Some "trimming " questions

[Skip Chernoff]

I have noticed that some planes go into a seemingly nice loop then right before the bottom of the the loop the plane makes  a quick turn tightening up the radius of the loop with no apparent input from the pilot. What causes this and how do you get a plane to stop doing it?

Secondly, what is "hinging"? I've heard the term used,but don't know its meaning.Thanks,PhillySkip

[Brett Buck]

I have noticed that some planes go into a seemingly nice loop then right before the bottom of the the loop the plane makes  a quick turn tightening up the radius of the loop with no apparent input from the pilot. What causes this and how do you get a plane to stop doing it?

   Assuming you aren't actually putting in input when you don't think you are (because of a panic reflex), the issue is usually that the airplane is tail-heavy.

Secondly, what is "hinging"? I've heard the term used,but don't know its meaning.Thanks,PhillySkip

  Rolling in corners, it can roll away from you (you see the bottom of the airplane in insides and top on outsides) or in towards you (see the top of the airplane on insides, and bottom on outsides). Usually adjusted, to first approximation, by changing the tip weight. More tip weight if it rolls in, less if it rolls out.
 
     Brett

[Skip Chernoff]

Thank you. This why I dig Stunthanger. You ask a question and you get an answer from someone who knows their stuff!!!

[Tim Wescott]

Let us know how it works.  It's not a bad exercise to take a junker plane and experiment with the CG.  You can usually fly them over a much larger range than you can fly them well.

[Skip Chernoff]

I went flying yesterday with a freshly built Brodak P40 (kit not ARF) .I noticed that when inverted I could see the bottom of the outboard wing from inside the circle. I had 3/4 oz tip weight ,I'll try it again with 1 oz and see if I can get that wing level. The plane also has adjustable trim tab on the outside wing. I can crank in a "bit" of trim if needed. I stopped trimming the P40 at the field because the adjustable lead outs kept changing position. Gotta fix that!

[john e. holliday]

You sure it's tip weight problem and not a warp?  Does the wing fly out board down when up right?  Could be flaps not lined up if it has them.  I know on my Hobo I had flaps not lined up.

[Mark Scarborough]

I went flying yesterday with a freshly built Brodak P40 (kit not ARF) .I noticed that when inverted I could see the bottom of the outboard wing from inside the circle. I had 3/4 oz tip weight ,I'll try it again with 1 oz and see if I can get that wing level. The plane also has adjustable trim tab on the outside wing. I can crank in a "bit" of trim if needed. I stopped trimming the P40 at the field because the adjustable lead outs kept changing position. Gotta fix that!

Is this the profile , same design as the ARF,,,,?
did you put dihedral in the wing? this airplane was designed to have dihedryl, and without it it will tend to fly tip down upright, and tip up inverted,, this is a vertical CG issue not sure you can fix that with a trim tab, or tip weight for sure,,
anyway, good luck with it

[Brett Buck]

I went flying yesterday with a freshly built Brodak P40 (kit not ARF) .I noticed that when inverted I could see the bottom of the outboard wing from inside the circle. I had 3/4 oz tip weight ,I'll try it again with 1 oz and see if I can get that wing level. The plane also has adjustable trim tab on the outside wing. I can crank in a "bit" of trim if needed. I stopped trimming the P40 at the field because the adjustable lead outs kept changing position. Gotta fix that!

  If it is different between inverted and upright, that is a warp or something akin to it. Tip weight error will cause it to roll in or roll out the same way all the time, warps cause it to roll differently depending on the direction of load. Adding tipweight will reduce any tendency to roll in at you but it will enhance any tendency to roll out away from you.

   It is much more sensitive  to rolling in at you than rolling out. That's because roll is limited/controlled almost entirely by line tension. Line tension is the only substantial source of roll stability . If you roll in, it loses tension, which also reduces the roll restoring force, making it roll more. That means you can tolerate excess tip weight or an outboard roll far better than inadequate tip weight or an inboard roll. So you can handle too much tip weight better than you can handle too little.


   Of course, there are other, more subtle, effects due to the coupling between roll and yaw and for the more experienced, you really need to get the tip weight and these other effects correct. Howard can confirm the fact that casual passers-by can tell when the tip weight is off by as little as few grams.   But if you are going to make an error, it is much better to err on the side of too much rather than too little. So you always want to start with a new airplane with a bit more than you think it will take, just to cover your bases.

