Setting a Slow and Reliable Idle

by: Jim Bronowski

To start with, many idle problems with non-pump equipped engines can often be traced to an improperly positioned fuel tank or a fuel tank that is too far from the engine. The center-line of the fuel tank should never be any higher than the center-line of the fuel jet and preferably 1/4 to 3/8 inches below. This helps decrease the siphoning action with a full tank of fuel.

The make of the glow plug also plays an important role. Any older design, cross-flow scavenged (ported) two-stroke engine should use an idle bar glow plug. Most of the newer Schnuerle ported two-stroke engines do not require an idle bar plug, but if idle problems are experienced, an idle bar plug should be used. If you aren’t sure whether the engine is cross-flow or Schnuerle ported, just look into the exhaust. If there is a baffle on the far side of the piston, the engine is cross-flow ported. If there is no baffle, it is Schnuerle ported. Some engines do have better idle characteristics than others due to differences in porting, timing, compression ratio, etc.

When it comes to the actual adjustment, there are two basic methods. The first is to start with the fuel tank half full and the idle speed set in the 2,500-2,700 rpm range. This is where a good tachometer comes in handy and is something every toolbox should contain, not just for setting idle speed but for proper richening of the top end as well. Then, use the “pinch test” (i.e. pinch the fuel line). If the engine dies immediately, the idle mixture is too lean and needs to be opened in 1/8-inch increments. If the engine speeds up and the idle improves, the mixture is too rich and the adjustment should be turned in or leaned.

If the engine is cowled in and the fuel line to the carburetor is not easily accessible, with a tricycle gear ship, lower the tail. If the engine dies immediately, the mixture is too lean. If the idle improves, the mixture is too rich. Remember to always make any idle mixture adjustments in 1/8-turn increments—not one or two turns at a time.

With a tail-dragger, make the mixture adjustments with the tail raised to a level position, being careful not to go so high as to have the propeller hit the ground. Then, lower the tail following the same procedure as with the tricycle gear model.
For the final check, accelerate the engine to full throttle. If it slows and sags and has a weak sound, the mixture is too lean and needs richening. If the engine sputters and spits out a lot of smoke, the mixture is too rich and should be leaned.

After a satisfactory idle and acceleration have been established, you can try lowering the idle speed to the point where the engine will remain idle for a prolonged period with good acceleration to full throttle. Again, the idle speed should be set with a tachometer and not by ear.

Many cases of an engine dying at idle are simply because of pilots who try to idle the engine too slow. It is nice to watch an engine tick over at 1,800 rpm, but an idle speed in the 2,200-2,500 rpm range is more practical and reliable.
Also remember, the heavier the propeller and the larger the diameter, the better the flywheel action. Increased flywheel action is always beneficial to a slow and reliable idle.

Repairing Warped Wings

By: Andrew Benjamin

Even though I have plenty of new planes around in various stages of completion, I enjoy acquiring used airplanes whenever I find one I like. It’s the flea market mentality in me I guess. Problem is, many times these planes aren’t completely straight like the Pica P-51 (was) I’m working on that I bought at the local club auction. I didn’t have an opportunity to check the incidences before I bid on it and then I put it away until now.  I just discovered it has a warped wing.

Here’s what I do if I find a warped wing and I thought I’d tell you about it in case you buy one (or build one) that isn’t exactly perfect. The Pica Mustang, like the majority of warbirds are made up of balsa sheeting over ribs and formers. In my case one wing tip was 1 degree positive over the other one. This isn’t a lot and some might ignore it, but I can’t live with any plane that isn’t real close to being perfectly straight. I not only bugs my sense of inner peace, but I don’t like necessary trim changes over the speed envelope.

I have a large dead flat and level work table I work from and I use digital and laser incidence meters mounted on homemade brackets of various sizes. I put the plane on it’s back with the wing secured and shimmed it level. Then I firmly secured the plane so it couldn’t move. I checked to see that the good wing half was at O degrees, the I went over to the warped side and cut an X through the bottom sheeting (built up wing) from the leading edge root to the trailing edge tip and from the trailing edge root to the leading edge tip. I accomplished this with a number eleven exacto blade and I purposely cut into the ribs underneath about a 1/4 inch. NOT THE SPAR THOUGH!

