Neutral Stability - W & B

[Kiwi]

Tony,

It looks like all of the guys replying here are experienced RV pilots. Perhaps a perspective from someone who isn't will help.

What you describe was my overwhelming impression when I was taken for a ride in an RV-6. It took me many months to get over this "disappointment" before I purchased a second hand tail kit. The aeroplane I flew is very similar to yours except it has a metal propeller.

What I have noticed is that many owners describe this characteristic as "sweet handling" or "responsive" rather than "neutral stability". I have concluded that you just have to learn to live with it (or even enjoy it) in order to experience the many other great features the RV6 has.

The owner of the RV6 I flew and I have discussed this at length and his experience is exactly as you describe. Something that he has noted that others haven't brought up here is that on the RV6 the whole control shaft set up is hinged off the back of the spar and the effect of it is to over-balance the elevators. i.e. you set them up with 100% balance without the controls connected and once you connect the elevators, the effect of the "falling" control shaft is to move the elevators up. As you load the aeroplane with "g" the effect is magnified providing an additional up-elevator input. When I was flying the aeroplane around in a steep turn I was actually holding a slight forward pressure, yet we are used to holding (in some aircraft considerable) back pressure in a steep turn. This builder has suggested putting a "donger" weight in the control circuit to counteract this. This is not an unknown design feature in some aircraft. Other aircraft use springs to either cure or prevent this occurring or to generate a particular control force gradient. Even the slow Super Cub has a spring in the elevator circuit to help with trimming and many of the experimental Cub guys have either experimented with this or added springs to deal with undesirable control effects.

Over to you,
Andrew.

[scsmith]

What we think of as 'sweet handling' is fairly light control forces, BUT NOT negative stick force gradient.

If you have to apply forward pressure to keep an airplane from slowing down, ESPECIALLY in a elevated-g situation, then something is not right and it should be fixed. In an elevated-g situation, this could be really dangerous-- the airplane should unload itself if you release the stick, not pull harder making more g's.

Several factors, as mentioned here, can cause negative stick-force gradient, but if an RV-6 is built right and within c.g., it will not do this. This is not normal.

Elevator trailing edge radius can cause vague handling - a tendancy to not want to trim, but to sit slightly off trim to one side or the other (meaning slightly nose up or slightly nose down 'out-of-trim') and it may kind of snatch - abrupt change in force. But I don't think it can cause a consistent negative stick force gradient. There are 3 things that can. One is reverse camber in the stabilizer/elevator. Not likely in RV's, the tail is symmetrical, and it would be hard to screw that up with the pre-punch kits. Depending on how the trailing edge bend is done, it may be possible for the upper surface of the elevator to be more flat, and the lower surface to be slightly bowed or curved. This would cause the problem.

One is relative incidence between the wing and tail.

The third is c.g..

Bob weights and springs are not very satisfying ways to fix it. If you can not find the cause, you can install a fixed tab on the trailing edge of each elevator, about 1.5" by 8", and bend it down 10 degrees. This will fix it. But try to find out what is not normal about the wing and tail rigging and the c.g. first.

Originally Posted by Kiwi

Tony,

It looks like all of the guys replying here are experienced RV pilots. Perhaps a perspective from someone who isn't will help.

What you describe was my overwhelming impression when I was taken for a ride in an RV-6. It took me many months to get over this "disappointment" before I purchased a second hand tail kit. The aeroplane I flew is very similar to yours except it has a metal propeller.

What I have noticed is that many owners describe this characteristic as "sweet handling" or "responsive" rather than "neutral stability". I have concluded that you just have to learn to live with it (or even enjoy it) in order to experience the many other great features the RV6 has.

