Flaps: What you need to know

Oct 01, 2012 12 Comments by

Sooner or later you may want to try your hand at flying a scale subject.  Since most full-size aircraft use flaps, many scale model also require them for true scale operations and function. A scale model with  flaps fully deployed is a cool sight. If you have never flow a model with then there are a few things to know about. There right ways and wrong ways to use them. This article should help you understand what’s going on.

In a nutshell, when flaps are lowered they change the wing’s lift and drag characteristics and lower the stall speed. By changing the camber of the wing, the lift and drag are increased for a given airspeed. As a result of these changes affect the speed that the aircraft can land.

Common Flaps

Though there are four basic types of flaps: plain split, Fowler and slotted. The plain flap is the most common and is simply a hinged portion of the trailing edge. It is usually hinged at the top of the control surface since it only moves in a downward direction. Super Cubs, Cessnas and other sport scale models use common flaps, to keep construction and function simple.

If you have never flown with flaps before, don’t worry. Flaps add flexibility to your model’s flight envelope, and it is a fun new experience. The major advantage is they shorten (and steepen) your landing approach by allowing your plane to fly more slowly in a nose down attitude. Here are some hints!

Do’s

  • Learn how your plane reacts to flaps at a safe altitude before attempting the first landing.
  • Reduce the throttle to around 1/3 and let the plane slow before dropping the flaps.
  • If used for takeoff, use only partial flaps.
  • Adjust the power to maintain the approach path. Flaps add drag and so will require more power.
  • Add power on a go-around and begin your climb out before retracting flaps.

Don’ts

  • Deploy flaps at high airspeed. The flaps may depart the wings or cause serious structural or servo damage.
  • Use flaps on the first takeoff and test flight. You must first determine how much deflection is correct for your model.
  • Use full flaps on takeoff. This adds a lot of drag.
  • Let the plane balloon and lose its airspeed. Adjust the elevator to keep the proper approach path.
  • Retract flaps when low and slow or you could settle onto the runway.

Deploying flaps may result in the plane pitching up or pitching down. The elevator must be used to compensate and keep the plane on the desired approach path. Another characteristic of flaps is that the first half of the flap’s deflection results in a greater increase in lift while the second half results in a greater increase in drag. Flaps also impart a large structural load on the plane and should only be used at a lower airspeed. Full-size planes have their air speed indicators marked for safe flap operating range.

Flap Facts

Since flaps provide more lift at slower airspeeds, you must be aware that when you retract them in-flight you will lose the lift and the plane could sink. For this reason, if you must do a go-around, make sure you increase power before retracting the flaps. Failure to do so could place your plane very close to stall speed before you can accelerate to a safe speed. This also applies to takeoffs with flaps. In most cases it is safer to take off with the flaps retracted or deflected no more than about 20 degrees. Larger deflections add more drag and can cause the plane to become airborne at too low of an airspeed.

Flying a scale model with operational flaps is a very rewarding experience. Not only do they look neat, but they also provide the same benefits as the full-size version.

Flaps impart increased loads on the wing and require attention during their installation. Make sure you use enough heavy-duty hinges on each flap and a heavy-duty control horn. There are many ways to actuate the flaps, including torque tubes and bell cranks. For large, fast or heavily-loaded models, the best way is to use a servo for each flap. These planes will also benefit from the flaps being locked in the down position preventing the airstream from blowing the flap back to the up position. This basically means that the servo arm is directly in line with the flap horn at full deflection and this takes the strain away from the servo. This is accomplished by turning on the radio and selecting full down flaps and choosing a servo horn position that is in line with the horn. Now, retract the flaps and make up the linkage from the servo to the horn. The amount of flap deflection is determined by the length of the servo arm; for more flap deflection, place the linkage farther out on the arm. The use of ball links may be required for smooth action and to eliminate binding.

Flap Deployment

The modeler has several options for the transmitter flap actuation method. The least desirable is to use a two-way switch, which only results in flaps up or full down. This is not very scale-like and could result in large pitch changes when the flaps are actuated. A three-position switch will allow the use of half-flaps for more scale-like flight. A knob or slider switch is another way to go and allows an infinite number of flap settings. The only drawback is that it is sometimes difficult to tell how much flap deflection is selected.

Another way to minimize the trim changes associated with flap deployment, is to use a slow servo speed. Many programmable radios now have the ability for you to slow down the response of specific servos. If the flaps take several seconds to lower, it minimizes the abrupt change in lift and gives the plane more time to settle down. Also, there are radio accessories called “Servo Slower” or “Servo Speed adjusters”, which allow the same thing but without the use of a programmable radio.

Flying with flap-equipped airplanes  is a great experience and just plane fun. Flaps allow you to operate your model from smaller flying areas and when it comes to scale competition, they allow you to full exploite your subject aircraft’s flight performance while giving you another flight option to add to your flight routine. Give it a try. It’s a blast.

