exposed to it and assured it wasn't a big issue. I'm not sure if my RV-6 does it or not. It was just different than the factory airplanes I had flown to that point.

I have found that with aft CGs the six also needs care in the landing. Just as Paul spoke about, it is easy to get into a too high nose attitude and the stick forces do get a lot lighter.

[mannanj]

Originally Posted by Ironflight

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

I once ferried a friend to a nearby airport to pick up his Barron after some radio work. He's 5ft 20in. tall and weighs 2XX lbs. I was right at the aft CG limit.

No problem with take off and cruise but on landing, (I chose to wheel land with a little extra speed) I had to use forward force on the stick to keep from over rotating. It was not a lot, but much different from the normal back pressure on the stick on landing.

The -8 is a different animal when landing with a heavy pax in back.

[Kevin Horton]

Originally Posted by mannanj

The -8 is a different animal when landing with a heavy pax in back.

And hopefully folks will first experience this in the flight test phase, using well secured ballast to simulate the weight of a passenger. That way you can do several flights, moving the CG a bit further aft on each flight, rather than doing it all in one big step when you first fly with a passenger. The first aft CG test flights should be flown solo, rather than with a passenger on board.

[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?

[Andy_RR]

Originally Posted by scsmith

...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.

Does this mean that an aft-CG 3-pointer would be easier/more controllable if you "over-trimmed" it for finals - i.e. trimmed it for a speed slower than your approach speed - meaning you'll have to fly the approach with some forward stick force.

Just thinking out loud here, would a servo tab* improve the stick-force gradient under these conditions, or perhaps just bugger up the forward-CG condition?

A

*no, I really don't want to have to build another trim tab!

[aeropunk]

Wow, this is a great discussion; thanks everyone for chiming in!

It sounds like the light stick forces with aft c.g. are a fairly common occurrence with RVs of all types, and pilots are simply making adjustments to their technique when flying with an aft c.g. The other CAFE RV performance reports seem to confirm this phenomenon as well (and RVs aren't the only planes that were displaying this).

I guess I'm still unclear about what's really going on here aerodynamically. Is the flow separating at the tail? What actually causes the lighter stick forces? (I guess maybe Steve did try to answer this question, but I'm probably too dumb to get it.)

Also, I noticed several people had comments like Larry's: "It was just different than the factory airplanes I had flown to that point."

Now I know these are "Experimental" aircraft, but if we were all real military/factory test pilots or engineers (and some of you guys are, no doubt), what would our recommendations to the designers be? Would it be, "Well let's just include a footnote in the POH about handling qualities at aft c.g.," or "Let's shorten the c.g. envelope a bit," or "Houston, we have a problem."?

Is there a danger of "deep stall" in this condition? I keep thinking of the Discovery Channel video of Mike Melvill doing the first stall series with SpaceShipOne and his "exciting" findings.

I don't mean to sound like I'm making a big deal about this, because you've all seemed to adapt well to it, and I'm not all that concerned about it myself, it's just that I hate it when I don't understand the "why" of something. Thanks again.

[RV8N]

Even when I'm solo and full fuel, my -8 has light stick forces when I get below 80 kts. I'm usually eyes-outside when I'm in the pattern and this is my first indication that airspeed is getting too low. It's time to get the nose down or add power (or both).

IO-360 (angle valve), Hartzell C/S, rear battery

Karl

[Bill Wightman]

Originally Posted by aeropunk

I guess I'm still unclear about what's really going on here aerodynamically. Is the flow separating at the tail? What actually causes the lighter stick forces? (I guess maybe Steve did try to answer this question, but I'm probably too dumb to get it.)

Also, I noticed several people had comments like Larry's: "It was just different than the factory airplanes I had flown to that point."

Now I know these are "Experimental" aircraft, but if we were all real military/factory test pilots or engineers (and some of you guys are, no doubt), what would our recommendations to the designers be? Would it be, "Well let's just include a footnote in the POH about handling qualities at aft c.g.," or "Let's shorten the c.g. envelope a bit," or "Houston, we have a problem."?

Is there a danger of "deep stall" in this condition? I keep thinking of the Discovery Channel video of Mike Melvill doing the first stall series with SpaceShipOne and his "exciting" findings.

I don't mean to sound like I'm making a big deal about this, because you've all seemed to adapt well to it, and I'm not all that concerned about it myself, it's just that I hate it when I don't understand the "why" of something. Thanks again.

Brian,

Aerodynamically, there is no problem. The RV's get light in pitch when the CG is aft simply because they don't run huge tail areas, and we do fly them right up to the practical aft limit (not limited by FAR criteria).

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.

As Steve pointed out, there are a number of factors related to the control surface design, the trim tab design, and the fixed surfaces themselves that contribute to stick force (or feel) characteristics. The designer can modify lots of things to change stick gradient, but IMHO the RV's are done right and I wouldn't personally advocate a change to the design to try and modify pitch feel in these airplanes.

As you've noted, we adjust our technique when we're tail heavy. Its part of the challenge of being a good sport pilot!

[RobByers]

Originally Posted by 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.

Yes, but you cannot measure d_CM/d_CL directly in test flight without the hand of God imparting a disturbance and having the aircraft connnected to a device to measure the moment generated. The restoring moment that is required due to a change in airspeed from trim can only be measure by the test pilot as a required change in stick position. i.e. as stated in the parlance of flight testing "Positive longitudinal static stability was "indicated by" the requirement for increasing forward stick with increasing airspeed from trim and increasing aft stick with decreasing airspeed from trim." Also note that it is stick position in the statement. Also, you can only discuss static stability of being positive, negative, or neutral based on these tests. You cannot discuss anything about the strength of static stability wihtout doing a different test. The many reasons that you cannont directly measure strength with this test are mentioned in the comment above with the additin of gearing ratios which are very important in helicopters. d_CM/d_CL is refering to both sign and strength of static staiblity. The graph can only provide insight to the sign of static stability.

Originally Posted by scsmith

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.

A very important point and one that student test pilots frequently misunderstand early in our course. There are no local areas of negative static stability or reversals of slope. You can only discuss static stability about the trim point and the slope of the line between trim at 140 kt to 70 kts is still indicating positive static stability. If you are concerend by the area between 70 and 90 kts you trim at 80 and do it again. Also it is better to plot position and force on different y-axes. They may actually tell different stories.

Originally Posted by scsmith

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.

The reason so much is made of force gradient is that humans are biomechanically not terribly accurate at determining positon as they are force as a means of feedback. In irreversible or partially reversible systems systems, a great deal of effort is expended in generating forces that make the aircraft feel statically stable even if the stick position gradient is flat or negative. We call this "apparent static stability." We can talk about position and force cues to off tirm speed conditions and whether or not that cueing is adequate, but we still can't say how strong static stabiilty is based on the plot.

[bobby]

In the RV-8 (or 8A) the pilot is sitting pretty much over the main wing spar. The CG of the pilot in the front seat would be a little aft of the spar so the arm is very short and a change in weight in the front seat shouldn't change the aircraft's CG much. When you put someone in the back seat the CG change can be substantial.

I've often wondered if it would be better to set it up to fly solo from the rear seat for CG reasons. Then when you take a passenger in the front the change in CG would be minimal. This of course would require engineering changes I'm not going to make and mine will be like the rest; pilot/solo from the front. I've just wondered about this from time to time.

In gliders we almost always fly with passengers in the front seat and solo from the front. Piper Cubs fly pilot/solo from the rear seat probably for that reason.