The owner of the RV6 I flew and I have discussed this at length and his experience is exactly as you describe. Something that he has noted that others haven't brought up here is that on the RV6 the whole control shaft set up is hinged off the back of the spar and the effect of it is to over-balance the elevators. i.e. you set them up with 100% balance without the controls connected and once you connect the elevators, the effect of the "falling" control shaft is to move the elevators up. As you load the aeroplane with "g" the effect is magnified providing an additional up-elevator input. When I was flying the aeroplane around in a steep turn I was actually holding a slight forward pressure, yet we are used to holding (in some aircraft considerable) back pressure in a steep turn. This builder has suggested putting a "donger" weight in the control circuit to counteract this. This is not an unknown design feature in some aircraft. Other aircraft use springs to either cure or prevent this occurring or to generate a particular control force gradient. Even the slow Super Cub has a spring in the elevator circuit to help with trimming and many of the experimental Cub guys have either experimented with this or added springs to deal with undesirable control effects.

Over to you,
Andrew.

[Bad Sheila]

How do you make an incidence level for my RV6 with a 4' level and a block of wood?

[scsmith]

There are pretty detailed instructions in the builders manual, and details on the drawings. Mine is for the -8 so I can't help you more specifically. Did you buy the airplane finished so no manual and plans? If so, Van sells them.

To give you some idea, though, on the -8, the stabilizer is mounted with a prescribed set of spacers under the spars, 1/8" under the front spar, and 3/16" spacer under the center joint of the rear spar. The wing incidence is set by leveling the fuselage longerons, and setting a 4-ft level on the wing, with one end bearing on the skin just at the spar seam, the other end resting on a wood block cut to a certain height, and resting on the skin at the rear spar.

Let us know what you find.

[N941WR]

Originally Posted by 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?
   ...

I recommend you:

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

Originally Posted by Bad Sheila

How do you make an incidence level for my RV6 with a 4' level and a block of wood?

Start with the easy stuff first, don't mess with the incidence until you get a current W&B. While you said your TW was weight was 70 lbs, mine is only 49 lbs. Granted, I have a an RV-9, not an RV-6 but they really aren't that different. The 30 pound difference back there could account for what you are feeling.

[L.Adamson]

Originally Posted by Kiwi

It looks like all of the guys replying here are experienced RV pilots. Perhaps a perspective from someone who isn't will help.

What you describe was my overwhelming impression when I was taken for a ride in an RV-6. It took me many months to get over this "disappointment" before I purchased a second hand tail kit. The aeroplane I flew is very similar to yours except it has a metal propeller.

What I have noticed is that many owners describe this characteristic as "sweet handling" or "responsive" rather than "neutral stability". I have concluded that you just have to learn to live with it (or even enjoy it) in order to experience the many other great features the RV6 has.

The owner of the RV6 I flew and I have discussed this at length and his experience is exactly as you describe. Something that he has noted that others haven't brought up here is that on the RV6 the whole control shaft set up is hinged off the back of the spar and the effect of it is to over-balance the elevators. i.e. you set them up with 100% balance without the controls connected and once you connect the elevators, the effect of the "falling" control shaft is to move the elevators up. As you load the aeroplane with "g" the effect is magnified providing an additional up-elevator input. When I was flying the aeroplane around in a steep turn I was actually holding a slight forward pressure, yet we are used to holding (in some aircraft considerable) back pressure in a steep turn. This builder has suggested putting a "donger" weight in the control circuit to counteract this. This is not an unknown design feature in some aircraft. Other aircraft use springs to either cure or prevent this occurring or to generate a particular control force gradient. Even the slow Super Cub has a spring in the elevator circuit to help with trimming and many of the experimental Cub guys have either experimented with this or added springs to deal with undesirable control effects.

My 6A is nothing like you've described. It's responsive alright, but trims very easily. It doesn't require as much back pressure in turns as the typical Cessna or Piper...............but will descend if you're occuppied... taking pictures and not paying attention. I know a couple of "9" owner/flyers who like the way mine handles. Certainly no disappointment here.

L.Adamson --- RV6A

[Bill Wightman]

Interestingly, I'm helping a friend work out precisely the same problem with another airplane (its an F1 Rocket). We checked everything, and I mean everything.

