Trim question

Ship is Skylark 56", foam wing, Silver Foxx 40, now up to 55 oz with tip weight and 1.5 oz tail weight and is still slightly nose heavy. 11.5 X 4 APC. Has around 20 trimming, shake down flights so far.

Doug Moon

It seems to me that in your method, success will be entirely dependant upon the relationship between the bellcrank center and the CG. My understanding is that it doesnt make any difference in the yaw trim where th bellcrank is. It depends upon the leadout position relative to the CG. If your method was correct, then we would have to move the bellcrank when we moved the CG.

Even if the leadouts turned a corner and went to the tail of the plane, as far as the flight trim, it just "thinks" forces are originating from the leadout position.

I'm not suggesting you dont get close your way--you've got WAY more experience than I--I'm just trying to understand the mechanics.

Ive done it in a similar way but dont let the leadouts "float"--get it so it hangs just a few degrees "Out" from vertical.

Am I all confused here??

Thnaks

No you are not confused at all. It is just that 90% of all stunters have the bell crank pretty darn close to the highpoint of the wing. Not way in the back or way out in front. In one of these configurations then the suggested method would not work. On a conventional design however it just gives you a real good starting point based on the cg in that verticle position at the time of the maiden voyage. I have used this method for several years now and it works well on conventional designs. Because the bellcrank is pretty much at the highpoint or a little aft on the wing. So having them in a straight line to start with usually puts it right in the ballpark. Really alot closer than that. To answer your first question. No you are not confused and yes thay all work together. It is just an easy way to bench trim it in the beginning.

Doug Moon

I reread the post again. If you had a stunter with the bellcrank in a position way out of the center then you would not loosen the leadouts and let it flow. You would simply hang it from the leadouts and make sure that the Trailing edge of the wing is almost 90 degrees to the ground. You would move the guides until you got it close. Take a well trimmed airplane and hanging up. You will see that it is really close to 90 degrees not quite but close. so you would be trimming for that slight angle which is what you need to be looking for anyway.

Hope this helps.

D

Thank you Doug--I was worried there for a minute!

In level flight with no up or down bias, the center point between the leadouts aligns with the CG of the model determining the yaw out or in of the nose. This is because both lines equally share the load/pull of the model. In the inside maneuvers, more of the load/pull is transfered to the up line. This moves the effective leadout exit point toward the up line exit. The model will then yaw to maintain the CG alignment with the effective leadout exit point. The further appart the leadouts the more yawing during maneuvers. If the up line is the forward line, the model will tend to yaw into the circle during inside maneuvers. If the lines are widely spaced the yaw will be more apparent. As the speed of the model decreases this yaw will also be more apparent (the top of the inside square and inside loops).

Another force that can have an effect during inside and outside maneuvers is the gyroscopic effect of the rotating prop. As a model turns inside maneuvers the nose of the model will experience an outward yaw. Outside maneuvers have a gyroscopically induced inward yaw. Positioning the up line forward (reversing the bellcrank ) can balance out yaw during maneuvers. Finding the right position for the up and down line is the trick. In general start by adjusting the position as others described above while hanging from both leadouts the model fuselage should be horizontal. And, I would add, keep the leadouts as close together as possible. Then adjust to flying conditions and preference.

Your mileage will vary
Bruce Hunt

This is also a good indicator of too much rudder. There is a happy medium.

My new fuselage for my Time Machine based plane is very stable (yaw wise) in the wind. Better than the previous fuse, which had a very large rudder. This may take a few iterations to achieve a happy medium on an unproven design.

By the way, Doug is VERY SHARP. His airplanes fly very well, across the board. He is very good trimmer. He only appears to be a dolt on the surface (GOTCHA)!

My advice. If you need large amounts of tip weight and line rake, try another prop or more nitro in your fuel (or a better-or bigger engine).

Two questions.

Are the hingelines sealed?

Where are the leadouts in relation to the CG?

Bruce-Very good information. Front leadout is up line, leadout spacing 3/4"

Brad-Best I can tell wheels look good in all conditions.Wish you could have made the Tulsa contest. As above, perfect weather and very well attended and very competitive in expert, intermediate and beginner as well as classic and old time. Again,I sure wish I could make the Dallas stunt/trim clinic.

