Neutral Stability - W & B

[gereed75]

Tony Your numbers look very close to mine. I was at 1090# with 70# on TW and I think an identical CG (numbers not here to double check). Pitch stability is generally not real positive, but nuetral and not divergent.

It will return to trimmed Airspeed after being displaced but not with a lot of authority. It tends to hunt around quite a bit. I would call it nuetral in pitch stability. My Trutrak auto pilot can hold altitude very nicely with only minor hunting.

My seats are aft of normal about 2 inches (tall pilot mod). I have flown with some heavy pax (280) and low fuel. Puts the CG maybe an inch or so ahead of aft limit. Again, you notice the more aft CG, the nose becomes much more responsive and sensitive, especially at low speeds for landing. Takes more forward stick to lift the tail - but it continues to fly just fine.

I agree with Andy, don't expect it to be hugely positively stable in pitch, but it should fly just fine.

When I change to a 406 ELT, I will mount it near the firewall to move the CG forward a bit. Take any opportunity that you can when placing equipment to get more forward CG.

Fly on!!

[DonPro]

Mine flew that way with a far aft CG. One more possiblity to check is to large of a radius on the elevator trailing edge. It sure effects the lightness of the ailerons on the stick. 1/8 radius is what is called for IIRC.

[Kevin Horton]

There are a number of possible variables here:

1. CG - Static longitudinal stability gets worse as the CG moves aft. I'd be surprised if the static longitudinal stability was neutral or negative at given the weight and balance data you have given us. Are you sure the CG is really where you think it is?
2. Control friction - If you pull back on the stick and release it, in an ideal world, with no friction, the aerodynamic forces on the elevator tend to move it back towards the neutral position. But, any friction fights against the aerodynamic force and tend to keep the elevator deflected. If you have enough friction it will overcome the aircraft's natural stability. I'd take a very close look at your flight controls and make sure you have as close as possible to zero friction.
3. Power - Stability of a single-engine tractor propeller-powered aircraft is affected by engine power. Stability is worse at higher power than it is at low power. On my RV-8 for example, at Vy at climb power with a passenger it has negative static longitudinal stability. If I trim at Vy with climb power, and slow a couple of knots and release the stick, it will continue to decelerate to the stall. If I trim at Vy with idle power, it has positive static longitudinal stability.
4. Airspeed - My RV-8 exhibited more positive static longitudinal stability at high speed than it did at low speed.

I recommend you:

1. Check for control friction. Correct as necessary.
2. Reweigh the aircraft with calibrated scales and recalculate the weight and balance.

[Bad Sheila]

Andy
It flies fine after being trimmed with tabs on the rudder and aileron. (The right wing wanted to drop)

When you "bump" the stick it returns OK.

The issue is only when you put the nose up or down it just wants to stay going in the same direction even with only the pilot flying?

We put a pile of batteries in the baggage compartment and it made it worst, confirming aft CG We put a 19 lb steel ring (flywheel!) around the starter ring gear plate which improved the situation.

Gereed75: My plane doesn't hunt much, luckily. Maybe I will switch my ELT from behind the baggage compartment to up front somewhere!

Other scenarios:

1) I will check the elevator trailing edge as per DonPro
2) The "Datum" used was 60" in front of the wing leading edge, is this correct?
3) Is it possible the engine mount (and therefore the wheels) could be the wrong one or positioned wrong?

Maybe I am being just too picky seeing as my numbers are in the "ballpark" as FLY6 says! I hate the idea though of having to add weight to my "Bad Sheila". (Aussie term for women)

Tony

[Terry Lutz]

It is difficult to tell via the web exactly what how the airplane acts during flight. From what I have read, the problem could be a couple of things going on at once. There is the possibility of friction in the control system, and I would look closely at that. And, from your description of the weight you have added and where you added it, you may indeed be more aft than you would like.

