Neutral Stability - W & B

Going nuts trying to sort out Neutral Stability in my new RV6 (11 hrs).

Put the nose up or down and it just keeps going the same way.

It is either the wing incidence or AFT C.G. I bought it half completed with the wings already drilled and haven't got the equipment to measure the incidence yet. I am inclined to think it is the W & B after talking to Vans.

My W & B seems all screwed up because according to Vans, mine must be the lightest RV6 around. It weighs in at 982 lbs. It has 70 lbs on the tail instead of about 55 lbs (according to Vans?)

Specs:

WEIGHT: 982 lbs
C.G: 67.13
ARM: 65920
Fuel: 70
Pilot/Passenger: 87.4
Baggage: 117
Range: 68.7 to 76.8

AIRCRAFT
0-320 - Wooden Prop - Lightweight starter motor - EFIS (All electric) - unpainted.

Does anyone have a similar RV6 with a weight and W & B similar to mine?

Any suggestions would be greatly appreciated.

Thanks

Tony (The "Bad Sheila" is my plane, not me?????)

Tony,
My -6 is 1015lbs(Aircraft Scales) with O360 and Wood Prop.I don't have a neutreal stability problem, but it is a bit light on the nose. 72lbs on the tail. If you need weight on the nose call Sam at www.sabermfg.com , he makes a crush plate just for this. I believe it's steel and is around 20lbs.

RV6 Aft CG

Thanks... he has a lot of good stuff that may fix the problem.

Another thing to check is control friction. Make sure that there is near zero friction in the pitch axis. Lube the bearings throughout, and make sure nothing is rubbing.

It doesn't take much friction around the neutral point to affect things.

RV6 Weights

Tony, I too have an O-320 with a wooden prop in my RV6. I have steam gauges which add some weight up front. I don't have the numbers in front of me but my empty weight was 1062# and my tail weight I believe was 72#. I found when doing sample W & B that it was impossible to get a forward c of g but very easy to have a c of g too far aft.

I will be able to check the my numbers tomorrow if you want any more numbers.

Hope this lets you know if you are in the ball park, Al

That's pretty light. Mine weighed in at 992 before paint. Over 17+ years and several additions including paint, avionics, and ER fuel tanks, I'm now up to 1040 lbs.

Mine is 999lb. with metal FP prop. Bare metal, so no paint. 70 lb on the tail at my weigh-in a couple of months ago too. I believe the empty CG was 70.something, but I haven't got the paperwork with me right now to check.

TW 70 lbs

It looks like my 70# at the TW seems pretty normal after several posts. Do any of you have the "neutral stability" problem?

If you don't then maybe my CG is OK and I need to look elsewhere, like my wing incidence.

For PCHunt: We had the problem you mentioned and fixed it with an new control stick mounting bracket that was drilled wrongly. We installed a new mounting bracket and control stick to fix the problem so that eliminated the problem you mentioned.

Thanks for all your input.

Tony
PS What is a "FP" prop?

Bad Sheila...

I doubt 'Wing Incidence' is anything to do with it - it's not a critical setting, nor is the relationship between wing and tailplane incidence.

I would go for CG every time:

Basic..982 x 67.13 = 65922
Fuel...150 x 70.00 = 10500
Pilot..180 x 87.40 = 15732
Total..1312.00.......92154 = 70.24"

Pax....180 x 87.40 = 15732
Total..1492.00......107886 = 72.31"

Both CG figures look fine from what you give - however, the "dual" case is further aft?

Are the "problems" you are experiencing genuine handling issues with the aircraft? Or does it fly nicely, but when you try an academic stability test, only then is it an "issue"?

Subject to the aircraft being "safe/controllable to fly", I might suggest varying the CG e.g. add a passenger, or ballast, and see how it alters in terms of "feel" as well as the more formal test.

I do know someone who built an RV-6A, IO-320 and very light wooden prop, and he added a heavy flywheel, both for the wooden prop and the CG - and he has IO and a nosehweel over you - you have a tailwheel at a long arm he does not.

Andy

PS

PS What is a "FP" prop?

Click to expand...

One that isn't "C/S" i.e. 'Fixed Pitch'

Attitude when Weighing Tail?

My tail wheel weight was 58 lb with the cockpit rails level and the tail raised. It is heavier with the wheel on the ground because the height of the main gear causes the cg to move aft as the tail drops - at least as far as the weight on the wheels is concerned.

Total weight was 1021 lb with YO-360-A1A, FP metal prop and no paint. There was also a 10 lb difference between left and right mains.

Jim Sharkey
RV-6 Phase 2

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!!

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.

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.

Pax W&B

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

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!

tail/wing incidence and stability

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.

stability

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.

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.

Trailing edges

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.

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 ).

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.

NOT NORMAL!

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.

Kiwi said:

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.

Click to expand...

Making an Incidence Level

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

do you have the instruction manual and plans?

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.

Kevin Horton said:

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.

Click to expand...

Bad Sheila said:

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

Click to expand...

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.

Kiwi said:

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.

Click to expand...

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

Working the EXACT same problem with another plane

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.

L.Adamson said:

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

Click to expand...

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.

near zero friction in the pitch axis

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.

RV6 Horizontal Incidence Leveling

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.

billdianne said:

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?...

Click to expand...

You make a good point that control system friction can be very hard to quantify. I think that whether or not the elevator reliable falls under its own weight, and the ease with which it does so, is probably a reasonable measurement criteria.

I suppose that you could, as you suggest, use a fish scale to measure the "breakout" force required to overcome the friction. But without a more specialized measuring tool I bet that it would be very hard to establish repeatable measurements that way. If you could make such a machine, it would be interesting to bring it to RV get-togethers and take measurements from a bunch of different aircraft and see what the range is.

...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.

Click to expand...

I know that when I was helping Steve Smith build his RV-8 (see Steve's post elsewhere in this thread) we also went to great lengths to reduce the elevator friction to the lowest possible level. I know that we played around with the spacing at the center bearing and at the drive arms; we may have even made custom spacers for them on the lathe. We eventually got almost all of the friction out of the elevator system. It wasn't completely frictionless, but the elevator would easily fall to the nose-down stop when released.

I know that on the glider I'm developing now, I've gone to very great lengths to keep elevator friction down to the absolute minimum. With modern sailplanes, the elevators tend to be very small and the hinge moments very low, so any friction or slop will mask what little natural elevator feel there is. Of the nine pivots in my elevator control circuit, there are only two that don't use ball bearings. All five push-pull tube guides use linear ball bearings.

Thanks, Bob K.

-6 weight

My -6 with an O-320-D1A and metal sens prop, is 1067# @ 70.8" EWCG with a tail wheel weight of 71#.

RV6 CG

Wow, so mine is not as bad as yours for aft CG because of the wooden prop. I feel better already. Thanks for the input.