Author Topic: Trim issue (Read 2787 times)

Brian Massey · « **on:** September 23, 2009, 10:58:27 AM »

I have just finished rebuilding a 40 year old Magician. It started out as a "rebuild", but all that's left of the original is the fuse and rudder. The new wing is about 1/4" thicker that stock, and I moved the stab/elevator back about 2 inched. The maiden flight showed the plane flew well, and turned amazing corners . . . with one "but".

When I give it "down" control, the outboard wing comes up; when I give it "up" control, I can see the bottom of the outboard wing (it goes down). I built the wing one rib longer on the inboard side, per the plans, but now wonder if the extra flap area is causing the problem.

Any thoughts?

Brian

Joe Gilbert · « **Reply #1 on:** September 23, 2009, 11:15:04 AM »

With the wing longer on inboard there is issues but check you tip weight that will cause it to do what you are seeing also.

Randy Powell · « **Reply #2 on:** September 23, 2009, 01:48:58 PM »

If it's moving one direction on inside control and the other on outside, it's likely a warp. If the wing moves the same direction on both inside and outside control (up or down) then it's a tip weight issue (or unbalanced lift).

Matt Colan · « **Reply #3 on:** September 23, 2009, 01:58:14 PM »

Check your tip weight, you may have too much in there. A warp in the wing could also cause it.

Brian Massey · « **Reply #4 on:** September 23, 2009, 02:07:58 PM »

Hi Randy; The wing did develop a warp while the silkspan was shrinking, but as best as I can "eyeball" it, the steaming process took it out. The plane flies straight and true in level flight; right side up or inverted. It only rolls on it's axis when given up or down control. I'll try removing some of the tip weight first and see what happens. Gary Anderson was watching, and he felt that the additional flap area on the inboard wing might be the culprit.

Brian

Dean Pappas · « **Reply #5 on:** September 23, 2009, 04:00:20 PM »

Hi Brian,
It probably is.
First make sure that the tip weight is right in both upright and inverted level flight.
If the tip banging is minor, then you can consider removing tip weight, but I don't think this is a good idea if the step above was done carefully.
Then trim the inboard flap (especially near the tip) to adjust out the problem.
You might even consider cutting the extra rib-bay worth of flap length at the tip from the rest of the flap and fixing it at neutral right off the bat. Make that neutral position bend-adjustable with a thick aluminum hinge and you are verging on high-tech!
Regards,
Dean Pappas

Ted Fancher · « **Reply #6 on:** September 23, 2009, 06:05:02 PM »

Hi Guys,

What Brian is describing sounds like a classic hinging situation which is very common on profiles with longer inboard wings (and the Magician has plenty of longer inboard wing. Almost certainly, no wingtip weight beyond what is there naturally from the engine and tank being centered about three inches inboard of the "center" of the wing is necessary.

We've also not asked one very important question when you're talking about Magicians. That question is: Brian, are the "flaps" fixed or are they movable coupled to the elevator? Many Magicians have been built with both options.

The underlying problem that causes "hinging" (dropping the outboard wing in insides popping up on outsides) is the result of the Center of Gravity being outboard of the spanwise location of the Aerodyamic Center of the wing. On a small wing like the Magician the two or three inches of extra wing on the inside is far more than required to balance out the airspeed difference due to flying on the tether. When you combine large amounts of excessive wing offset with a heavy engine and tank mounted outboard of the centerline of the already offset fuselage "hinging" is predictable and often in large amounts which is what I believe Brian is seeing.

Airplanes with excessive wing offset are very difficult to trim to keep the wings level in maneuvering--but it can be done if you're ruthless with your willingness to make it right.

One thing we pretty much know for sure. The underlying reason for your problem is almost certainly the arm between the CG and the AC (just think of AC as the spanwise "center" of lift of the entire wing--tip to tip). This is why it shows up dramatically in manevuers; because you're pulling lots of "g's" which act at the center of gravity. If you pull 10 g's (not a lot for a controliner) on your 2.5 pound airplane that is now the equivalent of setting 25 pounds lets say three inches outboard of where the lift supporting it is acting. This "will" make the airplane roll away from you on insides and toward you on outsides (speaking of the reactions in level flight).