Ted-Interesting thought on hingeline which is/was tight but not sealed. I'll definitely tape the hingelines and try to detect any differences. CG is now, with additional tail weight, even with or perhaps 1/8" in front of CG.

>...CG is now, with additional tail weight,
>even with or perhaps 1/8" in front of CG.

Are you referring to CG location on plans, or are you referencing Ted's question and stating location of leadouts in reference to CG?

We have used some hinges that broke, also. Nice and thin, but then, thin makes them nice and brittle. We are currently using DuBros with nary a failure, but do use an adequate amount with double hinges at the tip.

I do like the barrel type better than those flex kinds. "They" say they will take a million flexes. You and I know nobody counted on flight pressures when they did the testing.

Leonard Neumann

Leonard Neumann

Yup, Leonard pretty much caught what I would have mentioned. My personal feeling is that you might want the center-point of the leadouts at least an inch (but less than 1.25") aft of the CG.

Re the sealed hinge-line,I can only repeat Brett's oft quoted comment on "tight" hingelines..."I don't care how small the gap is, it's still bigger than an air molecule. I've said this approximately 12,472 times in the last couple of decades but it still bears repeating. Sealing hingelines can do nothing bad and can cure a plethora of trim problems. No stunt ship should see the air without this being done. I think it should be part of the construction process...generally the last step, of course!

The only caveat to the foregoing is that you must be very careful when taping the seals that you don't stiffen the control freedom. Tape only between the hinges and tuck each individual section well into the hingeline. Test the control action after each section is added and remove and redo if any restriction is felt.

One other question, do you know where the CG is relative to the average chord? In other words, balance the ship on a couple of finger tips with one finger halfway out each wing. Mark that point with a dot of ink. Now measure how far back of the leading edge that dot is and, also, how long the chord of the wing is at that same station from the leading edge to the trailing edge (yes, including the flap). The chord length will likely be around 11 or so inches at that point. The point at which the ship balances should be about 20% of whatever that length is...2.20" if the chord is eleven inches, for instance.

The technical aviation term for what you've determined is "the location of the Center of Gravity in percent MAC (Mean Aerodynamic Chord). A very good rule of thumb for basic "first flight trim" is to have the CG in %MAC at a point equal to the ratio of the area of the total tail (stab and elevator) to the area of the total wing (again, including flaps). For instance, if the wing area is 600 Square inches and the total tail area is 120 Square inches the tail is 20% of the wing and a good starting point for the CG will be at 20%MAC.

Check and see if your CG is roughly in the right area (within a couple of percent) and then make sure the center point of the leadouts are about an inch to an inch and a quarter aft of that. If you've a reasonable amount of tip weight and the structure is straight and of reasonable weight your basic flight trim is within a fraction of where you're at.

Other things are certainly important (especially the powertrain set-up and line length and lap times) but these fundamentals should never be grossly exceeded. If you have, you've done something else wrong.

Let us know how the Skylark shapes up.

My apologizes for not attending the Tulsa contest. I had every intention of attending with THE BIG GREEN U-ASS-A-MING with the new PA 65 onboard. Alas, it was not to be. I ruptured a tendon in my right bicep on Thursday, and as you may guess my flying for the rest of the year was in doubt. However, as of today I am back in the saddle, and I am actually flying a little BETTER, as I cannot move my arm much! Apparently, too much arm movement has had some effect on my shapes, as I have seen some immediate improvement. My technique is now very much with my arm cocked and directly in front of me at all times, (more like Werwage) as I am afraid to extend my arm.

I actually felt a little stupid with my previous reply and wish I had not said anything, as I felt my advice was weak and erroneous. I will, however, offer this NEW piece of advice.

Too much tip weight will cause the 45 elevation to be very light in a 90 degree corner both right side up and inverted. This is cause by the outside wingtip OVERFLYING the corner and causing an inward yaw. If you added tip weight recently and saw some effect (you said ½ oz) I would try going the other way from your starting point.

Also, try some nose weight, and readjust you handle to get the corner back. A rearward balance point can cause light tension at 45 degrees (and everywhere else). Nose weight is not a bad thing, but it takes a lot to make difference. A very rearward CG requires exceptional trimming (Brett, Ted, Dave settle down now…). A little nose heavy to start makes up for power and is a little more forgiving. You can always take it out later when your confidence improves.