   Once you get going and get confidence, you should search out the correct amount of tip weight by doing slight adjustments (1/8 ounce) either side of where you think it is correct. It is particularly tempting to run excess tip weight because it gives you more tension in general. But too much, even a little bit, can cause very ill effects in the middle of the corners and make them very difficult to do accurately and precisely. I evaluate it on every single flight, and at this point (having flown two nearly identical airplanes for close to 20 years now) I know right away when I need to change and how much, but doing it experimentally every now and then doesn't hurt.

      Brett

[rich gorrill]

Mark, I was with Skip when he was flying the P-40, you say it should have dihedral in the wing, I have seen a couple of the ARF's and didn't notice any dihedral, how much is it supposed to have? I'll have to call Skip or hope he see's your post to check the plans and find out if and why it was omitted. One big thing we noticed was when the engine quit it really went tip down on the glide. Maybe his trim tab was out of adjustment.

Rich

[Mark Scarborough]

basically I recall it being around 3/4"
all it does is raise the leadouts to align with the vertical center of gravity.
Hang it by the leadouts and see if it hangs straight or if it hangs towards the landing gear.. ( meaning the leadouts are below the vert cg

[Tim Wescott]

 If it is different between inverted and upright, that is a warp or something akin to it.

Initially, you find this out by flying upright and noting whether the wings are tipped.  Then you fly inverted, ditto.  It's harder when you've got dihedral -- in that case it's probably better to make sure the thing sits level on the ground and go by whether the wheels line up.  If you can, hold the handle up so that you're sighting down the lines and look at the lay of the wing (or wheels).  Then do the same thing inverted.  If you can't fly with your hand up there, get a buddy to watch the plane in both attitudes when you're flying at five feet.

If the tip is up or down the same in both attitudes, it's the tip weight.  If the tip is up when you're upright and down when you're upside down, or visa-versa, it's a warp.  You're best off fixing the warp, second-best tweaking the flaps, and take that trim tab off (or nail it down if it's built into the wing).

[Skip Chernoff]

Thanks all for jumping into the discussion. The plane was built from the Brodak Profile P40B kit. I'll check the plans ,but don't think it shows dihedral.
 
Brett I added nose weight and it smoothed out the loops as you suggested. The trim tab I can fix to the neutral position

Rich, I replaced the original music wire push rod with a carbon fiber one and the controls were much smoother.

Tim and Mark ,The wing doesn't appear to be warped and the flaps line up nicely with each other. I'm going to experiment with the tip weight.

[Dennis Moritz]

 Vertical Cg issue can be compensated for by an outboard tab. Check with Danny and Mike.


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[Skip Chernoff]

Dennis the plane has a 6 inch (approx) aileron out beyond the flap on the outboard wing. It's attached to the wing with 3 copper  hinges that can be bent up or down for trim. I also attached a rod with clevis and a small horn so I can make and hold small adjustments. I don't get to fly with Palko much,but the next time I'm out there with Danny I'll have him take a look.

Dennis,if the weather is ok let's get some flying in next week.

[Skip Chernoff]

Mark I will check the vertical CG as you suggested.Thanks,PhillySkip

[Tim Wescott]

Vertical Cg issue can be compensated for by an outboard tab.

At exactly one ratio of outward acceleration and airspeed.  Get it right in level flight and it'll be wrong overhead.  And visa-versa.

But, if you can't fix the underlying problem, you can at least improve the situation with a tab or by tweaking the flaps.

[phil c]

I have noticed that some planes go into a seemingly nice loop then right before the bottom of the the loop the plane makes  a quick turn tightening up the radius of the loop with no apparent input from the pilot. What causes this and how do you get a plane to stop doing it?

Secondly, what is "hinging"? I've heard the term used,but don't know its meaning.Thanks,PhillySkip

I'd say that most likely you unconsciously stop following the plane with you hand.  The plane will respond to much less than a foot of movement between the plane vertical and the handle, if you are holding the handle vertically.  Ted Fancher pointed this out a long time ago.  When you start a loop you have to move your hand just enough for a 45deg loop, and then follow the plane around the loop and neutralize the controls exactly the right amount as it is hitting level flight.  The pullout is probably less critical than the entry.  The judges are likely to have a harder time seeing if the plane dips a few inches after 5 ft, as long as it is a slow, smooth recovery.

