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.
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.
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.
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.
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.
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”.
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.