Gerry Yarrish

About the author

Senior Technical Editor About Me: I have a lifelong passion for all things scale, and I love to design, build and fly scale RC airplanes. With 20 plus years as part of the Air Age family of magazines, I love producing Model Airplane News and Electric Flight.

12 Responses to “Flaps: What you need to know”

  1. Bill Biglow says:

    Thanks for this discussion. My FlyZone select scale Corvalis w/ a AR500 or 600 Rx has only the gear slot availiable. That means only on or off. With a DX7 or 6i Tx, do I have some more realistic options?

    The flaps on the Corvalis seem to be mainly for show; they don’t seem to have much effect. So this may be for my next plane w/ flaps.

    On a personal style note. Could we be the first to declare a moratorium on using “It’s a blast” in any review of rc flying?

    Thank you, Bill

    • Chris says:

      The DX7 you can choose the switch to use for the 2 other settings of flaps. Read you manual and you will see you can select another switch, Like the 3 position switch, for flaps even on the gear channel. Then all you have to do is your programming of how much flap and elevator deflection you want to set up with. The 600 has an aux channel and if your using lights put those on the gear channel and use the flaps for flaps.

  2. Rick Rizza says:

    Great primer for flap use. You covered all the bases clearly and concisely. One thing I do that many don’t do is very useful with larger airframes like 3D models which have separate aileron servos and no flaps is program operational flaperons. Dropping the trailing edge (even just a few degrees) of any wing increases its lift at low speeds, which then allows a slower touch down speed, decreasing the wear and tear on the landing gear.

    Regarding Cessna flaps, those on the Cessnas I’ve flown are more like Fowler type then plain flaps, as they move back as well as down. At 40 degrees deflection (full flaps) a power off approach in a Cessna requires such an extreme nose down attitude to maintain your airspeed you feel like only the seat belts are keeping you from falling through the windshield! It’s a BLAST!! (wink wink to Bill! LOL)

  3. Gary Kapity says:

    Gerry,
    Very useful info, thanks. Would like to know if you can tell me what proportion of the ailerons I should allocate to flaps if the model does not include them?
    Thank you
    Gary

    • Terry Solesbee says:

      Two things to consider with flaperons.It does help the plane land slower. You will lose some of your aileron response depending on how much flap you apply and the effect gets worse as the plane slows.On my Great Planes F4 Phantom prop jet using full flaperons down the ailerons are ineffective on the final approach after the plane has slowed.I have to use the rudder to get it to turn, and that only works on that jet because the wing tips curve up.Your best off only using about 20% throw max. Then you should still have good aileron response on your final.

  4. Mike Callahan says:

    Hi Gerry, what you were saying about having the flaps on a two
    position switch no being very scale is very true except for a
    Supermarine Spitfire, their flaps were either up or down.

    Thanks for the useful info. Keep it coming.

    Mike

  5. Herman Burton says:

    Good article on the basics. I use flaps on my 60-size P-47 TopFlite kit-built warbird. In general, the flaps improve lift at low speed, and I can “crawl” in for a landing, instead of coming in hot.
    And the article does point out the problem of balooning if your airspeed is slightly too fast. I have found cutting throttle setting and letting the plane slow down prior to deployment of flaps keeps balooning to a minimum.
    Good article, Gerry. Thanks.

  6. Terry Bolin says:

    Flaps used to scare me but Now I have three scale planes with flaps..They are pretty much a must on war birds for landings. This aticle is all good information and will help alot.. I have found that on my brids, I dont have to add electronic down trim when flaps are deployed, just let the plane ballon and it will settle down..( on my planes with the way they are balanced). I just have to remember to keep the throttle up as stated untill I am ready to touch the grass on flair….Thanks Gerry for all you do!
    Terry Bolin, Neosho, Mo

  7. Jim Petro says:

    Am looking forward to your next installment on mixing automatic elevator trim change to neutralize the pitch effect when lowering flaps. We have “enjoyed” spirited discussions on this subject almost as much as debating the ‘down-wind turn’
    Jim Petro

  8. Phil Hultin says:

    On my Hangar 9 Twin Otter I have the flaps set up on a knob, but also on one of the three-way switches on my JR 8102 tx. The switch gives me about 3/4 flap and is much easier to handle than the knob during landing approach. I really wish tx manufacturers would put a flap slider in a location where you don’t have to get off the sticks to deploy, just as you are moving into final.

    I just put in a small down-elevator mix with the flap, kind of arbitrary but it seems to work well to keep the plane on a steady glide angle when the flaps are deployed.

  9. Franz Scaramelli says:

    Excellent tips!

  10. Paul Gunther says:

    I recently purchased a Black Horse T-28 Model with flaps, my question is the layout of the servos in the trays? Which way are they to be placed? I would like to see a photograph of a plane’s wing underside to see the setup of the servos..

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