The secret to eliminating the roll is easy. You simply move the AC and the CG closer together. That's pretty much the whole reason for adjustable tip weight boxes: to allow the pilot to fine tune that relationship. A well tuned stunt will have anywhere from zero roll in manevuers to just a bit of what you're experiencing a lot of. A tiny bit isn't harmful because it actually vectors the lift the wing is producing away from the pilot and can help just a little with line tension in difficult conditions such as dead calm air.

This (moving the AC and CG closer together) will be the ultimate solution whether you have fixed or movable flaps. The manner in which it may be "fixed" can be different based on having the movable flaps available as a tool to help the trim process or not.

There's lots of really good stuff to be learned about stunt from this discussion. It'll be worth paying attention to.

Speaking of really good stuff, by the way, Brian's real lucky to have caught Dean Pappas' attention with his question. He's one of modelings most aware and innovative guys. I've just finished reading the latest installment of his column in Model Aviation where he's talking about Neutral Points, Centers of Gravity and wing and tail Aspect Ratios. He's speaking primarily "about" RC stuff (where he's also highly thought of) but everything he talks about is germane to our CL stunters as well--especially because all those items act directly on the manner in which an airplane responds to pitch commands; which is the holy grail of stunt.

I'm kicking myself because I've been a little out of touch the last year or so with home projects and other interests and appear to have missed the first installments because I haven't been reading Model Aviation cover to cover for some time. If Dean was really nice he'd fire off a draft of them to me so I could "check him out". Only half kidding, Dean. I loved the latest article not only because it is hugely informative but also because it echoes some things I've published in the past and been given a hard time about from some quarters. There are few things more important (none that I can think of that are "MORE" important) aerodynamically for maneuvering airplanes than the subject matter of this months article. It's a dandy.

Ted FAncher

Howard Rush · « **Reply #7 on:** September 23, 2009, 07:03:54 PM »

Brian,

It looks like you live a few blocks from the biggest stunt contest in the West. Bring the airplane to Clovis Oct. 17 or 18, and we'll fiddle with it.

John Sunderland · « **Reply #8 on:** September 23, 2009, 08:50:51 PM »

Aside from the possibility of a warp, my guess is that Ted is dead on. If you need to repair this problem I would consider splicing in an extra outboard rib at only half one rib bay in outboard span, one inch I believe, and the assymetry issue will disappear for the most part, re-attach the tip, recover and then try it out. My Magician hinges a little but only due to the 1oz tip weight. 3/4 oz of tip weight should be just about right. I was confident one oz was good and glued it in permanently. I was wrong but never expected it garner as many cheap placks as it has.

Also as this is a classic era design, an adjustable LO wouldnt hurt if you dont have one already. The original layout is Ok for Fox or OS35 but anything heavier will require a move forward toward the LE. Constant cord designs can fly very well but CG can be an issue with modern power. If the LO position is off with a nose heavy condition combined with this asymetry as designed, you have problems of a multiple nature. I left the stab location as per the plans, skipped the movable flaps and glued them stationary. The addition of moving the stab back a few inches would be good if not for the extra kick you are probably getting combined with the excessive assymetry, possibly fast controls. Mine turns on a dime and is fairly stock externally. It has however, upset quite a few of my fellow contestants with there superior stunt designs in hand. And they are superior, but straight true and trimmed goes a long way in good conditions!

A combination of these things is probably causing your problem with the unnecessary assymetry as the main concern. Besides, you are going to get the best trim advice available and in person. Use it all!

john e. holliday · « **Reply #9 on:** September 24, 2009, 08:15:12 AM »

John; a fellow flyer my the name of Jim Kraft keeps blowing us away with a Magician powered with RedHead McCoy .35. As you said a well trimmed plane and practice go a long way. DOC Holliday

Brian Massey · « **Reply #10 on:** September 24, 2009, 09:24:23 AM »

Hi All; thanks for so many responses!

