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Author Topic: Flaps, Flaperons and Flapless Flaps  (Read 37 times)

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Offline Deerslayer

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Flaps, Flaperons and Flapless Flaps
« on: October 12, 2019, 11:16:34 AM »
Question: Why do I write this stuff? Because I think about it - a lot. Perhaps someone else out there has similar thoughts, or questions, and might be interested in one of these topics. So, here are some thoughts about "flippers".


Today, even many of the beginners' airplanes have Flaps, as well as independent servos for the Ailerons. In that aviation classic, "Stick and Rudder", the author refers to the control surfaces of aircraft as "flippers". I really like that, as it separates physical control(s) from their potential function. (Check out https://en.wikipedia.org/wiki/Wolfgang_Langewiesche to learn more about him.)

There are some useful ways to make use of the "flippers" on your aircraft apart from how we usually think of them. Here are some of my thoughts.

About 30 years ago, I acquired my first "computer" radio, i.e., a transmitter which allowed what was then the relatively uncommon technique of electronically mixing or coupling of channels on r/c planes. I immediately wanted to emulate what I had seen the "hot dog" flyers do, on my primary sport plane. I explored the idea of using the Ailerons as Flaperons, under certain conditions. This also enabled me to "mix" (I prefer the term "couple") the Flaperons to follow the Elevator input.

When Ailerons simultaneously move down, they are termed Flaperons. If they both move up together, they have become Spoilerons. In either case, they can still behave as normal Ailerons, even while displaced from their normal Neutral position.

The purpose of Flaps, or Flaperons, or a combination, is to alter the Angle of Attack (AoA) of the wing. In doing so, its Lift/Drag is altered. At low settings, say up to 20 degrees, employing Flaps or Flaperons should increase Lift along with a moderate increase in Drag. At higher settings, Lift will not increase very much and will quickly deteriorate at very high settings; meanwhile, Drag increases significantly. So, you can end up with a highly variable Lift to Drag ratio (L/D), tailored to your needs during different portions of your flight. For instance, to shorten a take-off, deploying low to medium (perhaps 10 to 20 degrees) Flaps and/or Flaperons can result in an extremely short pop into the air and an amazing rate of climb. This can be especially useful on a seaplane, that is otherwise difficult to yank off the water - as long as you couple the Flaperons in with the Elevator.

You can judiciously use this feature to improve your L/D, say, during a final approach with a dead engine, in order to save your plane. Landings, especially with a normally pitch sensitive aircraft, the approach and landing can become slow, smooth and precise with very little effort. It may even help solve the tendency of some planes, like WW1 fighters, to nose over upon landing. Overall, you can become a better manager of your flying!
 
I prefer to employ the Ailerons as Flaperons, even if the aircraft has Flaps. Furthermore, I do not use preset settings for Flaps or Flaperons, i.e. no "Flap switch" use. I prefer to use a Mix to couple their function to the Elevator, and possibly to the Throttle (to serve as Spoilerons). This gives the best of all worlds, i.e., continuously variable L/D of the wing for all conditions. Normally, this is dictated On/Off by a switch, as there may be times when such coupling is undesirable. This whole arrangement is like having an extra set of hands at your command. Try doing really tight touch-a-goes, gradually decreasing the total flight path and time and you will see what I mean! It can get to be very exciting and addictive! A great way to really learn about your aircraft and your own piloting capablities, as well as to improve upon both!

The way to set this up is to, first of all, throw away any Y-connector you may have to control the Ailerons. (Aside: Those things are tools of the Devil, having caused the loss of one of my own prized toys, as well as those of other flyers I have known.)

You must have independent channels, so that means at least a 6-channel Transmitter and Receiver for plane without Flaps. Then create a Mix with Elevator as the Master and the Ailerons as Slave, so that Up Elevator brings in Down Ailerons. Now, these "flippers" have a dual function - Ailerons and Flaps. As a starting point, set this to perhaps 30% and choose one of your Switches to select/deselect this function. The result is that, as you feed in Up Elevator, you will be feeding in a proportionate amount of Down Ailerons, i.e., you now have Flaperons!

Note: If you are not already using Exponential on your controls, this is a good time to start. Your flying will become more relaxed, more precise and you will be able to detect small changes in behavior of the aircraft. Some flyers seem to avoid Expo. By doing so, they are missing out on perhaps the best thing to have come out of the development of radios. Many of us, including the 3D flyers, will use 60% or higher Expo on Ailerons, and/or Elevator and/or Rudder.  Avoid so-called Dual Rates to reduce your control throws - all they do is cut down on your control authority, reduce the precision of what is left and may leave you sorely lacking in that little extra "oomph" just when you need it the most! Major Expo will give you all of the advantages, yet you have 100% control movements available at the extremes. 

