August 14

RC Transmitter Controls – Elevator


Hi again, go get yourself a cup of coffee (or tea) and lets look at another of your RC Transmitter Controls. This time we’re going to investigate the Elevator, that bit at the very back of your plane that moves up and down to alter the attitude of the model.

Pitching Up and Down

The complete assembly at the rear of the plane comprises a Horizontal Stabilizer (the fixed bit) and an Elevator (the moveable bit). Some aircraft have an all moving tailplane where the stabilizer is pivoted and moves about this pivot. Most trainer planes have a fixed stabilizer and moveable elevators so this is the arrangement we will concentrate on.

Elevator Control

What you need to understand is that the elevator is not used to make the plane go up and down! Before we deal with the elevator we need to look at what the complete tailplane (stabilizer and elevator) actually does. Many people are under the impression that the tailplane holds up the rear of the plane. in fact  on most planes the tailplane generates downthrust instead of lift.

How is this I hear you asking. Well, its all to do with plane’s Centre of Lift in relation to its Centre of Gravity. The centre of lift is the point through which all the models upward lift can be considered to act. The centre  of gravity is another point through which all of the models downward weight is acting. A stable model is one where the centre of gravity is in front of the centre of lift. This means that under normal conditions the lift is acting behind the centre of gravity, it tends to push the tail of the model up resulting in the plane wanting to dive into the ground. To prevent this happening we have to create “down push” on the tail to keep the nose up. This the job of the tailplane assembly.

This effect can be physically tested in your workshop. Usually the centre of pressure (centre of lift) is about half  way between the wing leading edge and trailing edge (50% of wing chord). Support your model at this point under both wings. It will immediately fall nose forward because the centre of gravity is in front of where it is supported. Push down on the tail end of the model treating it like a see-saw to make it sit level. The force you use to achieve this balanced position is the force required of the tailplane to keep the plane level.

What the Elevator rc transmitter control does

Quite simply the elevator moves up and down to change the effective aerofoil  cross section of the tailplane. This varies the amount of balancing down-force delivered by the tailplane. As a consequence this variation in the force pushing the tail down you can make the model adopt differing pitch attitudes from nose down to nose high depending on how much you vary the force. So what the elevator does is control the pitch-attitude of the model. Here is a picture of a transmitter showing the two main control sticks, the Elevator and Aileron stick is on the right.

Transmitter Controls

From a practical aspect, raising the elevator creates more tailplane down-force pushing the back of the model down and so the nose goes up. If the elevator goes down, tailplane down-force is reduced and the planes natural nose down tendancy dominates and the tail comes up.

Putting Theory into Practice

You’re flying along straight and level and you decide to squeeze a little up-elevator. What happens now? You have increased the down-force produced by the taiplane causing the model to “pitch-rotate”  and the whole fuselage pivots around the Centre of Gravity (CofG), sending the tail down and the nose up.

Now we must consider the implication of this pivoting and its effect on the wing. Providing the wing is firmly fixed to the fuselage (as it should be) then the wings’ Angle of Attack  will increase. This will increase the lift coefficient produced by the wings’ aerofoil section and so your wing will create more lift.

Angles of attack

Before this our model was flying straight and level because lift was equal to model weight. Now we have a surplus of lift over model weight so the effect is to bring about an upward acceleration that pilots call Climb. If you recall our previous post we decided that an increased throttle setting was what caused our plane to climb!

Let me finish and you’ll see more clearly. Because of our elevator change the model is sitting slightly nose high and and as a consequence the wing is presenting a larger area to the on-coming airflow. This has  created more drag and as we haven’t touched the throttle the forward thrust from the engine is still the same. This now means that the drag now exceeds the thrust and so airspeed decreases, slowing our model down.

Things are getting a little complicated now so stick with me and concentrate.

I’m going to say this twice:

LIFT IS PROPORTIONAL TO THE SQUARE OF THE AIRSPEED.

Once again:

LIFT IS PROPORTIONAL TO THE SQUARE OF THE AIRSPEED.

Got that? Good, hang on to it. What does this mean from a practical viewpoint? Well what it means is that quite small changes in airspeed can cause larger changes in lift.