Here is some more info on the plane:

The flaps are movable, coupled to the elevator;

There is tip weight; but I'm not sure of the exact amount. I took the wing tips from the old plane and used them. There looked to be about an ounce of lead on the old tip.

I did install adjustable leadouts. I have never used them before and wanted to experiment. They are currently set about where the plans called for the leadouts to be.

I'll be flying again Sunday and will remove the tip weight prior to then. I like the idea of "fixing" a portion of the inboard flap; if the tip weight doesn't correct the issue, the flap will be next.

Has the Golden State Championships date and place been set for sure?? I have not read anywhere that they are a "go" for sure. If so, I'll probably enter, but only so others will feel good about their skills.

This Magician we're discussing is the plane I flew in competitions back in the '60's; I took one 3rd place at a contest at the Sepulveda Basin in '68 or '69; that was and is my high water mark. If entered again, no one has to fear appearance points.

Brian

Howard Rush · « **Reply #11 on:** September 24, 2009, 11:32:36 AM »

North Clovis Educational Center, Fresno, on October 17-18.

www.aeromaniacs.com

Howard Rush · « **Reply #12 on:** September 24, 2009, 11:34:58 AM »

If you don't have an adjustable tip weight box, you can stick modeling clay on the outside of the airplane. I'll have clay and a scale at the contest.

Brian Massey · « **Reply #13 on:** September 24, 2009, 05:01:26 PM »

Well the deed is done; at least the first deed. I removed .7 ounces from the outbopard tip; which was all the weight.

I'll fly Sunday and we'll see what happens.

Ted; I was impressed that YOU resonded to this post, now to find that YOU are impressed that Dean Pappas chimed in . . . well, I'm really impressed now.

I've heard through the grapevine you are a good flyer, but Dean I'm not that familiar with. I tried (and will try again) to read his latest article, but I'm one of those "Mathimatically Challenged" that really likes plain (plane?) english. Dean is way over my head. Having said that; Dean, you need to understand that alot is way over my head; at least according to my wife.

Brian

Bill Little · « **Reply #14 on:** September 25, 2009, 12:30:34 AM »

Hi Brian,

I am hoping Dean, and Ted will see this as I have a question about your problem for them: How about a wart on the outboard flap? This would increase the flap area and should help neutralize trhe problem. Or am I missing something? (it is very late here right now.

) I have seen temporary tabs added and adjusted to get the desired results then affixed permanently. I especially like the clear plexiglass ones.

Sounds like the habit of building in more outboard flap area from the get go helps alleviate the problem. (

)

Big Bear

Brian Massey · « **Reply #15 on:** September 25, 2009, 08:02:07 AM »

Hey Big Bear;

I kinda like that idea. I may try adding a severeal sq in tab on the outboard flap before trimming the inboard side.

I hope Ted and Dean add their .02 cents worth.

Brian

Ted Fancher · « **Reply #16 on:** September 25, 2009, 08:12:59 PM »

Let me answer that in just a few words.

More of my competition ships have ended up with a "wart" on the outboard flap than haven't.

(These words don't count) The ship that I took to Shanghai for the 1994 World Champs had an adjustable "wart" that slid in and out of the flap. Mine are generally about two to three inches long and a half inch or so wide. It isn't rocket science. It's all part of the balancing act of CG and AC. The tab (or additional area built into the outboard flap) allows the CG to be a little further out and the greater flap area on the outboard side keeps the wing from hinging. This allows the a little extra tip weight to be carried which can be helpful with line tension in some maneuvers. Most all of mine have been added after the fact because I felt during the trimming process that the airplane was solider with a little more tip weight which cause some modest hinging. The use of the tab allowed me to keep the tip weight and eliminate the hinging.

Ted

Brian Massey · « **Reply #17 on:** September 25, 2009, 09:18:53 PM »

Hi Ted;

Thanks for your .02 cents worth; I have a pretty good mental picture of what you're saying, but if you have a digital picture that would help me a lot!

Brian

Bill Little · « **Reply #18 on:** September 28, 2009, 08:07:18 AM »

Thanks for the reply, Ted.