I put the digital incidence meter back up on the warped wing tip and twisted the now pliable wing tip until it read 0 degrees and noted the movement in the “X” for reference. I then flipped the incidence meter upside down on the wing with the brackets near one end so that the long home made aluminum extrusion extended beyond the leading edge about 2.5 feet. I hung a 1/4 full gallon paint bucket on the right place of the aluminum extrusion to leverage the twist out of the wing in the opposite direction a little beyond the 0 degrees I was aiming for and went to bed to let the wing undergo “traction therapy” all night.

This morning I took the warped wing half out of “traction” and it measured a perfect 0 degrees like the other one. I then generously wicked thin CA in the “X” cut lines in the sheeting and now I have a perfectly straight wing that’s just as strong as before. I just need to sand the glue joints a little and no one needs to know the difference (except the world wide web) after it’s finished. This method works on sheeted foam wings also, but one doesn’t need to cut an X . A \ cut in the correct orientation will work to straighten it and be sure to use foam compatible CA in the later case. Of course the best bet is to build the wing straight in the first place. Thought I’d share this old trick in case anyone want’s to loose that asymmetrical aileron trim.

PS. Some say the test of a good builder is not how well he builds the first time but how well he fixes his mistakes! I still prefer to get it right the first time however but I also like to know how to fix mistakes.

Protect Hinges

by: Gene Davis

Petroleum jelly often has been used on pinned hinges to prevent epoxy glue from sticking to the hinge joint; however, it is difficult to get just the right amount on the hinge and to make sure the hinge is completely coated.

A very cool way is to melt the petroleum jelly in a small dish such as a dessert dish (an oven safe type, of course). Use only enough to melt to a depth of about 1/6 of an inch. Fold the hinge and dip the pinned end into the melted jelly.

Remove and touch the hinge to a paper towel to remove excess. In a couple seconds, the petroleum jelly cools and has penetrated the hinge. You now have a completely coated hinge joint that epoxy will not stick to.

Prop Charts

By: Scott Cannon

Prop Chart For Two – Stroke Engines

Alternate Propellers Starting Prop Engine Size
5.25×4, 5.5×4, 6×3.5, 6×4, 7×3 6×3 .049
7×3,7×4.5,7×5 7×4 .09
8×5,8×6,9×4 8×4 .15
8×5,8×6,9×5 9×4 .19 – .25
9×7,9.5×6,10×5 9×6 .20 – .30
9×7,10×5,11×4 10×6 .35 – .36
9×8, 11×5 10×6 .40
10×6,11×5,11×6,12×4 10×7 .45
10×8,11×7,12×4,12×5 11×6 .50
11×7.5, 11×7.75, 11×8,12×6 11×7 .60 – .61
11×8,12×8,13×6,14×4 12×6 .70
12×8,14×4,14×5 13×6 .78 – .80
13×8,15×6,16×5 14×6 .90 – .91
15×8,18×5 16×6 1.08
16×10,18×5,18×6 16×8 1.20
18×8,20×6 18×6 1.50
18×10,20×6,20×8,22×6 18×8 1.80
18×10,20×6,20×10,22×6                                        20×8 2.00

Prop Chart For Four – Stroke Engines

Alternate Propellers                                                Starting Prop Engine Size
9×5,10×5 9×6 .20 – .21
10×6,10×7,11×4,11×5.11×7,11×7.5,12×4,12×5 11×6 .40
10×6,10×7,10×8,11×7,11×7.5,12×4,12×5,12×6 11×6 .45 – .48
11×7.5,11×7.75,11×8,12×8,13×5,13×6,14×5,14×6 12×6 .60 – .65
12×8,13×8,14×4,14×6 13×6 .80
13×6,14×8,15×6,16×6 14×6 .90
14×8,15×6,15×8,16×8,17×6,18×5,18×6 16×6 1.20
15×6,15×8,16×8,18×6,18×8,20×6 18×6 1.60
18×12,20×8,20×10 18×10 2.40
18×10,18×12,20×10 20×8 2.70
18×12,20×10 20×10 3.00

Pre-Flight Checklist

Recent comment and responses observed on several large scale email lists led us to collect this information. NASA would certainly appreciate your help if you have additional information to contribute to this Pre-flight safety check list.

With the wing removed, check the following items

  • Are batteries fully charged? Check with E.S.V.
  • Is engine broken in?
  • Aircraft must have pilot’s identification, AMA number, telephone # and address in it.
  • Check for any visible cracks or tears.


