RC Model Aircraft Aerodynamics – the Basics

Aerodynamics Basics to Improve Your RC Planes Flying

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This short piece is to explain some of the aerodynamic forces that affect your flying and what they mean. Yes – it is theory but it will be worthwhile anyway.

Turning

Why does an aircraft turn when it is banked? Why does an aircraft lose altitude when it is banked?

The answer to both of these questions is the same and the diagram on the right illustrates it.

When the wing is parallel to the ground all the lift acts directly against gravity. However, when the plane is banked the lift doesn’t fight gravity directly. It pulls up perpendicular to the wing, but that is no longer straight up.

We can think of this lifting force as acting in two directions. Some acts against gravity (less, however, than when in level flight, so the plane may lose altitude). Some of the lift acts in the direction of the horizontal, pulling the aircraft into the turn.

Stalling

Many of us think of stalling as simply that speed where we lose lift and the plane noses down. Of course, the whole story is a little more complicated.

Stalls happen when the wind stream striking the wing passes the critical angle of attack (see Wikipedia for more detail if you want). This is normally 14-16 degrees.

The picture on the right tries to illustrate this (click on it to see a blown up version).

As you pass the critical angle of attack you get what is called a separated air stream (go and look at Wikipedia if you want more details). This separated air stream (where air passing over the top of the wing doesn’t meet air going under the wing) causes turbulence and destroys the low-pressure system above the wing that normally gives lift. That’s why you stall.

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Given how difficult it is to measure true airspeed in our models anyway, the more reliable indicator of stalling is to watch the way the aircraft behaves in the air. If at neutral attitude she can’t maintain altitude then you are potentially heading for a stall (of course you may also be descending to land under power…).

Adverse Yaw

There’s a pretty good explanation of this one on Wikipedia which is worth a look which explains the forces involved. I’m going to put it slightly differently because I’ve never seen adverse yaw that prevents an aircraft from entering a bank (I’m not saying it doesn’t exist) – adverse yaw is a yawing force (so a force that rotates the plane around its CoG on an axis drawn from the top to the bottom of the plane {what the hell do I mean? Think of your plane cartwheeling – that’s the axis of rotation “yaw” describes}) that causes the nose of the aircraft to fail to track true when you bank the aircraft using the ailerons.

So if you roll the aircraft and it tends to go nose high or nose low (so if you from level flight you roll to perpendicular and the fuse is not parallel to the horizon) you are getting some adverse yaw.

Now – if you’ve never really noticed this before chances are you don’t get it, or don’t get it enough to make a difference.

The correct way to fix adverse yaw is with differential ailerons as the first option (so the aileron that comes up comes up further than the one that goes down goes down – there’s a good pic on the Wikipedia page – check it out). If you’ve got a computer radio and your ailerons on different channels then chances are you have a differential function you can use. If you don’t the same thing can be achieved by carefully adjusting the neutral position of the servo arm (assuming ailerons are on different servos).

Although your servos hopefully throw the same arc by changing the neutral position in that arc you can make the push/pull motion in one direction larger than the other. Set them up so that in the direction you want the most throw the servo travels through the point perpendicular to the servo. Hopefully, the picture on the right makes this all clear.

If you only have one servo to control both ailerons then I guess you are going to learn how to use the rudder to compensate – have fun with that.

Induced Lift and Aerodynamic Lift

Aerodynamic lift is the low-pressure system created above an airfoil as a result of the Bernoulli Principle. You need something shaped like an airfoil to produce it. Just as an interesting aside, it’s the same effect which allows sailing boats to sail into the wind. While they are sailing into the wind the sails take a shape similar to an airfoil. However, rather than trying to create a low-pressure system on the outside of the surface of the sail, the goal is to create a low-pressure system just in front of the leading edge of the sail, to pull the boat forward. Just like an airfoil they also have a critical angle of attack which if you exceed (by trying to sail too close to the wind) you will take away the “lift” and the wind will spill from the sales.

Aerodynamic lift is the one that fails when you stall. However, you get more lift for less drag from aerodynamic lift than from induced lift.

Induced lift is the equal and opposite reaction when an airstream strikes an object and bounces off. A simple cross kite is kept in the sky by induced lift – the airstream hits the kite and bounces down. As a result, an equal an opposite force pushes the kite up.

Flatties (models made from depron etc. with flat wings) fly only on an induced lift. They are light enough that they do not need much lift (and half the time they have their nose in the air hovering anyway, so who needs a wing?).

Planes with a “built up” wing (an airfoil) use a combination of aerodynamic and induced lift to fly. For example, when a powered plane approaches to land with its nose slightly up it is using the induced lift (as well as aerodynamic lift) to achieve a very slow but very stable flight. However, the amount of thrust required to hold the aircraft in this attitude is surprisingly high (given how low its airspeed is). That’s because the induced lift comes at the expense of large induced drag. Likewise when the aircraft climbs out. It is getting a lift from both aerodynamic and induced sources. However, as the plane transitions to stable level flight, the induced lift all but disappears (as does the drag it creates) and the aircraft enters its most efficient flight mode.

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