Lets take an example. If we reduce our airspeed by only 10%, our lift goes down by almost 20%, nearly twice as much! If as a result of this reduction our lift falls below that required for level flight then the model weight will exceed the lift resulting in a downward force. Newton’s law of gravitational force tells us that our model will take on a downward acceleration and the model will start to descend.

The interesting observation here is that our use of elevator has given us both climb and descent! It can’t be both so which is it?

Up or Down or What?

To get a true answer to this question we need to know a great deal more about the aerofoil of our wing, an area I don’t propose to go into here. Different aerofoils have different lift to drag ratios at different angles of attack (AoA). these ratios will effect the way our plane behaves.

If for a given aerofoil the lift grows faster than drag then our model will climb but if drag grows faster than lift then our model will descend. Now true as these statements are, apart from some very specialised aerofoils,  the Lift:Drag ratio for many common ones is pretty constant and we can assume that the two effects often cancel each other out.

The actual outcome from changing the angle of attack slightly is that the model will experience an initial amount of climb which is not sustained. As the drag increases and the airspeed falls off the model will end up flying straight and level again but with a slight nose high attitude and at a slower airspeed.

The result is that we can determine the main function of the elevator is to change the angle of attack. It determines that, for a given amount of thrust, the proportion of lift due to airspeed and the proportion that is due to the wings aerofoil effect resulting from its lift coefficient.

  • Up Elevator = less speed-generated lift and more aerofoil lift
  • Down Elevator = more speed generated lift but less aerofoil lift

Having said all this, there is a limit to the amount of angle of attack change you can bring about using your elevator. You can’t keep getting more and more lift by increasing the angle of attack. The defining point of the limit is the Stall Point.

Wing airflow

In this diagram the upper wing aerofoil is moving through the air creating lift whereas the lower wing aerofoil has reached an angle of attack that has caused the air to delaminate from the upper surface of the wing. this results in a failure of lift generation and the plane will stop flying. This usually happens when the angle of attack reaches around 15 – 20 degrees.

As we increase the angle of attack the lift increases smoothly until the critical angle is reached at which point it decays almost instantly. The sudden nature of this transition  necessitates the avoidance of this critical angle of attack. We have now discovered another function of our elevator. Not only does it determine our angle of attack, it also causes our model to stall.

Understand this: “Aeroplanes do not stall because they are flying too slowly. They stall because the elevator stick is too far back and the resulting angle of attack is beyond the critical value. this can happen at any airspeed and in any attitude”.

In Conclusion

The elevator is probably one of the most important rc transmitter controls. It does not control height, it controls angle of attack and the proportion of lift you gain from different sources. It is possibly the most important control. You can fly without one aileron and you can fly without a rudder but you can’t fly without an elevator. Always be aware of the potential for excessive elevator use pushing you into the stall danger zone.

Next time I’ll discuss the ailerons in detail. Don’t forget to pop in on my website: www.rookiercflyer.com, you’ll find lots of useful information to help you.

Keep practicing.

Colin

 

 



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Posted August 14, 2015 by Colin in category "General Flight Training

12 COMMENTS :

  1. By Alfred James on

    what really amazing post! I see that you enjoy this theme, so much physic in this planes, and totally because that’s exactly how a plane work, not matter if it’s a toy or a really huge one. Do I want to know if you have those planes? I would like to know the opinion of an expert as I can see you are, what I have to choose if I want to buy a new one?

    Reply
    1. By Colin (Post author) on

      Thank you for your kind comment Alfred. Yes I do fly model aircraft. I have several , all are powered by electric motors.

      Can I suggest you visit my main website http://www.rookiercflyer.com and read through the full content. It will take you from knowing absolutely nothing to choosing a suitable model and progressing through to learning to fly it, where to fly and everything you will need to do it properly.

      After this you can take in my posts that help you progress to more advanced subjects.

      Good Luck,

      Colin

      Reply
  2. By Michel on

    I have only ever tried to fly an RC Plane once and I almost crashed it. I think to fly one of these planes successfully you need the equivalent of a driver’s license.

    But watching these guys fly these planes, they make it look so effortless and fun. Does the rc transmitter control elevator make the plane any easier to fly? .