First flight:

Deselect this Flaperon Mix, take off, get some reasonable altitude and with the plane trimmed for reasonably slow flight, select your Flaperon Mix. Play around with pulling Up Elevator, see how the plane behaves. Do a few loops at various speeds, with and without the Flaperon Mix engaged. You should see a major difference, with much tighter loops and yet no greater tendency to tip stall while the Mix is operating.

Slow the plane down close to stall, then work the Elevator to see how it behaves. You should notice that the plane can be flown significantly slower with the Flaperon Mix than without it. Play around with it, have fun and learn about your aircraft's flight envelope.

Land as usual, with or without the Flaperons engaged. If you do have them active, plan to keep your landing speed low, then do a really good hold off as you begin to flare. You should be able to do a great, fully controlled landing at low speed and in a shorter than usual distance. 


Next flight: Select the Flaperon Mix for take-off. Take off as usual, don't haul back too aggressively on the Elevator or, if you do, be prepared for a surprisingly quick and steep take-off.

You have now seen the three major advantages of using Flaperon Mixing: Short take-offs; slower landings; and, some tighter aerobatics.

So, what next?

Experiment with the amount of coupling you use. I have upwards of 60% on some of my planes, and you may find that you have a wide range of values to try out. Have fun, learn things, discover you plane's inner soul!

But, wait! What about the other trick with Ailerons, i.e., creating Spoilerons?

If you set up the other half of your Mix to raise the Ailerons in unison as you push Down Elevator, you now have Spoileron capability. A cautionary word: This may, on some planes, result in a rather aggressive drop in altitude and/or pitch change that may concern you, so start with a small amount of coupling, say 20%, until you have tried it out. Assign this to a switch.

Another option for having Spoilerons is to couple them to the Throttle, so that bringing the Throttle down will gradually raise the Spoilerons. Set it up to be switch-selectable and don't use really high amount, at least at first. Perhaps try a 20% or 30% mix. Now you have an option to descend rather rapidly, which could be useful in doing the really hot, really cranked-in  touch-and-goes. It is best to make sure that you have your Elevator-to-Flaperon Mix active at the same time. 

Some may have noticed that I have shown up at our field over many years with some airplanes of my own design which have unusually shaped Ailerons. This is an approach which give one the benefits of having a wing without Flaps becoming as useful as one with separate Flaps, without using more servos/channels. These flippers will gradually taper down from the last, say, 10% of total Aileron length to perhaps 1/2 of their width. The innermost 30% or so of the control surface will become significantly wider than the rest of the surface, blending in via a nice curve. This inner section of our "flipper", being within the high energy region of the propwash, does not need to be very large to have a major effect upon the airflow and to contribute greatly to downwash and Lift. The blending in of the enlarged section to the reduced portion is intended to reduce the disruption to the airstream that an abrupt change would have. My theory!

Here is the effect of this unusual Aileron profile, as used through 20+ years and  many generations of these planes:

These flippers are now Flaperons/Spoilerons. You can get at least as good effect as you would with separate Flaps, but with simplicity and automation. They can be switched On to be coupled to the Elevator as Flaperons for take-off, playing around in the air and landing. At the same time, or  preferably by a separate switch, they can be coupled to the Throttle to serve as Spoilerons. That can make for dramatically steep approaches and spot landings.

The roll rate of this configuration has proven to be much quicker than with straight Ailerons, with all mixing turned off. Additionally, you have full span Flaperons, with at least as significant effect as with separate Flaps, especially when coupled to Elevator.

But, you say, "What about tip stalls using Flaperons?". Well, in my opinion, that should be of no concern. Unlike many years ago, with certain glow planes, a modern well-designed airplane has little tendency to tip stall. Especially a light, electric one. Using Flaperons coupled to the Elevator does not increase the probability of tip stalling. If you do not believe me, set it up and do some testing in flight, at a couple of mistakes high. Convince yourself.

Incidentally, I see flyers who have never deliberately stalled their aircraft. Shocking! These folks are often the same ones who approach a landing at a blazing fast speed, fight the aircraft down onto the ground, possibly bunny-hop a few times and even visit the shrubbery at the far end of the field, complain about their aircraft being overly "pitch sensitive", and so on. They are afraid of stalling the thing, I guess.
From my full scale gliding days, and throughout my R/C flying, on the first flight of any aircraft, as soon as it is trimmed OK, I do power-off stalls. You learn a lot in doing this, including what speed is reasonable to land at, whether the plane has a tendency to roll or spin at close to Stall, how might it recover if you do inadvertently stall it, how good this aircraft is at low speeds, etc. 

This approach can be tried with virtually any aircraft and a reasonably current radio, at no cost and no permanent changes, just a simple Mix or two and the flip of a switch.

Get to know your plane! Try things! Learn stuff! Have more fun!
« Last Edit: October 14, 2019, 06:06:08 AM by Deerslayer »
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