Tech Talk - Aerodynamics 101
by Bob Wilson, President
"OK, Jim has been bugging me to get something started under the heading of "Tech Topics" so I'll go ahead and get the ball rolling. Hopefully, somebody will want to argue and will get involved so that this column can continue on and to start things going, I will discuss Aerodynamics. There should be enough meat there to start this thing off.
One thing about full scale flying is that what you don't know can kill you. We call it "eating your lunch or buying the farm." It's not so important to flying RC models. No one is in danger (usually) and the most that can happen is that you are out a model and some money.
Let's start with stalls. On many occasions I have watched RC pilots lose an engine and attempt to glide back to the runway and in doing so, stretch the glide to the point that they stall the airplane and snap roll into the ground. Often with resulting damage. All airplanes, models or full scale, have a glide ratio, that is, they will fly forward for X number of feet for every foot they lose in altitude.
Some of our models are slick and very efficient with low drag and will glide quite far and others, models with lots of drag, will glide less efficiently. In full scale flying, we were taught to keep the nose down, establish the proper angle of glide and hope for the best. (usually a pasture) The same rule applies to our models. With the loss of the engine, keep the model gliding at its best glide ratio and never slow it down enough that it will stall. At this point, your instincts will tell you to pull back and maintain altitude but this is when you will incur a stall and either snap or dive into the ground. It takes a lot of discipline to keep the model flying and when you do slow down only do it when you are just about to touch down. Understand that even if you are in tall grass or weeds, you must resist the temptation to pull on the stick and stall.
So what is a stall? A stall is when the airflow over the wing breaks away from the surfaces of the wing. In other words, the wing ceases to support the aircraft and is no longer flying. Bernoulli's Law states that when a wing is flying, it crates a vacuum over the top of the wing and a high pressure area along the bottom of the wing and this Law continues to be argued even to this day by some of our top aerodynamicists but the simple fact is that when you slow down too much, the wing is unable to support the 1 G load and falls out of the sky.
Every airplane has a most efficient glide angle and speed. Even our models. There is a speed at you can glide at which will give you the most distance before touchdown (splat!)
With an engine out, you hold this speed and only slow up just before touch down. Again, your instincts will tell you to pull up but you must resist that instinct because it is wrong. Accept the inevitable and that you are about to land in the weeds and may incur some damage but that is better than fighting it and snap rolling into the ground where you will do far more damage.
Newbies often believe that the wind is "pushing" an aircraft. In short, the wind does not push an aircraft, be it a model or full scale. If someone wants to pay for renting a Cessna 172 I would be delighted to take them up and demonstrate that the wind does not indeed push an aircraft. To understand this, think of the wind as being a very large conveyor belt and that the model or full scale airplane is flying along on this conveyor belt. When one is riding the basket on a full scale balloon, there is no wind blowing over the balloon. You can smoke a cigar and watch the smoke drift up. The balloon simply flows with the wind on that same conveyor belt.
Airplanes and balloons do not know the wind is blowing.
If an airplane, full scale or model, is flying into the wind it will fly at exactly the same speed as when it turns downwind. Understand I am not talking about speed over the ground, only speed in the air.
Yes, if you fly downwind in a 20 knot breeze, you will be going 20 knots faster over the ground, but not in the air.
At OTX, we often have rotors. Rotors are comparable to ocean waves and they are caused by air flowing over trees and other obstructions near the ground. When the wind is out of the south you can see their effects easily because a south wind blows over trees and other obstructions. We can also see the effects of thermals. When your left wing tips up you can be sure that there is a thermal off to the left of the model or if you are trying to land and close to the ground, you may be seeing the effects of a rotor. In any event, the wind is not pushing the model but you are seeing the effects of general turbulance in the form of disturbed air, rotors and thermals.
One of the reasons I like to make short carrier style landings is not only to avoid losing sight of my model during final approach but to avoid prolonged fights with turbulance during the approach.
I'm sure that most of you have noticed that the turbulance seems to end just about the beginning of the runway at OTX and this is probably because you are getting away from the line of trees.
Unfortunately, wind has always and always will be a problem for our models. The problem is exacerbated by the fact that we have to land cross wind almost all the time. The bright side of all this is that dealing with winds and crosswinds will make a far better pilot of you. You learn to make the model do what you want, not the reverse.
Enough for now.
Macon Aero Modelers, Inc.
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