Friday, November 19, 2010

What Is 3D?

Hey Guys! I thought that I might use what I learned over the course of my flying career to touch on the subject of what 3D really is, and what can help us to be successful at it. I guess the common definition of someone who flies 3D is someone who is completely out of their mind, so that makes me a perfect candidate! I’ve learned that there actually is a scientific definition to the term “3D flying” and that there is a science to it. Now I’m going to use the stuff I’ve learned and try and put it here without confusing you… or myself.

3D is classified as the flying of an RC airplane through extreme maneuvers, mostly high alpha, where the wings are in a complete aerodynamic stall. Although this may be puzzling to a full scale pilot, the realm of RC makes this possible due to the ability to have greater power to weight ratios... and also the fact that nobody will die if the plane crashes... which is very common.
           First thing I would start with is a descent power to weight ratio. A good one to start with is 2:1; for every one pound of airplane you have two pounds of static thrust.

Another thing is to have an airplane that is capable of extreme control throws. This allows the most amount of vectored thrust to be re-directed to make the plane move in a desired direction.

Also, having a propeller with a large diameter is a big help. Without an adequate diameter, 3D is near impossible. I own a Precision Aerobatics Extra 260 48". This airplane has enough vectored thrust for almost every 3D maneuver with a 12x6 prop on a 3S LiPo battery.

A friend of mine owns the same airplane, but was flying with a smaller diameter prop, a 9 or 10 inch, and 3D was impossible. Even though he was flying with a 4S LiPo, which provides more oomph, the plane did not have enough thrust for 3D maneuvers... but man did it sound cool as it was fumbling around looking like a chicken with its head cut off as I attempted to 3D it. The plane could not 3D at all! The small prop made it almost impossible to pull out of a hover, and extremely hard to do a rolling harrier. 

              Having a big prop increases the amount of vectored thrust being blown over the control surfaces. During most 3D maneuvers, the only air flowing over the airplane and its control surfaces is that provided by the propeller. If you have a big prop, then that increases the amount of vectored thrust, and therefore, your control surfaces have more air to move which gives you greater control over your aiplane at different attitudes. If you have a prop that is too small... say bye bye to 3D, because it will be next to if not completely impossible!

            For a beginner, having an airplane with a large amount of wing area will also help with some 3D maneuvers. The maneuver called the harrier benefits from having a larger wing because it disperses the weight of the aircraft over a larger area, therefore making the air lift less weight per square inch. This lifting force is called wing loading.

         Say your airplanes has a 15 oz per square foot wing loading. That means that due to the size of the wing (wing area) and the weight of the plane in ounces, the wing has to lift 15 ounces per square foot of wing.

           Another example of a 3D maneuver is hovering. Obviously, the wings are not producing any lift because the plane is pointed up at 90 degrees. For this maneuver big wings are not needed, and in fact, in some cases it may be better to have smaller wings so that the wind does not influence the aircraft as much. During a hover the only things holding the plane in the air are the power to weight ratio and the control surfaces moving to direct the vectored thrust to keep the nose up at the desired attitude. 

             One more example of a 3D maneuver is the blender. This maneuver can be performed basically anywhere in your flight, and it is a spectacular maneuver. You give full rudder, full aileron in the opposite direction, full down elevator, and full throttle. Having a smaller wing benefits this maneuver as well, as there would be less are resistance during the maneuver. One goal to reach for is to get an airplane with a moderate size wing, such as the 3D Hobby Shop Extra 300 SHP, to learn 3D. Then move on to more advanced airframes.

           Next thing is Center of Gravity. The Center of Gravity, or CG, is the place on the wing where the airplane balances during flight.  For 3D, a slightly aft CG is preferable. There are several reasons for this.

First, is that most of the 3D maneuvers are performed at a “high alpha” attitude, or extreme nose up; so if the nose is up, the tail has to be where? Down! So having more weight back there helps with pilot workload.

Second, having weight in the tail provides the aft end of the plane with more weight, which means more inertia or momentum. This is a good thing. For maneuvers such as an Enema (yes that is its actual name) or a flat spin, you need the extra weight in the tail to provide the momentum needed to swing the tail end around.

Having a tail heavy CG also increases the tail surfaces’ authority. By placing the CG farther aft, the elevator and rudder become more effective because the tail has more leverage since the tail surfaces are now closer to the CG. This is about the opposite of the leverage we are used to; the longer the handle on the wrench, the easier it is to tighten the nut. The reason why the aerodynamics work like this is because the airplane is going to try and rotate around the CG, and now the tail of the airplane has less distance between it and the CG, which means that it can travel less of a distance for the same amount of pitch or yaw; ergo, a quicker response and greater control authority.

Okay... that's enough for today. Happy Flying!

I've attached a video below. It contains many 3D maneuvers.

3DHS Extra 300 SHP Huckin' from Thomas Kitt on Vimeo.

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