Tech Talk - Aerodynamics & Wind
by Bob Wilson, President, Macon Aero Modelers
I hear a lot of confusion about winds and their effect on aircraft and thought I would add a couple more comments to my earlier discussion about aerodynamics. The confusion seems to be in the non-pilot group who have never had training in full scale aircraft. My offer to demonstrate that wind does not increase the speed of an aircraft and that the wind does not push an aircraft still stands. If anyone wants to pay to rent a full scale aircraft I will be happy to demonstrate this at any time.
When landing a full scale aircraft in strong winds we usually add about 10% to our airspeed, and depending on what sort of aircraft it is, pilots often elect to not use flaps at all and if they do, it is usually no more than 10% flaps. This especially applies in a cross wind since the first thing you want to do is get the airplane firmly planted on the runway so that you can steer it though the cross wind.
The reason we increase our landing speed in strong winds is that close to the ground, we are subject to wind shear, and the extra speed keeps us from stalling in the event the wind suddenly dies on us. If you approach at 60 Kn in a 25 kn headwind and the wind suddenly dies on you, you may find yourself in a stall condition which may cause you to buy the farm.
The problem exists because wind, close to the ground, is subject to a number of factors. Wind shear being one of them but turbulance that can disrupt the landing approach is the most frequent occurrence. When you get close to the ground you will be picking up turbulance. In the mountains, you can pick up up and down drafts (turbulance) from the mountains at much higher altitudes. Generally, in flat and level country, turbulance disappears once you get some altitude on a good day yet the wind may be moving at 50 kn but you would never know it in the aircraft except that you are covering more, or less, ground than you planned for.
Obviously, our rc models don't fly at 3,000 or 4,000 feet and we are stuck in lower altitudes of perhaps a couple of hundred feet and it is in this lower altitude that we encounter turbulance
At OTX, we encounter turbulence while on a landing approach and this is due to the trees, fences, overgowth and maybe even the stuff growing in the nursery next door. In addition, wind close to the ground tends to do something similar to ocean breakers at the beach and rolls across the ground, creating more turbulance. In fairly light winds, the wind may die entirely and you find yourself short of the runway or the aircraft stalling on landing.
In addition to all this, we have the effects of thermals. Thermals rise even in cold weather and the time of year has little efffect on thermals because thermals are the result of warm air rising and cold air dropping. It's the relative difference in temperature that creates a thermal. Glider guiders all know this and actively hunt for thermals to stay in the air.
No model airport is free of turbulance because of the low altitudes we are flying at. It exists any time there is wind blowing and we can always expect to run into it under those conditions.
;There are two safe ways to land in turbulent winds. One is to keep your speed up slightly and use most of the runway. When I break this rule, I always end up in trouble. The other method is to keep your landing pattern in close for the simple reason that the less time you spend in turbulance, the better the landing will be.
When I drag out too far on final approach, I not only have more difficulty in seeing the aircraft in front of that mountain the other side of 441 but I encounter more turbulance. This is where practicing the "carrier approach" comes in handy and even the largest of our models can safely do a carrier style landing and still have enough runway.
Of course, large light aircraft such as the U-Can-Do or the Yak 54 with their light wing loadings will react more to turbulance than a model with a higher wing loading.
My often quoted statement that "aircraft don't know the wind is blowing" still stands as any trained pilot will tell you but it does react to turbulance, gusty winds and wind shears. Lets not confuse turbulance with wind at altitude. Some of you may not be too interested in all this but the fact is that aerodynamics is a most fascinating subject and while you may not have to deal with it as a full scale pilot, I can tell you that all of our champion 3D and IMAC pilots all have a pretty darn good understanding of the physics of aerodynamics."
That's this installment.
Macon Aero Modelers, Inc.
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