Finally after all else failed, I checked the tail incidence and bingo - we have a problem. The horizontal tail is mounted too positive. We're gonna do some more flying and testing soon to evaluate a possible fix that doesn't involve mucking up the empennage fairing. We will try a fixed tab like Steve recommended.

This F1 has an extended nose (a destabilizing factor), LOTS of HP, and the EVO wing, which doesn't produce as much downwash on the tail as the short wing. As a result, the airplane balances perfectly but is dynamically unstable: it will not damp out pitch oscillations over time. Instead, it diverges. It can be trimmed at 230 knots and there is no pitch trim requirement from that speed all the way to the landing flare.

And, I totally agree: any of the RV airplanes built to plans and operated inside their (kit maker) approved CG envelope should absolutely have a positive stick gradient and should trim up on speed without issue.

[Kiwi]

Originally Posted by L.Adamson

My 6A is nothing like you've described. It's responsive alright, but trims very easily. It doesn't require as much back pressure in turns as the typical Cessna or Piper...............but will descend if you're occuppied... taking pictures and not paying attention. I know a couple of "9" owner/flyers who like the way mine handles. Certainly no disappointment here.

L.Adamson --- RV6A

Your's Larry has an O-360 and constant speed prop, which makes all the difference. I have concluded (party from your contributions) that an O-360/Hartzell RV6 will fly very differently from an O-320/wood prop RV6, and I'd like to take the opportunity to thank you for your contributions.

This issue in general terms has been discussed many times on this forum. My summary largely reflects what others have written on this forum. I can even lay my hands on a pilot report by Ed Kolano who seems well respected in the US, that describes this characteristic in the RV7.

My contribution was intended to illustrate this Tony's experience may largely be a perception. A Cessna 182 for example is going to appear to be far more stable than an RV6 and if that's what you're used to flying, that is how you will judge it. It would be a better data point if an experienced RV pilot of a similarly configured RV6 flew Tony's aircraft and pronounced it unsatisfactory or otherwise. I seen this a few times, pilot A flies an aeroplane and thinks it unsatisfactory, then pilot B experienced in the type flies it and simply says "they all fly like that".

Over to you,
Andrew.

[billdianne]

How do you measure friction on the ground? If I lift the elevators by hand should they fall back under there own weight? Can you use a fish scale to measure the pull at the stick?
When I first installed the elevators they were very stiff. I literally spent hours getting rid of all the friction I could, aligning the bearings, torquing the bolts to the minimum setting and making custom spacers for the center bearing. It made a huge difference but I still don't know how to quantify what is considered to be normal.

[WenEng]

Bad Shiela,
What Steve Smith said. For the RV6, 1 degree positive wing incidence angle is the typical goal.

RV6 Wing Incidence Leveling:
Level the top longeron running along the open cockpit. (between station 30 and station 55.75).
Place long level on wing. One end on top of forward wing spar skin butt joint line.
Place a spacer measuring 3 1/32 inch (3.03 inches) on top of wing's rear spar rivet line. Your level should read centered or 0 degrees with a digital level. You get the 1 degree positive result because you use a 3.03 inch spacer to read a centered level bubble.

Horizontal Stabilizer incidence leveling.
It is difficult to see the tooling holes on the stabilizer, but they are supposed to be parallel to the top fuselage longeron. Keeping the top fuselage longeron leveled, put a 1/8 inch spacer on top of the stabilizer forward spar area and a 1/4 inch spacer on top of the stabilizer's rear spar area, your level should read centered or 0 degees.
There are variations of that for builders who had a slight error in the wing incidence (example being wing 1/2 degree too positive), where then you would hope to see the same error in the stabilizer incidence.

Thats the quick answer, and there are probably as many variations to perfect as there are RV6s. Personally, I would focus on CG and friction first and then go take these measurements for your own education.