I recommend you approach it a little more precisely. Establish your CG as accurately as you can, then go fly to check the Phughoid characteristics. Trim the airplane for hands off with power for level flight and about 10 knots slower than you would normally cruise (140 knots or so). Then raise the nose a few degrees and let go. Wait and watch. Keep the wings level with your index fingers pushing only on the side of the stick, and don't put any input in pitch. Don't touch trim or power. If the airplane is statically stable (what you definitely want), the airspeed will slow and the nose will eventually fall below the horizon. The Phughoid cycle can take one to two minutes, so be patient. The nose will fall and speed will increase to 10 knots or so above the trim speed. Then it will oscillate up again. And down again, and so on until it stabilizes close to your trim speed. That indicates positive static stability, even though it is a dynamic characteristic. With an aft CG, the Phugoid cycle time will be longer than at forward CG, so you can roughly check static stability with this method. If the airplane has neutral static stability, the nose will stay where you put it, and airspeed will decrease toward the stall.

Normally, you would check static stability by measuring stick forces at airspeeds of +10, +20, and -10, -20 from trim speed. But the forces on our nice RVs are pretty light, and it is difficult to measure forces with something like a fish scale.

The main point is that you should approach the problem methodically, rather than by try this, try that. It could be that you have a statically stable airplane with friction in the control system, although Van's pitch control system is very well designed to minimize friction. What I doubt is that engine mount/thrust line or horizontal tail incidence is causing the problem. They mainly have an effect on trim position. One of the other responders mentioned that power in a tractor propeller configuration is statically destabilizing, and he is spot on correct. So make sure you do all your checks at the same power setting, fuel weight, and airspeed for an accurate comparison of just changes in CG.

Have fun and fly safe!

[scsmith]

A couple of points:

First, as several have said, friction can be a big effect. A common place for unexpected friction is at the center elevator bearing where the two elevator horns come together. I had to machine a custom spacer to get that connection to be tight on the inner race of the bearing, but not bind at all.

Second, there is widespread confusion between stick-free stability and classical "static longitudinal stability" which is defined as stick-fixed. Stick-fixed stability is predominantly affected by c.g., also with some dependence on power setting. You could not fly an airplane that is unstable stick-fixed.

An airplane that has stick-fixed stability can still have stick-free instability, which also shows up as negative stick-force gradient. This means that as you slow down, you have to apply forward stick pressure to keep it from continuing to slow down more. This can be pretty darn unpleasant, but you can fly it (just can't take your hands off the stick). A number of things can influence the stick-free stability of a stick-fixed-stable airplane. One important one is the relative incidence between the wing and the horizontal tail. Wing aspect ratio also affects it, because it changes the downwash induced on the tail, changing its apparent incidence angle.

So, you should check your wing and tail incidence.

You can assess stick-fixed stability by measuring the stick POSITION for trim at a range of airspeeds. You can assess the stick-free stability by measuring the stick PRESSURE for trim at a range of airspeeds.

[DENMACRES]

Check to see if elevator is not binding. If the center hinge is not properly shimmed the bolt will pull the elevators together and make the outer hinges bind. You have to unhook the push pull control. I should be free and balanced.

[Bill Wightman]

Wing - tail relative incidence does have a major influence on stick free dynamic stability. If there appears to be a lack of stick force gradient with airspeed, it could be the horiz stab is mounted too positive. Check the incidence of the wings & horizontal stabilizer. Relative to the wing, the h-stab should be mounted at least one degree negative (LE down on the h-tail). Then check and see what the plans call for.

[jthocker]

My money is on under bent elevator trailing edges. You should be able to lay a straight edge on the elevator spar to with 3/16's inches from the trailing edge. Trailing edge "geometry" has an unbelievable effect on handling characteristics. We squeeze the light aileron to fly hands off. Too "fat" ailerons and the plane will be twitchy. Go too far in squeezing the aileron and the plane will fly very stiff (like an RV9 with its riveted traing edge's). I'm guessing the elevators probably react the same way.

[szicree]

Speaking as someone who built and sold a nearly complete -4, I would strongly advise carefully checking wing and h-stab incidence before horsing around with anything else. It's quick and requires nothing more than a decent 4 foot level and a little block of wood. To fiddle around with other rigging/weight adjustments would only be a band-aid if these are not first established within proper limits. Personally, there's no way I'd be doing first flights in any plane unless I'd checked these myself or the plane had been built by someone with a known successful build history. I'm not saying the other suggested fixes aren't ultimately going to be the right path, just that proper wing and tail incidence is not something that you want to take on faith. If you don't believe, just search these archives for stories of honked up rear spar drilling (I'm in there ).