Broadway Bod Busters

Trimming a control-line combat plane

By Don McKay
Edited by Sarah Watke with contributions from Jeff Rein and others who remain anonymous.


This is written for Control Line combat-type planes, but the physics apply to other types of control-line airplanes.

When you buy or build an airplane you make choices. This article will help you understand the impact of some of those choices. Because we fly various types of models, not all of the issues discussed will apply to each type of airplane. Types of combat planes include: Vintage Diesel combat (D-Bat) with built-up construction, Fast Foamies and composites, FASI ready-to-fly (F2D RTF) foamies or built-up, and 1/2A types.

For Combat, you need to build and trim the plane so it will do at least three inside and three outside tight loops, at "full throw" into the wind, while keeping line tension, and the airplane should not "stall." Additionally the in Fast and 80MPH events, the fuel shutoff must remain open.

You need to trim the plane for "full throw" because it is natural in the heat of combat to over-control the plane by flying using full-up and/or full-down. Many times at a contest someone (including me) says something like, "It flew fine when I tested it, then bad in the match." That was because I was flying lock-to-lock in the match. If the plane does not fly well upwind, the shutoff will burp or kill the engine. An example of this, I set up my plane for a Nelson 36 that flies at 120 MPH, then changed to an OS Max 25 for a 80MPH contest (both engines weigh close to the same) and found out that, with less power, the plane handles very differently and tripped the shutoff.

Things that affect the plane's capability are a complex combination of:

  1. Wing Warps
  2. Balance Point or Center of Gravity (CG)
  3. Wingtip Weight
  4. Line Leadouts (rake) Sweep Back
  5. Elevator Control Throw
  6. Size of Elevator
  7. Static and Turning Mass
  8. Wing Area and Offset
  9. Engine Offset
  10. Alignment of Engine with Wing
  11. Drag
  12. Airfoil

1. Wing Warps, or the lack thereof, is very important, so you need to fix these first before adjusting the other items listed.

Wing Warps (wing not being straight or being twisted) are a main reason a plane does not fly well. A warped wing will cause the plane to fly level with the outboard wing "Tip Up" or "Tip Down" and can cause the lines to go slack during loops-usually one way or the other but not both. The warp acts like an aileron set in the wrong position. Warps must be fixed before adjusting anything else and before trying tight loops, or you could end up with slack lines and the plane "coming in" at you.

Visually inspect the wing for warps and correct any warps before you ever fly the plane. A good way to do this is to prop up the plane to view, from 10 feet away, the relationship of the trailing to leading edge. Another method is to place the plane on a "known" flat, level surface. Using the same height spacers, prop up the wing tips. Make sure the leading edge is parallel to the surface. Then measure the distances from the flat surface to the top of the trailing edge at identical locations. There should be no measurable difference.

In either case, fine tuning needs to be done by flying the plane right side up, then inverted, and looking at the attitude of the plane. Usually the plane will fly with the outboard tip slightly "tip up" or "tip down". This is caused mostly by small warps that cannot always be seen through the visual methods above.

I mark on the inside wing tip with a "Sharpie Marker" what needs to be done to make the plane better when I get back to the shop. Then I mark what I think I fixed, so I can check it again the next time out. You can clean up your Sharpie notes with rubbing alcohol.

The best way to eliminate the warp (twist) is to twist the wing in the opposite direction of its warp. While holding the plane in this reverse warp position, reheat the covering on the top and bottom.

Airplanes built with foam trailing edges tend to get warped easily but are easier to fix, in the field, using the "Russian Warp" method. If you squeeze foam it compresses, and about 70% of the compressed foam stays when released. To do a "Russian Warp", sandwich the foam trailing edge between your thumb and fingers, about one half to one-inch back from the trailing edge. Then, while squeezing, slide your fingers down the trailing edge toward the wingtip, reshaping the foam to correct for a warp.

Whatever method you use to correct for warps, they usually require you to over-correct a little, because the reverse warp you put in will straighten out slightly right away and more over time.

Warps can recur over time so always fine-tune your planes prior to each contest. Watch how you transport and store your planes to prevent warps. If you leave your plane out in the hot sun, the plane can warp from the heat.

At the field you can correct minor warps by taping or gluing aluminum trims tabs on the wing tips. These trim tabs can be cut from a beverage can. You can also get a 1000-watt DC/AC converter that connects to a car battery or lighter outlet. With this converter, you can operate most covering irons and make corrections in the field.

2. Balance Point or Center of Gravity (CG)

The balance point or CG is very important. The farther forward of the airfoil highpoint you balance the plane, the more stable the plane flies, but the less responsive it is to control. Nor does such a plane turn as well as a properly trimmed plane. You never want to have a plane balance behind the high point of the airfoil as it will be very unstable.