Engine and Fuel Tank Area

  • Blunt faced prop hub (acorn nut) or rounded spinner required.
  • Sand off sharp edges of prop and balance it prior to installation.
  • Paint prop tips either white or yellow for visibility.
  • Is there any prop damage i.e. cracks, nicks?
  • Is propeller properly attached? Facing correct direction and installed about the 2-3 o’clock position upon the compression stroke.
  • Does engine have proper (needs definition) muffler?
  • Is engine mounted securely to engine mount?
  • Is throttle arm and push-rod connection secure?
  • Is glow plug correct length, type, and installed tightly against it’s compression ring?
  • Is engine mount securely mounted to fuselage?
  • Are all engine components secure? Lock washers – Locktite – etc. Notice if there is any oily black residue on or near the engine mounting screws/bolts, or muffler mount as this is a sure sign that something is loose, or metal-to-metal rubbing against each other.
  • Is there any apparent damage to fuel lines?
  • Is fuel tank isolated from vibration?
  • Fuel lines should not be too long to be caught in prop. Also the fuel draw line should be as short as possible with a slight amount of slack, to maximize fuel draw. Is there a fuel filter installed?
  • Check to see that the fuel tank clunk falls freely.
  • Is fuel tank mounted correctly in relation to engine needle valve? Center of fuel tank centered or within 1/2″ of needle valve gives optimum performance?
  • Is there an engine kill switch?
  • Is cowl secure?


Receiver and Servos

  • Is receiver securely mounted and isolated from vibration? Should also be placed into a plastic bag to keep fluids out of receiver (fuel).
  • Are all servo tray screws snug against the grommets, not crushed?
  • Are all servo arms and wheels secured with a screw to the servo drive shaft?
  • Are push rods securely firmly secured to servo arms. If ball links are used are they peened on or was a drop of CA placed onto the threads to secure nut.
  • Are servo arms adequate in thickness and material? Giant scale needs strong ones!
  • Are the servo rails secured to the fuselage? Are they made from spruce, or plywood?
  • Is the power switch in the aircraft mounted so it can not be accidentally turned off? It is suggested that if you use a rod connected to an internally mounted switch, you mount the switch where “in” is “on”.
  • Are all connections secure? Did you use “twist ties’ to lock servo plugs together?
  • Is antenna in good condition w/o cuts, breaks and frayed wire exposed? Never, ever tie a knot in the end of the antenna to affix to the fuselage or vertical fin.
  • Was a grommet or fuel tubing used to line the exit hole in fuselage for antenna wire? Never fold excess length of antenna wire back on itself. If long and trails behind the airplane, you should run antenna through a control horn hole to secure it from loosely flying about behind plane.
  • Are the holes in the horns aligned with the hinge line?
  • Differential throw?
  • Is wing free of warps?
  • Does it have the proper amount of wash-out?
  • Does the wing balance laterally?



  • Is there any apparent structural damage?
  • Are hinges adequate and pinned?
  • Is covering secure?
  • With a firm but gentle pull, will the elevator detach itself?
  • Are control horns secure and in good condition?
  • Do the holes in the control horn align with the aileron hinge line? Otherwise, you will have differential throw.
  • Is there a gap between the elevator and the horizontal stab?



  • Is there any apparent structural damage?
  • Are hinges adequate and pinned?
  • Is covering secure?
  • With a firm but gentle pull, will rudder detach itself?
  • Are control horns in the horn aligned with the hinge line?
  • Differential throw?
  • If tail dragger, is steering tail wheel isolated from rudder?


Landing Gear

  • Is landing gear firmly attached to air-frame or wing?
  • Are wheel collars secure? Flats filed into music wire prevents slippage. Mount so set screws are at the 6 o’clock position.  Main gear should have a slight amount of toe in. This is especially true for conventional (tail dragger) gear.
  • Are the wheels aligned with the wing?
  • Is the landing gear in their proper location fore and aft?
  • On tricycle gear, does plane rest level or preferably with a slight nose down or rake?
  • Is stance too narrow? ( Distance between gear legs)


Pushrods or Cables

  • Are all clevises in good shape?
  • Can they be pulled free from the threaded end?
  • Does each clevis have a keeper?
  • Are push-rods and clevises of adequate size and strength to prevent flex?
  • Are push-rods supported?
  • Are cables multi-strand and without slack?
  • Are there any “z” bends in push-rod wire where it exits air-frame? This condition induces flexing of control surfaces. There should be a straight shot from control  horn to servo, unless cabling is utilized.
  • Do cables have a guide around pulley?
  • Are cable ends properly swagged?
  • Do turnbuckles have safety wire?