As pointed out though, there can be lots of other causes.

[Brett Buck]

Mark I will check the vertical CG as you suggested.Thanks,PhillySkip

   There's not much you can do about it at this point, just tweak it normally, and build the next one with the wing nearer the CG.

    Brett

[Tim Wescott]

   There's not much you can do about it at this point, just tweak it normally, and build the next one with the wing nearer the CG.

    Brett

Oh, there's always a way.  Although your suggestion is probably best at this point.

[Geoff Goodworth]

Bob Reeves and I ran into this dihedral issue when we were designing his profile Shoestring using the ARC/ARF P-40 wing.

The P-40 kit has dihedral on the wing—1/2" under each tip.

The ARC/ARF has no dihedral.

Also, one of the leadout rake calculators says that the mid-point between the leadouts should be 0.45" behind the CG. I don't know whether the leadout position will have any influence on the issues discussed above.

These examples were both built by my flying mate Dennis Percival.

[Skip Chernoff]

Geoff you are correct about the dihedral .I just checked the plans that were supplied with the plane. My plane does have the dihedral built into the wing.....I never noticed it.

[Dennis Moritz]

Skip. Yes let's fly. The ARFs have no dihedral. Tab compensates ok when trimmed right. Mike and Danny did this. Danny does very well that way. P40 is one of the best flying ARFs. Pat Johnson designed the kit and the ARF, I believe.  He recommends some dihedral. A half inch up or three quarters at the tips. Every one I saw was an ARF and straight. Mike's P40 ARF is electric. Turns very tight. Outboard tab bent down.


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[Dan Berry]

Put heavier wheels on it.

[Skip Chernoff]

Dan and Dennis,thanks for your input.....I'll try the heavier wheels next time out.

[Dan Berry]

Dan and Dennis,thanks for your input.....I'll try the heavier wheels next time out.

That's what I was advised for my Hurricane.

[Brett Buck]

Dan and Dennis,thanks for your input.....I'll try the heavier wheels next time out.

   Next time, use dihedral. Weight is not the end-all and be-all it used to be, but if you can avoid dead weight (i.e. weight that is there just to be heavy) you can put more weight here it might do you some good, like beefing up the structure. A tiny bit of dihedral to get the leadouts in the right place would be a much better in general and has minimal ill effects.

     We have done experiments where we added as much as 8 ounces - half-a-pound - of lead to a 35-sized airplane, with a tremendous improvement to the performance. But a much more productive use could have been made of that 8 ounces by making the wood thicker, the spars stiffer, etc. Of course, there are a lot of factors involved in determining what weight you want, and it's a lot easier to add it afterwards, than try to take it off, so you need to know what you are doing before you intentionally add structure.

     Brett

[Dan Berry]

Nobody like to add weight but if it won't fly without it there is little recourse.
Watching someone try to fly a tail-heavy plane because they hate to add nose weight is a case in point.

[Brett Buck]

Nobody like to add weight but if it won't fly without it there is little recourse.
Watching someone try to fly a tail-heavy plane because they hate to add nose weight is a case in point.

  Of course, but the post I was responding to was regarding what to do the next time. Once it's built, you are mostly stuck, and you just put on whatever weight is required. The case of tail-heavy, in particular, is my favorite example of the sort of insane and bizarre emphasis on weight as a performance issue. The late Jason Pearson built a Barnstormer with his first full car paint finish, and it wound up extremely tail-heavy. He tried to fly it anyway, and of course it was a disaster, although he did get it down safely. I told him he had to balance it where it needed to balance, so, grudgingly, he did it. It took *6 OUNCES* in the nose to get the CG right. He flew it and it flew much better as you would expect, with no real problems. He nonetheless hung the airplane up, because he *knew* that it was too heavy, despite the fact that it flew perfectly well.

