Heavy Stunt

By Bruce Hunt

January 2009

Flying in the wind

During the 2007 Fall Follies contest in Salem, flying in the wind was a challenge and only three Expert level pilots put up a single flight each: Paul Walker, Bob Smiley and myself. Walker's first flight in winds gusting to 20mph from the south and over a line of trees looked almost effortless. I wish I had that flight on video so I could use it as a reference of how to fly in those conditions.

Since my model was in excellent trim and had flown Classic well enough in the wind the previous day, I took my shot as the second flight. Launching just to the right of down wind, the tail immediately lifted at launch requiring a jump into the air to avoid having the prop hit the pavement. Flying upwind, the turbulence from the trees and the cars parked at the side of the field threw the model around like a leaf. There was very little tension up wind and the erratic gusts required stepping back and holding level flight higher than it would have been on a calmer day. Going into the inside loops the engine picked up RPM, a bad habit the LA46 had when flown without a warm-up flight. Adding the engine speed to windup from the wind required flying much larger loops that were still among the fastest three loops I've flown in competition. Now I knew the engine would continue to run away during the inverted flight and outside loops. While it helped to have some additional speed during the inverted laps the bumpy air up wind led to higher inverted laps. Biasing the outside loops a bit to the right of downwind and making the sizes larger helped control the outside loops even if they were over in record time. Exiting from the outside loops the engine finally slowed and I breathed a sigh of relief knowing that the timing on the square maneuvers would be more controllable. Such was the case all the way through the horizontal eight.

Now it was time for the vertical eight, hourglass and overhead eight. If you've flown these maneuvers in wind before you know they can be challenge. My strategy was to fly them big and keep the wing from being perpendicular to the wind at the top. Going up I made sure that the transition from the inside to the outside loop maintained a climb and going down I made sure the transition from the outside loop to the inside loop was flat. The hourglass was more vertical with the top more rounded and behind the head. The overhead eight has for some reason been one I like to fly in wind. Bringing the inside and outside loops down and lower than 45 degrees, I let the plane fly into the wind. I knew that patience would bring the model back to downwind where a turn to overhead would set up the next loop. This was one time I did not want to make the mistake of flying the overhead eight in front of me. For the clover leaf, I concentrated on keeping the first inside loop high and big making sure I kept plenty of altitude to work with. Pulling out of the clover into the wind I cut the wingover short after it crossed overhead. Now the trick was to make sure the model landed downwind. With a hard tank all that it required was to do a cut-off loop when the model was just passing downwind. With the start of the cut-off loop I started to walk backwards adding energy that would be required to pass up wind on the way to a downwind touch down. That was the plan, but the additional energy resulted in the model gliding past downwind where it settled to the ground almost vertically with a three foot roll out.

That flight was the impetus for writing this column. After reviewing some of the comments by Ted Fancher and Brett Buck on flying in the wind during the 2003 NATS, I will summarize the key ideas. I will cover three areas that relate to improving flights in the wind: maneuvering, trim, and design. Obviously by the time you take the field at a contest on a windy day, design considerations will be of little value. But now is the time to consider the issue when you are building your next model.

One of the main design features that gives the modern stunter an edge in the wind is its increased tail volume. That is to say, models that have larger horizontal stabilizers combined with longer tail moments can be trimmed with a center of gravity closer to the center of lift without becoming unstable. In the wind, this translates into a reduced tendency to "wind-up," the gain of speed during a maneuver, and the improved ability of the model to turn with less control input. A nose heavy model, one with the center of gravity forward of the center of lift will be more stable in level flight but will also have more windup in windy conditions. The wind pushing on the wing during downwind maneuvers acts like the wind on a sailboat when it tacks into the wind. The nose heavy model wants to follow a large turn but the wind is trying to reduce the size of the turn adding energy and speed to the model. The faster the model travels the more force it requires to turn. This becomes a limiting factor that defines the smallest radius turn the model can make, sometimes referred to as the "Netzeband Wall".

Another design issue that affects engines turning larger props and or props turning at high RPM is gyroscopic precession. The effect we are concerned with is a tendency for the model to yaw outward, turn out of the circle, on inside maneuvers and yaw inward, turn into the circle, on outside maneuvers. In wind, the yaw effects the tension felt at the handle during maneuvers adding to the tension on inside maneuvers and reducing it on outside maneuvers. In wind, the loss of tension at the top of vertical maneuvers such as the vertical eight and hourglass can be a heart stopping moment. Engine setting and prop selection can minimize the gyroscopic effect but it can also be reduced with leadout position if a reversed bellcrank is installed during construction. Using a reversed bellcrank, puts the down line to the rear and farther behind the center of gravity of the model. During outside maneuvers, the model yaws out slightly to align the down leadout exit from the wing with the center of gravity. The leadout position, when set to counteract the gyroscopic precession, can eliminate the tension variation by trimming the model for zero yaw. The use of a 4 inch bellcrank will also provide the slower controls and extra leverage needed for improved maneuvering in the wind.

The last design option that can provide help during windy flights is the ability to do a cutoff loop at the end of the flight. The cutoff loop allows the pilot to assure the position of the landing on the downwind side of the circle. The use of a metal tank in the model makes this maneuver an option when it is needed. Plastic "clunk" tanks do not, as a rule, provide for a reliable cutoff loop.

