Big Jim's Answers to Questions

What makes an airplane fly?

We might as well get this one out of the way.  Let it be known though that to this day there is still some debate as to what actually makes an airplane fly.  I think it's safe to say however that it 's the wings.  No, it's not the engine.  The engine pulls the thing around but the wings are what keep it in the air.  "But how?" you ask.  Well, first I guess we better discuss the forces that act upon an airplane in flight.  They are generally referred to as...

Thrust: which pulls the aircraft forward.

Lift: which pulls the aircraft upward.

Drag: which pulls the aircraft backwards.

Gravity: which pulls the aircraft downwards.

Gravity and drag are the forces we are trying to overcome with lift and thrust.The propeller provides the thrust which moves the plane forward.  The forward motion causes air to pass over and under the wing.  The shape of the wing causes the air going over the top to have to travel farther than the air going under the bottom.  This means it also has to travel faster. The faster moving air exerts less pressure on the wing than the slower moving air on the bottom.  The higher pressure on the bottom actually pushes the wing up into the air.

Airfoil Illustration

That's your commonly held explanation for what makes a wing fly.  There's more to it than that however.  The angle at which the wing is in relation to the oncoming rush of air has a lot to do with it also.  Proof of this is the fact that a wing designed to lift in one direction only can be made to fly upside down if the angle of attack is steep enough.

What does it mean when an airplane "stalls"?

Well it doesn't mean the engine quit.  It simply means the wing quit flying.  For a wing to generate lift there has to be air flowing over it at a minimum speed and in the right direction. If you take an airplane from level flight and tilt it upwards at say 45 degrees it will begin to climb but probably slow down also. As the speed of the aircraft (and the air flowing over the wing) decreases, so does the lift generated, until finally there isn't enough to support the aircraft any more and it starts to fall from the sky.   This is called a "stall".  If both left and right wings stall together then the nose of the aircraft will drop first, as the speed increases so does the lift generated and the airplane will begin to fly again. If one wing stalls before the other then the aircraft will tip sideways and possibly enter a spin.  Then the pilot will have to assist in getting the plane flying again by stopping the spin first.

What is the difference between air speed and ground speed?

If an airplane is flying in completely still air there is no difference.  However, if the wind is blowing and the airplane is flying into the wind then the ground speed will be less than the airspeed.  In other words, if the airplane is flying at 20 mph air speed into the wind, and the wind is blowing at 20 mph, then from the ground the airplane will look like it is standing still. That's 20 mph air speed and 0 mph ground speed.  However, if the airplane turns around and is now flying with the wind, it will appear to be moving very fast from the ground.  That's still 20 mph airspeed but 40 mph ground speed.

If the engine quits in flight will the airplane crash?

Not necessarily.  As explained earlier, the airplane needs forward air speed for the wings to generate lift.  An airplane can keep moving forward without the engine by pointing the nose down just a bit and flying "down hill".  Sort of like a car who's engine quits.  It can keep going as long as it's down hill. Of course you can't go down hill forever, so it's nice if there's a place nearby to land.  Some model airplanes can glide quite awhile without the engine where as others have to be landed immediately.

My engine says I can use an 11-5 propeller or a 10-6. What do these numbers mean and how do I know which one to use.

The two numbers refer to the diameter and pitch of the propeller. In the USA these would be in inches, so an 11-5 propeller would have an 11 inch diameter and a pitch of 5 inches. The pitch means that the angle of the blades is such that with one full revolution the propeller would pull the aircraft forward 5 inches (in theory). The higher the pitch the farther forward the plane should move per revolution. Increasing either one puts a larger load on the engine so if you're going to increase one you need to decrease the other. Which one to use depends on the plane and what you want to do with it. Assuming a constant RPM, the larger diameter is going to produce more thrust, or pull harder. The higher pitch is going to produce more speed, or pull faster. So, if you have a biplane with lots of lift and drag you'll probably want a propeller with a large diameter to overcome all the drag. You don't need to go fast because of all the lift. However, if you had a sleek jet type model with small wings and not much drag then you would go for higher pitch to get as much speed as possible. You don't need lots of thrust since there isn't much drag to overcome.

How much does a model airplane cost?

A typical model airplane kit that would be suitable for a beginner will cost around $75. The engine for this plane will also cost around $75. The 4 channel radio that you'll need to control the plane will cost about $150. Add to this a few odds and ends such as propellers, fuel, glo-plug battery, etc. and you'll need about $350 total to get going with what would be considered a standard beginner's setup. There are cheaper airplane packages out there - many of which are made of styrofoam or plastic. I've seen some of these that fly ok, and others that don't fly very well at all. If it looks like something you'd buy at a toy store - you probably want to stay away from it for serious flying.