The more nose-heavy your plane is, the more control (surface and degrees) it takes to turn the plane. The larger control surfaces and the more degrees of throw you use, the more drag is caused, and that slows your plane down in maneuvers.

A 1/8 difference in CG can be noticeable in the handling characteristics.

Since the tank or bladder starts out full and ends up empty, it should be placed on the CG to avoid changing the CG as the tank empties.

Because the engine is the heaviest part of the airplane, it gives you the greatest flexibility in setting the CG. It is easiest to build a slightly nose-heavy plane and add a little weight to the back of the plane. Because of leverage, if a plane is "tail-heavy" it takes a lot more weight added to the shorter nose to make it right than just adding a little to the tail. If your plane is a little tail-heavy, you can also take weight off the tail by cutting down the elevator, but that can cause other problems. You can also move the elevator forward or otherwise lighten the back of the plane.

3. Wingtip Weight

The outside wing should be built to weigh a little more than the inside wing. The outboard wing takes more abuse because of hard landing and crashes than the inside wing so it works to overbuild the outboard wing stronger and heavier. For a built-up wing, weigh each rib. Starting at the outboard tip, put the heaviest rib first, then work your way to the inboard using the lighter parts.

Too much wingtip weight can cause the plane's outboard wing to stall in turns and can cause wobble or yawing.

Coins work well as wingtip weight or to correct a nose-heavy airplane because coins are consistent in weight. Place weight on the CG unless CG adjustment is needed. Weight is often added temporarily to counter the effects of extra windy conditions. At the field weight can be added with tape to the outboard wingtip. This is one of the easiest things to adjust quickly in the field and easiest to undo.

1 ounce (US) = 28.35 grams

Coins weights in grams:
10 cents -- Dime -- 2.2 Grams
1 cent -- Penny -- 2.6 Grams
5 cents -- Nickel -- 4.8 Grams
25 cents -- Quarter -- 5.7 Grams

4. Line/Leadouts Sweep Back (rake)

Line/Leadouts sweep back (rake) lets the plane fly pointing out of the circle slightly. More sweepback adds more line tension.

Line Sweep on planes with internal controls is hard to change once the plane is built and finished, so it is important to do this correctly the first time. It is easy to relocate where the line guide is on the wingtip on planes with external controls, like combat planes with the H&R-type external bell cranks/shutoffs. Before completing an airplane you can suspend it from the ceiling by the leadouts. Obviously the engine and stab need to be on. Observe which way the nose is pointing and adjust the leadouts guides position to get the rake set.

It is also important that the lines/leadouts exit the wingtip at the middle point of the airfoil cord (dead center of the wingtip) as opposed to the top of the wing. Otherwise, the plane will fly "tip up," which makes it hard to trim for real warps.

Generally the slower or underpowered the plane is, the more leadouts sweep back should be used. Too much sweep causes the plane to slow down and can cause the outboard wing to stall during turns, causing the plane to wobble. Brodak makes a 2-Line Adjustable Leadouts guide. It works well to find the optimal wing rake on a new design and can be used on some kinds of combat wings like Diesel.

If you think your wing is straight (no warps) and your outboard wing weighs more than the inside wing, and your plane still comes in, or the lines go slack when you turn in both directions, then you might need more line rake. You can overcome this problem in the field with some engine offset or more wingtip weight.

If the plane comes in or goes slack on the lines when you turn in one direction only and not the other, then chances are you still have a warped wing.

If your plane pulls hard in all maneuvers, then maybe a little less line rake would increase speed without sacrificing adequate line tension. I use a lot of Line Rake on Diesel Combat planes that have low power-to-weight ratios. Speed and Racing planes, that do not do maneuvers, use very little line rake for maximum speed.

5. Elevator Control Throw.

Limiting control throw is the preferred way to prevent over-control. Most planes need between10 to 25 degrees of throw, both up and down. Being able to adjust controls after the plane is finished is preferred. If you need more throw than 25 degrees, it means something else is wrong, like the CG is too far forward.

The easiest thing to help with over-control is a control handle that has a narrower space between where the lines hook to the handle. Wider spacing means more control with less hand movement. On _A planes, I use a handle that has only 3 _ inch spacing. On my FAI planes, there is only 3-inch spacing. On Diesel and Fast/80MPH ships, there is 5-inch spacing due to large 3-inch bellcranks used on those planes. The correct spacing at the handle helps with general control but does not prevent over-control that happens due to the "Heat" of combat. Only limiting control or throw prevents this.