After the model is built, it can be trimmed as needed to allow for windy conditions. The center of gravity should be moved back as far as possible while maintaining stable flight and smooth landings. This will reduce windup and the resulting tension increase during maneuvers. As already discussed the model should be trimmed for zero yaw with leadout positioning. An additional benefit to zero yaw will be felt during vertical maneuvers as all the energy of the engine will go into the climb instead of being wasted on unnecessary tension.

Assuming a well trimmed model for normal conditions, other trim issues that should be considered in wind include a wider line spacing at the handle and a slight increase in tip weight. The handle spacing adjustment counteracts the increased tension during maneuvers with an increase in control response. Tip weight helps keep the model from rolling in on the upwind side of the circle and counteracts some of the wind induced outward roll. Prop pitch and RPM adjustments can also be made to reduce acceleration in maneuvers. A lower prop pitch will reduce the rate at which the model changes speed as it changes direction. A high pitch prop accelerates the model faster than a low pitch prop. Lowering the pitch and raising the RPM can reduce the effect of wind induced windup while maintaining the power necessary for overhead maneuvers.

Once the design and trim issues have been addressed, the pilot's flying strategy and maneuver positioning in the wind make a significant difference in the quality of a flight. In general, the objective is to maintain tension, avoid windup and conserve energy above 45 degrees. Tension is a particular issue during takeoff and landing. In both cases, the model should be flown to have its slowest speed downwind and highest speed upwind. A take off upwind, the downwind side of the circle, has the risk of nosing the model over as up control is applied to lift the model from the ground. On the upwind side of the circle, the tendency is to have the model jump into the air. In general, a good strategy is to launch close to the downwind side of the circle. Once the model is in the air, the pilot must pay close attention to the exact location of the wind. In windy conditions, the direction of the wind, while gusty at times, maintains its direction much better than on calmer days. I find the wind by its feel on my face and consciously mark its direction by noticing objects in the background. Then as I turn 90 degrees to the wind, I consciously mark downwind off my left shoulder again using objects in the background. Now that I know the location of the wind, I can fly the reverse wingover by making a turn to vertical starting before reaching the upwind point. I want to avoid turning to vertical after passing downwind. Starting the turn before the upwind point allows the model to use the energy of the wind as it climbs overhead while reducing the wing area exposure to the wind. The stronger the wind the more critical it becomes to make this turn so the model is exactly parallel to the wind as it goes overhead. The same applies to the second wingover where the turn to vertical is started just before reaching downwind. A slower, less abrupt, turn to vertical will use more of the wind's energy to get you to the top. In general the heavier your model and the lower the prop pitch the more gradual the turn to vertical should be.

The next maneuvers that cause pilots problems in wind are the inside and outside loops. Here the strategy is to start the maneuver past downwind and open up the loops. The theory at work is to let the wind slow down the model and open up the loop as it turns down the back side of the loop. At the bottom of the loop the model is flying more into the wind and losing any speed it gained in the first half of the loop. By keeping the loops open the pilot can fly more controlled loops and counteract the wind's efforts to contract and speed up the loops. Another strategy that can help is to step into the loops. This has the effect of opening the loop and giving the model more distance to fly. In other words, slowing the model down.

In the wind, the horizontal maneuvers need to be flown with their intersections directly downwind. The strategy here is to make sure the top of maneuver is stretched out so that the wind doesn't compress the maneuver and move the next intersection past downwind. The horizontal maneuvers should also be kept open with the pilot moving into tension and away from slack.

The vertical maneuvers present a unique problem in wind in that the pilot wants to maintain both energy going up and tension on top. In the vertical eight, it is important not to fly the top of the inside loop horizontal but to maintain a slight climb through the intersection and transition to the outside turn. The slight climb through the intersection helps maintain the energy of the model going up which translates into more speed and tension going into the outside loop. Then the outside loop should be flown large and past vertical. This positioning reduces the exposure of the wing to the wind and helps set up and maintain the height of the intersection going down. The hourglass should also be flown to position with the top past vertical using soft corners and a short top to minimize the effect of the wind. Both maneuvers can be entered with additional energy provided by "Whipping," leading the model and pulling it as the pilot walks backwards.

The last overhead maneuver is the overhead eight. As wind increases it is flown with less a single point of intersection and more as back to back "D's". The initial climb is flown up to and past vertical on the initial climb before turning to a 45 degree return to upwind. The key here is to wait for the model flying at 45 degrees to come back upwind where it is quickly turns back to vertical and past overhead into a 45 degree outside turn back to upwind. The key strategy is to keep the inside and outside loops large and wait for the model as it flies back to the upwind position.

The cloverleaf must be flown to maintain the size of the upper leaves and the horizontal center line above 42 degrees. The risk is that the wind will push down the top causing the horizontal line to be too low. Making the upper leaves larger helps maintain the energy of the model and the final climb to overhead where the model is immediately turned back to level flight instead of following the line back to the upwind side of the circle.

The landing is very dependent on where the point of touchdown is located. As the wind increases, it becomes more important that touchdown be close to downwind. As mentioned earlier, this is where the ability to do a cutoff loop comes into play. Doing a cutoff loop downwind, pull out low and whip the model through upwind to the downwind touchdown point. This will be a fast rolling touchdown where it will be important to immediately apply down elevator to keep the model on the ground as it rolls into the wind.

All of this can be quite scary during a contest when you have not practiced in windy conditions. So get out and practice even when the conditions aren't ideal. The experience will always give you a feel for the limits of your model and the confidence needed to push your limits as a pilot.

This page was updated Jan. 8, 2009

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