Intro to Electric Ducted Fans (EDF) Part 4 by Jay Raydo
In this installment we will talk a bit about some flying characteristics specific to jets. The most important thing to remember is energy management. Without a prop to provide an air blast over the flying surfaces, the jet is reliant on airspeed alone to maneuver. Also, consider the relative inefficiency of the EDF in general compared to a propeller driven aircraft. Itís roughly synonymous with driving your car in 4th gear all the time. It takes a few seconds for the jet to respond to a change in power. The fan unit will spool up almost instantly, but we arenít talking about that because changes in airspeed donít happen instantly.
One must be cognizant that a jet has to be more or less flown around 3-5 steps ahead, especially at low airspeed. When on the step, itíll respond to control inputs very well. But you must still plan ahead. When you come out of that loop or cuban 8 with the power full on, at some point in the very near future, you are going to have to turn around before the jet is in the next county. At slow speeds you may find yourself getting behind the power curve. Let it go too far and you may not have enough power to recover without a serious loss of altitude. Plan your power changes for what is going to happen, not what is happening now.
Being typically highly loaded, jets are notoriously violent in their stalls both at high speed and low. Remember, a wing can stall at any speed; it all hinges on the angle of attack that the wing sees while itís moving through the air. I can do some of the prettiest snap rolls with any of my jets just by burying the elevator stick straight back. No rudder or aileron needed. Should you find yourself in a stall with a jet while maneuvering, relaxing the elevator will allow the wing to get flying again. Unpucker and dig your shorts out. If you get into a stall during a landing approach, keep the visual picture in mind while you are building the next jet as a reminder to not do that again. I have yet to fly a jet that doesnít do a wicked tip stall when too slow on landing.
There is a small adjustment you can make on a jet that has ailerons and a radio that supports mixing the two ailerons. If you slightly reflex (raise) both ailerons, maybe an 1/8 inch, the tip stall can be significantly tamed. Think of it as a very mild crow setting. I have all of my applicable jets set up on the right slider to bring the ailerons up a few degrees. I set them when rolling out on final after the gear are out and the flaps are transiting to full. Speaking of which, if you use a radio that supports servo speed on the flaps I highly recommend slowing the transit speed to 5-7 sec for full travel. Not only does this look realistic, it keeps the jet from jumping around on you during configuration changes. Slowing down, setting up the base to final turn, and keeping the altitude reasonable is enough on your plate; you donít need the jet hopping all over on you while the flaps and gear come out.
Letís back up a bit and discuss taking off. There is plenty of opportunity for disaster here, but this is also a time to shine as a pilot. Nothing looks sillier than a jet screaming down the runway veering left and right, bouncing around, and showing no signs of wanting to lift off. Grass runways just make this worse, not to mention at some point you have several or more pounds of very expensive jet running out of room while attempting to break a land speed record. Two key factors at work here: One, remember how small the control surfaces are? Even at full takeoff speed they donít create much lift in any direction. If your landing gear are a bit too far behind the CG of the jet, it will take an inordinate amount of lift by the horizontal stab to lift the nosewheel. Letís say you do have enough throw on the elevator to force the nose off the ground, as soon as the jet gets airborne that throw is going to be waaaaaay too much and the jet will appear to suddenly jump off the ground and head for the heavens. Full power, gear down, flaps down to some degree, and severely nose up while not having much speedÖ guess what? You could show some flair by doing a dirty roll on takeoff, but maybe the trees are approaching fast.
The second thing going on here is that most full scale jets point very slightly nose down while sitting static. We, as modelers, want to emulate that. The problem is that the models themselves arenít privy to the Reynolds numbers that the full scale guys are. The moral here is that the jet sitting with a nose down attitude tends to stay ëstuckí to the ground any time it is moving forward. The faster it goes, the more stuck it gets. Add to this a sensitive nosewheel steering setup, and look out. Our jets like to have a light neutral or positive (nose up) stance on the ground. This will let the jet fly itself off the runway when it is ready, or will allow a smaller, more precise rotation to liftoff that looks pretty. Now instead of being startled that your jet is dirty, slow, at full power and climbing, you can enjoy pulling the gear and flaps up while initiating a beautiful turn. Here again it is beneficial to have the flaps come up slowly to avoid any sudden pitch changes that look goofy and might be a bit unsettling to the pilot.
When to throw the gear/flaps/speedbrake out? Follow what full scale pilots do. Each full scale aircraft has upper limits on how fast they can be going before they canít get the airplane dirtied up. Slow down to a normal pattern speed before extending the flaps and gear. It is much less stressful on the servos driving the flaps when the airspeed is under mach 1. Don't use the gear and flaps as speedbrakes. The flaps provide more lift at lower speeds. If you have trouble getting a slippery jet to slow down, the throttle management needs some work. Again, the whole ëplanning and staying ahead of the jetí thought process works wonders. Consider thisÖyou are turning downwind from your last burner pass with the intention of landing. If you toss the flaps out before the jet slows down some itís going to jump quite a bit, no matter how slow they come out. Being a highly loaded wing, each surface is going to have a lot of responsiveness. I have witnessed the flaps being put down when the jet was really moving, when all of a sudden it rolled over and split síd into the ground. The flap control horn pulled out of one flap and not the other resulting in an asymmetric deployment of the flaps. Ouch.
The smaller foamies that fly on a higher ratio of wing than power can be horsed around more without dramatic consequences. Everyone has watched my F-16s float about and carry on at ridiculously low speeds. With a light wing loading any aircraft can be flown in this manner. But even the F-16 will tip stall something fierce if you arenít paying attention. Watch the next time you see it doing a low slow pass, or an elevator descent. You will see the tailerons fluttering about. No gyros here (I havenít figured out how to put a gyro in with elevon mixing), just paying attention to the wingtips. At every wiggle, there is a correction.
Some tricks I have learned: Tone down the nosewheel steering, a lot. Only a few degrees either side of neutral are needed. Use dual rates to get the turning radius you want for taxiing. Keep the wheels no more than an inch behind the CG. Easier said than done, especially since some models canít be easily changed. Set the jetís gear up so that the nose is slightly up when at rest. Slow the flaps down if you can, and when flying at a field like the clubsí, use as much flap deflection as the airplane will allow. Small wheels, heavy wing loadings, and a fast approach add up to bent gear. The more you can reduce the speed at touchdown, the better. Fly 3 or 4 steps ahead of the jet. Remember how much room even a small jet can use.
I am sure there is more on this topic, and as I think of other points to make l"ll be sure to include them in future installments.