Start Engine and perform the following checks:

  • While engine is running at full throttle, hold the nose of the plane so that it points straight up for at least 5 seconds…or longer. Then hold the nose down for the same period of time. the engine should keep running in all positions.
    Caution: always point exhaust away from everyone and everything. If engine stops, try richening the fuel mixture, it may be too lean. if that doesn’t solve problem, do an inspection of the fuel line and fittings for a leak.
  • Does engine quit running at low throttle trim? This is a safety requirement!!!!
  • Does engine have a reliable idle? Does engine hesitate when quickly throttled up? This usually means the setting is incorrect. When pinched off at idle, the engine should speed up after two to four seconds, then stop. If it quits immediately as you pinch off the fuel line, the setting is too lean. If it continues to run on and on, it is too rich.
  • Perform a vibration range check with the engine running at all speeds. If range decreases significantly, some part of your mechanical systems may be mounted too rigidly. The receiver may be mounted too firmly in the fuselage. Loose engine mounts, and mounting screws will contribute to radio failure by the vibration they create. Vibration must be kept to an absolute minimum!

Control Line Scale

by: Fred Cronenwett

Profile, Sport, Designer and F4B-FAI

Control line scale is an event that can be simple or complex as you want it to be. There are several AMA events and also include 1/2a scale that is flown as a club event. In general there are very few kits advertised as CL scale models, so CL scale pilots take radio control kits, plans and models and convert them over to CL scale. Brodak is the only source that I know of that advertises a CL scale kit. The events can be very simple to the extremely complex that require extensive research and building experience. Throttle control is expected in all events except for 1/2a scale. The AMA rules were changed in 2013 to allow 2.4 Ghz to be used in CL scale competition. You can still use 3-line and other forms of “Down the wire” electronic controls so there are lots of options when it comes to throttle control and other features. The best way to stated is to get a scale ARF that you can document, 2.4 Ghz controls and try a fun scale contest. Get some input from an experienced CL scale pilot on some pointers on the documentation and the flight portion and you will do well. Once you have added the leadout guide and the bellcrank to control the elevator you have converted the RC scale model to a CL scale model.

1/2a Scale
This event uses .061 or smaller engines such as the cox or other .049 and .061 motors that are on the market. These models do not have throttle control and tend to be small. This event is currently flown in Milwaukee, Tucson, St Louis, Muncie (Nats & FCM contest) and at the Brodak contest. Check with the contest CD for a set of rules before you build a model for this event. In general you start your motor(s) and takeoff and then land. This is a simple and fun event. Rules can be found on the Brodak web site and on the Tucson club web site (

Fun Scale
This event does not have Builder of the Model rule. This event also has very few points for static and the bulk of the points are with the flying portion of the event. The documentation for fun scale does not have to be as extensive as compared to profile or sport scale. The flight portion is the same as profile and sport scale where you have to takeoff, 10 level laps, 6 options, realism and landing. This is the only event you fly an ARF in. You will find beginners and experts flying this event because it is a fun event. To do well you would need to have throttle on the model. Look on the AMA web site for the rules for fun scale.

Profile Scale
This is where the fuselage and any nacelles are no more than 1” thick. You will find this to be an event that is very popular and competitive. Static judging in this event is100 points (max), then your flight points are another 100 points (max), so you need to do well in both to place in this event. The flight portion is the same as fun scale that require takeoff, 10 level laps, 6 options, realism and landing. The documentation will need to be done to the same level as sport scale. Outline is judged with a critical eye, also markings and color are also judged just like in sport scale at 15 feet.

Sport Scale
This event is where you build a full body fuselage and nacelles. Throttle is expected to do well and if the full size had retracts you will be expected to have retracts on the model if flown at some contests. The flight portion is the same as profile scale and the static judging is done from 15 feet. The cockpit is not judged and the judges will be critical of any changes in the outline, color and markings.

FAI (F4B) Scale
F4B is the international scale event that has very high standards and some consider this event to be “Museum” scale where every detail is expected in the cockpit and surfaces. This event is no longer flown at the world championships so the USA no longer sends a team for this event. However the NATS still hosts a F4B competition for those who want to enter the event. You will need to get hold of the FAI rules before building a model for this event. They have lower model weight limitations and other differences as compared to the AMA rules that you would need to understand.

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