    Reply
    1. By Colin (Post author) on

      Thank you for your comment Michel. Most rc planes have at least three controls to make them go in whatever direction the pilot wishes. These are Ailerons, Rudder and Elevator. A plane can fly without either Ailerons or rudder but an Elevator is essential.

      To answer your question, yes the Elevator does make a plane easier to fly. If you take the trouble to visit my website http://www.rookiercflyer.com and follow all of the content you will fully understand how a plane flies. My book discussed in the “Research” page will give you everything you need to understand to learn to fly an RC plane.

      Reply
  3. By TheDopestMatrix on

    this is so much science in one article it’s insane. I think the elevator is something that is there to react to the controls of the remote, right? And it tilts the plane the way you want it to go?? Im just trying to get the big picture of what you were trying to say.

    Reply
    1. By Colin (Post author) on

      Hi There,

      Thanks for commenting on my post. You are partly correct in what you say. The elevator does react to the input from the Remote or, as we prefer to call it, the Transmitter. By moving the elevator you either increase or reduce the ‘ANGLE OF ATTACK’ of the wing. This in turn increases or reduces ‘LIFT’ and the ‘INDUCED DRAG’ that results.

      Applying UP elevator increases lift and drag which reduces the forward speed of the plane. Applying DOWN elevator reduces lift and drag so the plane speeds up.

      In the first instance, to compensate for the slowing effect of the drag, extra power is required to maintain forward speed and the plane starts to climb.

      In the second instance, because lift is reduced the plane will descend as well as speeding up so a dive results.

      Basically, at a particular throttle setting, Elevator controls speed whilst with the elevator set at level flight setting, throttle controls climb and descent.

      Reply
  4. By HolisticJB on

    Hi Colin

    Wow you have an impressive site and so much information.

    I have never owned a remote control plane, but have often watched others fly them and been so impressed but what can be achieved.

    I’ll be honest and say I was one of those who thought the tail was there to give elevation and keep the plane up, but with your explanation of the centre of gravity point it makes total sense…and embarrasses me that I have forgotten all of my high school physics!

    This is definitely being bookmarked and sent to family. My Dad and nephews are made keen on planes. The boys have RC planes and will love this and my Dad enjoys everything to do with flying…even decided to build himself a gyrocopter.

    Thanks for an educational and informative read!

    Jason

    Reply
    1. By Colin (Post author) on

      Hi Jason,

      Thanks for your encouraging comments. I’m glad you found my site content informative. I’m also glad you feel it is suitable to share with your Dad and nephews.

      Please tell them that if they need any help or advice I am happy answer any questions they have. The website is laid out in such a way as to provide a complete program of help and support for the newcomer willing to learn.

      Colin

      Reply
  5. By Thorpe on

    Thanks for the article. It was really thoughtful and well written. It shows that you truly love this stuff and that passion is translated through your words. The pictures were really well thought out and placed in an appropriate manner. How long have you been flying planes? In your opinion what is the best RC plane on the market?

    Reply
    1. By Colin (Post author) on

      Hi Thorpe,

      Thanks for the comments on my post. Can I suggest you visit my website so that you can read up on every aspect of flying model planes. There is a page in the menu entitled “Suitable Planes For Beginners”. Go take a look at this and you’ll find some good suggestions for a first plane plus everything else you need to know.

      Cheers, Colin

      Reply
  6. By JellyB on

    Hi Colin

    I have never really paid attention to model planes as I always thought that’s a boy’s world! But I must say I’m extremely impressed with the way you’ve written this article and I’ve never heard of most of what you’ve written. Now I’m suddenly interested and would like to give this a go. I do have a nephew who like to play with toy planes so I may get him a model one and take your tutorials as a guide! Thank you for this. Very educational and informative.

    Cheers!
    Jelly.

    Reply
    1. By Colin (Post author) on

      Hi Jelly,

      Thanks for your kind remarks. I’m so glad I’ve managed to attract a female reader and give you an insight into my favourite hobby. There are many women who fly rc planes so you won’t be alone. Learning with your nephew should be fun but I strongly recommend you get him to join a club somewhere near to where you live. It can save you so much money in the long term and you’ll both get great encouragement from the members.

      If you need any help, don’t hesitate to get in touch, I’ll always try to help.

      Colin

      Reply

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