Building the MachNone
A Sky Pup Construction Project
Having flown Sky Pup SN 2028 for over 60 hours in various wind conditions, I want to pass on some observations. My Pup has a tailskid and I operate mostly from unpaved grass landing strips. Also I have installed smooth tires on the wheels in the hope that they might slide sideways a little on landing if needed. Your Pup may handle differently so the following suggestions should be taken only as personal observations on my part. I had minimal prior experience in 3-axis controlled aircraft and this may have been an advantage when flying the 2-axis Sky Pup. The casual observer might think the Pup has no crosswind potential at all. I think you will be surprised that with proper technique, it can handle more crosswind than some so called 3-axis ultralights.
Briefly, for crosswind landings make a crabbing approach with plenty of airspeed and gently straighten it out just before touchdown. Wheel landings are best and you should keep the tail up and the aircraft rolling straight ahead until it slows down.
The important thing you must have is plenty of airspeed. A faster approach reduces the amount of crab angle needed and gives quicker control responses which may be needed for turbulence near the ground. A faster landing also reduces the amount of drift the landing gear might be exposed to. In a strong 45 degree crosswind the groundspeed will be reduced considerably anyway. Do not “kick out” the crab suddenly or the upwind wing will lift and result in a one-wheel landing. It’s easier than it sounds because there is almost always a noticeable wind gradient very close to the ground. The wind gradient does much of the work for you. Gradually reduce the crab angle as you descend through the gradient in whatever amount is needed to maintain a straight ground track. It all happens fairly quickly but with practice it becomes a smooth natural transition. For me this occurs at about 6 to 10 ft. above ground during the roundout portion of the landing. The actual flare should be very minimal as this is supposed to be a wheel landing. I always leave some power on and if the wings aren’t level or the aircraft isn’t aligned with it’s ground track I have two choices. I can add enough power to fly level and try to straighten it out or I can climb back to pattern altitude and try again.
Now for the most important tip in this whole article. Often during or after touchdown the upwind wing will start to lift and the Pup will start to yaw into the wind while still rolling fast. This usually indicates that you are landing too slow, flaring too much, or letting the tail down too soon. At this point there is a moment of confusion because your learned response is to use the rudder to lower the wing. That action will yaw the Pup even further from it’s direction of travel and place excessive side loads on the gear. Instead, use the rudder to keep the Pup going straight and lower the wing by getting the stick forward and keeping the tail up. This reduces the angle of attack and destroys the extra lift produced by the upwind wing. There isn’t much time to think about it when it happens and the wrong response will make the situation more unpleasant in a hurry. In strong crosswinds the Pup will tend to curve into the wind as you slow down. When possible you can try to land at an angle to the runway to reduce the drift correction needed. On narrow runways I find this difficult so I usually make a straight in approach. Direct 90-degree crosswinds are the hardest. My personal limit is about 10 mph for a direct crosswind and 15 mph for a 45-degree crosswind. On narrow paved runways my limits are lower. Wide runways with no obstacles allow you to land in almost any direction.
For crosswind takeoffs use the same techniques as for landing. Keep the tail high and delay liftoff until both wings are ready to fly. If you takeoff too soon the Pup may sink back to the ground in a drift and damage the gear. If the upwind wing starts to lift in spite of forward stick then let it. Use the rudder to keep it rolling straight ahead and wait a few more seconds until the other wing is ready to fly. As you lift off allow it to crab into the wind and climb straight ahead over the runway. With the ample power provided by the Rotax, my Pup can easily handle more crosswind on takeoff than on landing.
Don’t get the idea that these techniques work perfectly all the time or even most of the time. Many ultralights would not survive the occasional hop, skip, and jump takeoffs and landings I have made in crosswind conditions. Most of the credit goes to the rugged and extremely stable landing gear of the Pup. Its wide gear track, the low overall CG, and reluctance to groundloop, or drag a wingtip, result in very forgiving and tolerant landing characteristics. A traditional ultralight with small diameter wheels on a tricycle gear and a relatively high center of mass seems tricky in comparison to the Sky Pup.
Taxiing the Pup in a strong crosswind is another matter entirely. Because of the generous aft fuselage side area the tendency to “weathervane” into the wind is strong. I have made good landings and takeoffs in crosswind conditions in which I could not safely taxi. Turns at the end of a narrow runway can become impossible in a strong crosswind if the grass is deep or the ground is rough. Sometimes you just have to swallow your pride, climb out, and push it by hand. After all, it’s the flying that really counts.
A steerable tailwheel would probably handle crosswind conditions better. I like the skid because it is rugged, simple, trouble-free, and may help reduce the landing rollout. A tailwheel is needed if your CG is well aft (7 inches or more) as it becomes difficult to lift the tail enough with forward stick for taxi turns. Also realize that as the CG moves aft, the groundlooping tendency will increase. If you do install a steerable tailwheel, please make provisions so that side loads on the tailwheel will not be transferred to the rudder horn or the hinge fabric. A builder at Oshkosh said he is using a swivel tailwheel from a skateboard which he says works fine. Conventional wisdom is that a free swivel tailwheel would be the least stable configuration but it may depend upon the individual pilot, aircraft and operating conditions.
In closing, I do not wish to seem to encourage anyone to attempt difficult crosswind landings. There are times when it can take 3 or 4 landing attempts to get back on the ground when turbulent conditions are a problem. Sometimes you get lucky and the landing attempt occurs at the same time as a lull in the wind. You may have to land in a nearby hayfield or go to an alternate airport if the wind changes or increases during the flight. I do admit to planning cross-country flights based on the wind direction and the runway available at my potential destination. Doesn’t everybody? In actual practice I routinely fly with several high performance ultralights which have far greater crosswind potential due to their full span ailerons. The average ultralight pilot however wisely chooses not to use that potential because of the skill required and for fear of damage to their fragile $6000 to $8000 flying machine. For many recreational pilots the trade-off for this largely unused potential is the added concern for stalling and spinning while in slow flight.
Here’s one last tip about crosswind landings. Beware of cold winter days and flat clean terrain when there is little or no wind gradient to diminish the crosswind component. It can be quite a surprise as even light crosswinds are challenging.
I would appreciate very much hearing about your experience.
Good luck, and Have Fun.
(Sky Pup News No. 16)
Crosswind Landing Tips