Limiting throw can be done in several ways including:

6. Size of Elevator

The larger the elevator, the more effect it has and the tighter the plane will turn. So if your plane turns too tight, stalls, or the lines go slack, and you have made the adjustments discussed above, you can cut down the elevator. Cutting down the elevator also lightens the tail by moving the Center of Gravity (balance point) forward, making the plane more stable. This is one of the easiest things to adjust quickly in the field by trimming the back of the elevator with a knife, but hardest to undo.

The farther back the elevator is the more leverage it has in turning the plane. On some types of planes, like those that are attached with a boom, it is sometimes easy to adjust where the elevator is located and balance point (Center of Gravity - CG) by shortening or lengthening the boom.

Where you locate the hinge point on an elevator can also affect how much control you get. A "cantilevered" elevator (where the hinge point is back from the leading edge of the control surface) has a lot more control than one that hinges on the leading edge.

7. Static and Turning Mass (also known as weight)

I'm going to spend some time on this factor because most people do not understand the effect, yet it can be very important. There are two types of Mass to deal with, "Static" and "Spinning."

It is well known that 1/2A and FAI combat wings turn tighter than Fast Combat (36 powered) ships. The reasons for this are:

Overall Airplane Weight

The lighter the plane, the easier it turns and the less power it takes to accelerate to full speed out of maneuvers. This is especially important with underpowered planes like diesel combat types. Build planes using a gram scale so you are aware of every gram of added weight. The lighter the plane, the more it is affected by windy conditions. You also want to build planes strong enough to take crashes.

Spinning Mass

Lighter props and engines let the plane turn better than heavier ones because they limit the "Gyro Effect." The "gyro effect" of the spinning mass is that the spinning weight wants to maintain its current alignment in relationship to its axis and thus fights any attempt to turn the plane. If you ever played with a Gyroscope, you would understand how powerful this force is. If you try to turn a spinning Gyroscope against its axis, there is a lot of resistance and if you force it to turn, it S-L-O-W-S down the spin. That equates to robbing horsepower.

Relationship of General Mass to the CG

An added effect is that a heavy prop changes the balance point C/G of the airplane, thus making the plane more nose-heavy, which further limits the ability to turn tight but makes the plane more stable.

General Rules regarding Spinning Mass

Ideally, you want your plane to balance and handle well with the least amount of spinning mass as possible. This lowers the overall airplane weight and spinning mass.

You can't change the Spinning Mass of the engine crankshaft. You can use a lighter weight aluminum prop washer and nut to make planes turn a little tighter. This will move the CG back and decrease spinning weight.

The reverse is also true. Sometimes your plane is too tail-heavy and does not perform smoothly or is not stable enough. In my field box, I carry heavy props and a thick steel weighted prop washer. This works with the regular prop washer and nut, which weighs an additional 30 Grams (a little over an ounce). I also have a thinner, steel-weighted extra prop washer that works with regular prop washer and nut that adds 16 grams (1/2 Ounce). There is also available, at hobby shops, a chromed brass spinner/nut you can get that weighs 94 grams (over 3 Ounces). I do not carry this nut because my planes are never that bad. However that much added weight may be useful when training new fliers or could make the plane real stable for something like a balloon-bust event.

You can make the spinning mass work for you. I carry both Heavy APC and lighter Master Air Screw "Series 2" Prop for 80MPH, Fast, and Diesel combat. As a result I sometimes use a heavy APC prop to tame a touchy plane. This has a dual effect: 1) it increases the gyro effect and 2) moves the balance point forward. Most of the time I use a light-weight Air Screw "Series 2" Prop or wood props for my planes. I keep notes and mark, on the wingtip with a "Sharpie" marking pen, which prop should be used for that plane. I always test a plane for the first time with a heavy prop because I don't want any unnecessary surprises.

Weights of some combat props in Grams ­ Note the BIG difference:

1 ounce = 28.35 grams

Used on ... prop ... grams

80MPH ... 8/5 APC Composite Nylon ... 20.0

Diesel ... 8/5 RevUp Wood ...7.0
8/5 Zinger Wood ... 8.5
8/5 Master Airscrew Series 2 Composite Nylon ... 11.4
8/5 Master Airscrew Series 3 Composite Nylon ... 12.0

Fast ... 8/6 APC Composite Nylon (Bored for a Nelson) ... 19.2

Diesel ... 8/6 Master Airscrew Series 2 ... Composite Nylon ... 11.4
8/6 Zinger Wood ... 9.1
8/6 Glass Tornado Blue GRS Models #22 ... 17.0
I'm not sure I would use Wood or MA series 2 props on Nelson 36 as they may throw blades.