Bill

Ward Van Duzer · « **Reply #19 on:** September 28, 2009, 10:06:41 AM »

Brian,

Ted's "wart" is a trim tab on the outside end of the outboard flap that's not bent for trimming. i.e. it's straight!

They sure do work.

Ted: I love the idea of an ADJUSTABLE "wart"! Thanks...

Ward

keith varley · « **Reply #20 on:** September 28, 2009, 01:55:22 PM »

I love Ted's description of how adding tip weight increases line tension because the resultant hinging, vectors the lift away from the pilot. Now follow me as I describe a hypothetical case.
If I add tip weight ,to get line tension, then I add a wart to get rid of the hinging that results , What have I actually gained? Perhaps just a heavier plane and a stylish wart.Is there some way that the added tip weight helps line tension apart from the hinging/vectoring process. Keith Varley

Ted Fancher · « **Reply #21 on:** September 28, 2009, 04:15:35 PM »

Quote from: keith varley on September 28, 2009, 01:55:22 PM

I love Ted's description of how adding tip weight increases line tension because the resultant hinging, vectors the lift away from the pilot. Now follow me as I describe a hypothetical case.
If I add tip weight ,to get line tension, then I add a wart to get rid of the hinging that results , What have I actually gained? Perhaps just a heavier plane and a stylish wart.Is there some way that the added tip weight helps line tension apart from the hinging/vectoring process. Keith Varley

Hi Keith,

Bet you thought you got me with that one, didn't you!

Loss of line tension is often (usually) associated with a loss in speed. We try to avoid it as much as possible but it does happen despite all of our efforts. When the CG is displaced spanwise from the thrust line there is a moment produced between the two whenever the airplane accelerates. Thus the further outboard of the thrust line the CG is located the greater that moment will be to direct the airplane toward the end of the lines restoring the tension lost when the airplane slowed down. One of the biggest reasons that equal panel wings are very popular is because they require a goodly amount of tip weight to counteract the higher speed of the outboard panel. That weight moves the CG outboard from the thrust line which is helpful not only in helping with line tension in maneuvers but also has a dramatic effect on the ability to maintain line tension during the launch on windy days.

The greatest acceleration the ship ever experiences is right at launch. With the CG outboard Mother Nature provides additional tension during the launch when there is little or no pull on the lines due to centrifugal force. The CG wants to lag behind the thrust line which will cause some yaw to the right creating tension. In addition, the yaw to the right vectors the thrust away from the pilot thus compounding the effect.

It's a really good idea to have the CG outboard of the thrust line and if you utilize a longer inboard wing it is worth paying attention to.

With the CG inboard of the thrust line the opposite effect takes place and can be quite disturbing. Think of an All American, Sr on takeoff and its well earned reputation for wanting to come in on the pilot. Because the AA, Sr. has that three inch longer inboard wing to get it properly balanced for flight the spanwise CG has to be somewhat inboard of the thrustline. The net effect of that is an inward yaw on take off that can be truly disturbing. The only appropriate fix for the AA, Sr. built to spec is to offset the engine enough that the thrust line (from the top view) passes through or inboard of the CG.

It's a real effect and well worth considering in the design or trim phase. I use some wing assymetry in all of my designs but am opposed to significant engine offset. As a result many if not most of my "serious" stunt ships end up with the wart.

Anyway, I think that's right.

Ted

keith varley · « **Reply #22 on:** September 28, 2009, 05:18:09 PM »

Thanks ,Ted. That was very well wiggled out out of .Anyone can see that a lot of acceleration takes place at the moment of launch. After thinking it over , and realizing that despite our attempts to provide a one speed flight ,with our huge lumbering engines, there still has to be some slowing at corners and during climbing.For instance during a wingover , as the plane is reaching the zenith , it must accelerate to gain any speed that was lost during the initial climb. Also after the second turn of a square loop ,for instance, I can see perfectly well that the original momentum is gone. It is good to know that the CG being outboard of the thrust line is providing some extra line tension. Never realized that before ,or thought of it at all, and it is comforting. Keith