   We have cut up airplanes (including an airplane that had already won the NATs) and fixed issues (and it later won the NATs AGAIN) when it was reasonably possible. Or rather, I talked Ted into doing it. David's first NATs-winning airplane has something like 4 saw kerfs in the rudder, where he progressively sliced mostly through with a Zona saw, shoved glue in the slot, and then bent it over to rmove rudder offset on his (non-adjustable) rudder. Fly it, see how it works, try some more. Many of those were done at the field, cut-and-try. Having the universal boon of Hot Stuff makes building and adjusting a much different deal than it was.

     In this case, you would more-or-less have to build a new airplane to add dihedral, and trying to lower the leadout guide is also probably impractical or impossible to get it to work without binding up. So, yes, do what is necessary now, and fix the issue more effectively on the next airplane. Note that this is a classic example of the sort of evolution needed to develop a design, you trim until you get to a problem that is effectively impossible to trim out, and then you devise a change to remove the limitation.

      Brett

[Skip Chernoff]

I'll do whatever it takes to make a plane fly better. I don't build them for display. I'll cut into them without thinking twice. Having said that, everything that I'm building now has hatches to allow me to make critical trim adjustments. The P40 in question here I didn't build,but in all fairness the builder he did a very nice job. Thanks ,Phillykip

[Skip Chernoff]

Here is a photo of the P40 in question...

[Tim Wescott]

I'll do whatever it takes to make a plane fly better.

Has anyone suggested measuring it for vertical CG problems, have you, and what were the results?  Sorry if I'm repeating something someone else has said -- the thread is long, and I'm stealing time from my morning routine to write this.

If you hang it by the leadouts and look at it nose- or tail-on, then it should show no roll.  This is easy to tell on a perfectly straight-winged plane, harder when there's taper, and a pain and a half when there's dihedral.  If you really feel you must you could hack the wing out and add in dihedral -- but if you include redoing that nice finish job, you're talking about nearly as much work as it'd take to build another one from scratch.

[Dennis Moritz]

Heavier wheels. Windy in one of his videos demos a similar method.


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[Skip Chernoff]

Tim, the plane does have the proper dihedral. I'm going to hang it up now and see what it does.

[Tim Wescott]

Tim, the plane does have the proper dihedral. I'm going to hang it up now and see what it does.

If the outside tip is straight below the inside tip (in roll -- you should expect it to be a bit in front in yaw), then all the worrying we've been throwing at you about vertical CG is just farting with consonants, and you have a garden-variety warp.

[Brett Buck]

If the outside tip is straight below the inside tip (in roll -- you should expect it to be a bit in front in yaw), then all the worrying we've been throwing at you about vertical CG is just farting with consonants, and you have a garden-variety warp.

  And in any case, tweaking it out is probably the right answer. As noted previously.

   Brett

[Skip Chernoff]

I checked the vertical cg as suggested and the plane "hung" straight down with the tips in good alignment.The nose was slightly down on the yaw access. When the weather breaks I'll trim the aileron (tab) on the outside wing and report back to you.

[Tim Wescott]

I checked the vertical cg as suggested and the plane "hung" straight down with the tips in good alignment.The nose was slightly down on the yaw access. When the weather breaks I'll trim the aileron (tab) on the outside wing and report back to you.

I think I mentioned this already, but I'm going to repeat it in more depth.

Aileron taps are great if your back is against the wall, but they do funny things to the lift/speed relationship, so they tend to be "one speed" devices.  This isn't as bad if your problem is a warp than if it's a vertical CG issue, because to a first order the roll moment from a warp and the roll moment from the tab follow each other.  However, because the tab concentrates a bunch of lift all at one point, and a warp distributes it over the whole wing, there's still going to be some funny second-order dependencies of the roll moment on the angle of attack -- meaning that if you get it adjusted for a sharp turn it'll be off in a round maneuver and in level, or visa-versa.

If you can, tweaking the flaps is better, because that distributes the roll moment over the whole wing.  It'll still do weird stuff: a warped up wing with tweaked flaps will still tend to want to contribute it's own personal touches to your pattern.  However, if the flap differential isn't more than 1/8 inch up or down in total (i.e., up 1/16 on one side while down 1/16 on the other), then you're probably good to go.