1/2-A ... 1/2A Red ... 4.6/3 GRS Models #34 ... 4.6
1/2A Orange ... 4.6/3 ... GRS Models #36 ... 3.5
1/2A Yellow ... 4.6/3 ... GRS Models #33 ... 3.25
1/2A Purple ... 5.1/3 ... GRS Models #31 ... 3.5
1/2A Green ... 5/3 ... GRS Models #35 ... 3.8
All 1/2-A Props listed are Fiber Glass Composites

FAI ... Fiber Glass Composites props all weigh about the same. ... 4.9 to 5.1

8. Wing Area and Offset - Relationship of inside to outside wing length/area.

Generally it is accepted that the inside wing should be a little (one inch or 5%) longer than the outside wing. When you do this, the inboard wing creates more lift than the outboard because the inboard has more area. This helps to support the weight of the lines and prevent the plane from kiting in (lines going slack) when gliding in for a landing. The drawback is that this increases drag and weight on the inside wing (explained later). If a plane still "kites in" while gliding down, then remove the outboard wingtip and/or add more wingtip weight.

9. Engine Offset

Engine offset prevents the bad effects of line drag and holds the lines tighter in turns. The slower or lighter the plane, the more important offset is. Engine offset is used mostly on 1/2-A (light) and Diesel (slow) planes. The drawback to engine offset is that it slows the plane down and can cause the outboard wing to "stall" in tight turns, making the plane wobble or yaw.

Engine offset is one of the easiest items to adjust in the field. Offset can be added at the field by adjusting how the motor mounts bolt to the airplane. This is easy to do with 1/2A mounts and some other mount types. If the mounts cannot be adjusted, then try adding washers or other spacers between the engine lugs and mount on the forward two bolts. A drawback to using washers to adjust the engine offset is the possibility of warping the crank case slightly and damaging the engine. In Diesel planes with permanently attached maple mounts that can not be adjusted, I build the offset into the mount so that the engine case fits squarely with the mounts, but I still get 2 degrees of outboard engine offset.

10. Alignment of Engine with Wing.

If the wing is not in line with the engine, meaning it has "Up Thrust" or "Down Thrust," it will fly stable right side up or upside down, but not both. In one mode or another, it will tend to "hunt" (not want to stay level). You want to avoid Up Thrust or Down Thrust on any plane that does maneuvers. If the plane flies stable when upright but "hunts" when inverted, it likely has down thrust on the engine. For a speed or racing plane, that flies only level, a little Down Thrust (1% or less) can make the plane faster as the wings hold the plane up, and the engine is pointed straight ahead for maximum speed.

11. Drag

The drag on an airfoil section is composed of air pressure and skin friction. At low lift coefficients, skin friction dominates.

From what I heard (and this may be overstated) the drag of a 1/10-inch diameter rod or tube, is the same drag as a 1-inch good airfoil. In short, a tube or rod could be up to 10 times the drag of a good airfoil shape of the same thickness. A square spar has much more drag than a rod or tub of the same thickness and many times more drag than a good airfoil.

Sharp edges and protrusions (like needle valve, nuts, loose covering and bolts) that stick into the airflow, cause significantly more turbulence and drag compared to airfoil shape.

To cut drag:

The inside wing has more drag than the outside wing since the lines add significant drag, and the inside wing is usually 1" longer. To counter this, the below factors offset the effect of that drag and hold the lines tight:

12. Type of Airfoil Used

Sharper leading edges make the plane faster and turn tighter compared to rounded leading edges used on most planes today. But sharper edges also make the plane more unstable. The rounded leading edges used on high-performance combat planes today take advantage of the improved engine performance to get the speed, while the lighter engine mass allows for tighter turning.

Airfoils that are flat for an inch or so across the highpoint (crown) of the airfoil and balance point tend to be more forgiving compared to airfoils smoothly rounded over the highpoint on the airfoil balance point. With a "flat" airfoil, you get more stability and less critical CG without sacrificing turning. You do give up a minimum amount of speed due to the less-than perfect-airfoil. Most good combat planes used today have a flat airfoil crown.

13. Location of where the Streamer hooks to the plane.

One of the easiest adjustments in the field to prevent slack line is to relocate where the streamer attaches to the airplane. The streamer adds a lot of drag to the plane. That drag can be used to work to your advantage. The AMA rule book allows you to connect the streamer to the plane within 3" of the center line. If your line sweep is not enough or your plane comes in at you during maneuvers, just move the connection point of your streamer farther out on the outboard side of your elevator. In Vintage Diesel Combat we usually hook the streamer to the Compression Screw of the engine. That would be about 2_ inches off the center line. The only problem with this is if you get your streamer cut away your plane will handle differently.

Conclusion I do not know, or have not remembered to include, everything that affects airplane handling, so if you have additional input feel free to contact me. I'll keep this article updated and give you credit for any of your ideas used. Don McKay Email to:

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This page was upated Aug. 30, 2009