As for Dallas this weekend I SMELL GOLD! I am MOON-ERIZING my pattern, you see. I have probably picked up at least 2-3 points overall. I figure my improvement ratio is 1 point per 6 gallons of fuel.

Just reread this entire thread and can't believe I didn't ask this earlier.

How long are your lines, what diameter and what are your usual and preferred lap times?

>...Also, try some nose weight, and
>readjust you handle to get
>the corner back. A
>rearward balance point can cause
>light tension at 45 degrees
>(and everywhere else). Nose
>weight is not a bad
>thing, but it takes a
>lot to make difference.
>A very rearward CG requires
>exceptional trimming (Brett, Ted, Dave
>settle down now…). A
>little nose heavy to start
>makes up for power and
>is a little more forgiving.
> You can always take
>it out later when your
>confidence improves.

This is an area where I get a little "jumpy", too. There is a certain point to locate the CG where the plane is going to fly best. There is no reason to move the CG forward of this point. What Ted posted as a "rule of thumb" for first flights is very true. Set your CG back from the leading edge of the average cord of the wing about the same percentage as the tail area is to the wing area.

On a straight wing, measure anywhere. On a tapered wing, generally measure at a point about halfway between the fuselage and the wing tip. Wings like a Mustang with its added root section, or the Spitfire with its eliptical wing will be slightly off, but you will still be close. You will NOT get it too far back doing this, and you will NOT lose tension doing this. (Assuming you have done your pretrim correctly in other areas as well.)

People talk about moving the CG back later when your confidence improves. The problem with this is, if the CG is too far forward, the plane won't turn. With the CG properly placed you can narrow the handle spacing and then widen it as your confidence improves without changing the trim on the airplane. But we need to get the plane into its best trim possible as soon as possible, and that won't happen when it is too nose heavy.

How many planes have been lost because people tried to do a loop that it just wouldn't do because it was too nose heavy? When the CG is in the RIGHT place, it is much easier to fly.

Leonard Neumann

I have been reluctant to move leadouts backward as I have good to heavy line tension in level flight and hear bad things about leadouts too far back. This is an excellent post by both Ted and Leonard in that it gives inexperienced trimmers who are not really sure what they are observing a formula for a starting point and adjustments with adjustable leadouts with varying CG's etc. than original plans.

By the way, I initially started with 62' center to handle .018's but have gone to 63' .015's. Lap times with what I percieve to be an ideal run, a little gurgleing with plenty of smoke in level flight but definite leaning and power surge when nose goes up, runs 5.2 to 5.4 with 11.5 X 4 which seems very comfortable. 11 X 5 produced some runaways early in trimming process and have not tried them since.

I'll fix the flap, move the leadouts back and try to get it in the air again as soon as I can-weather forecast is for high winds for the next few days with another front passing early in the weekend-and will definitely report results-Thanks

Jim

What Ted posted
>as a "rule of thumb"
>for first flights is very
>true. Set your CG
>back of the center of
>lift about the same percentage
>as the tail area is
>to the wing area.
>
>
>
>Leonard Neumann

A question here...I have been following this closely but noticed an apparent discrepency. Leonard indicated locating as a percentage aft of center of Lift. I think Ted indicates aft from LE.

I suspect that (But not sure) the answer is placing the CG some percentage FWD (if I remember correctly)of the Center of lift ..but the lift center is difficult to precisely locate. It also moves around with angle of attack.

Was this just a typo? or are you coming you comming at the problem from different ends?

What is the "rule of thumb" for location of CG related to center of lift? does the center include the tail surfaces?

How does the rough location of the CG, per Ted's percentage method, allow for or get modified by, various tail moment lengths?

Just wondering....

Curt

Was this just a

The cg should be located a percentage back of the leading edge of the average cord. On a straight wing, this is no problem. Measure anywhere. On a tapered wing, measure half way out. If you have an eliptical wing or some such, you are on your own--guess! (Half way out to the tip on most wings will get you close, and this is a starting point, after all.

Thanks for correcting me. I don't want to lead people astray (which my goof definitely would have).

Leonard Neumann