The very best thing, if your trim tab is straight but you need more flap tweak, is to find the warp and kill it.  It doesn't take much warp to do this, so you may not be able to see it.  I can look at a wing a dozen times and not see a warp, only to have one of my fellow competitors see it.  This would make me feel really bad, except that I've been in the position of being the guy saying "hey buddy, you're wing's warped, see?" to some pretty astute people, so I'm not crushed.  If I had a plane that was balanced vertically, and which needed a good deal of flap tweak, I would fix the damned warp even if I couldn't see it -- having both wings warped equally is probably MUCH better than having one warped plus flap tweak.

[Skip Chernoff]

I'll tweak the flaps if need be. At present I've got a Strega almost ready to go and a very nice Ares that's waiting for filler coat. I traded a Gieseke Nobler kit for this plane so I could have a profile to fly at Brodaks in June. If I can't trim the P40 into flying shape I'll give it to one of the "newbies" to learn on. I'm not sweating it. I'm pretty sure I can get her going.

[Mark Scarborough]

you know, if it comes to actually being a warp, why not remove the warp, there are ways

[Skip Chernoff]

I've checked it on both wing panels with my incidence meter at the root and out at the tips.Both wing panels are identical and straight as can be. Thanks,PhillySkip

[Tim Wescott]

I've checked it on both wing panels with my incidence meter at the root and out at the tips.Both wing panels are identical and straight as can be. Thanks,PhillySkip

The next time you feel motivated to flog the issue, check both panels in the middle of the half-span.  If one TE (or LE) has a wow in it the warp won't show if you only measure the roots and tips, but it will show in flight.  Or just sight down the LE and the TE on each wing -- if you've got a good eye and you know what you're looking for, you can see an incredibly small deviation.

[Brett Buck]

The next time you feel motivated to flog the issue, check both panels in the middle of the half-span.  If one TE (or LE) has a wow in it the warp won't show if you only measure the roots and tips, but it will show in flight.  Or just sight down the LE and the TE on each wing -- if you've got a good eye and you know what you're looking for, you can see an incredibly small deviation.

   Or, alternately, just tweak the flaps and move on.

     Brett

[Tim Wescott]

   Or, alternately, just tweak the flaps and move on.

Oh, now, where's the fun in that?

I could see myself, if I had such a plane in my possession, purposely not sighting down the LE and TE, because if I did see a warp I'd be consumed by the urge to fix the damned thing.

[Skip Chernoff]

After adjusting the outboard wing trim tab I got the P40 to fly level both  upright and inverted.

[Tim Wescott]

After adjusting the outboard wing trim tab I got the P40 to fly level both  upright and inverted.

The next step is to see how it flies in round insides and outsides, and then square insides and outsides.  If you're really lucky, it'll track dead straight to the lines at level, in the rounds, and in the squares.  If you're not, then you'll need to make some compromise involving the trim tab tweak, the tip weight, and the line rake.

[Skip Chernoff]

I'm waiting for a day that I can get in a few back to back flights. It's been cold and windy here. The P40 is getting better each time out.

[Mark Scarborough]

Oh, now, where's the fun in that?

I could see myself, if I had such a plane in my possession, purposely not sighting down the LE and TE, because if I did see a warp I'd be consumed by the urge to fix the damned thing.

Or to do things like take a flyable sacrificial airplane and totaly go overboard on "repairs" making if vie for the title of the NEW worlds most labor intensive arf flight streak

[Tim Wescott]

Or to do things like take a flyable sacrificial airplane and totaly go overboard on "repairs" making if vie for the title of the NEW worlds most labor intensive arf flight streak

Hey!  I resemble that remark!  That thing's on my bench now, ready for some 'coat so that it'll be flyable yet again.

[Brett Buck]

After adjusting the outboard wing trim tab I got the P40 to fly level both  upright and inverted.

    Very good.

     Brett

[Skip Chernoff]

Brett, I got a couple of flights in yesterday in the cold. I'm now focusing on the cg. Taking a little of the nose weight out at a time. The outside loops are gorgeous. I settled on 61ft lines eyelet to eyelet. I also found an 11x5 Xoar prop worked well on my La40 giving me 5.1 5.2 lap times. I did add a 1/4 oz more tip weight. It was windy so I'm not sure of the line tension overhead ,but suspect I'll need to move the lead outs rearward a bit. Thanks for your encouragement....can't wait to get to that Strega and Ares I've got sitting there waiting for filler coats....PhillySkip