StolSpeed Aerodynamics - Performance Enhancement for Light Aircraft

Fisher Dakota Hawk

Second test of plane with entire wing covered with Feathers.

All airspeeds unless noted as GPS are direct readings off the airspeed indicator. They are not corrected for angle of attack or or other possible calibration errors.

During first test I found it not possible to get a full stall at 1000 rpm and slow deceleration with full back stick. Although airspeed finally stabilized at 34mph indicated on Air Speed indicator.

Before second test I measured full up elevator to be 18.9 degrees. The maximum design up limit is 25 degrees. I increased elevator up throw to 22.9 degrees.

The next flight test I found the following:

Slow deceleration with engine set at 1000 rpm I still could not get enough angle of attack for a full stall but did get more. Finally speed stabilized at 31.5 mph ! Yes 31.5mph! At this speed there was still aileron control but it was diminished to about the same as it was prior to VGs at 40 mph.

I let the airspeed increase to 35mph and found very good aileron response. Rolls from left to right to 30 degrees either way while keeping the ball centered were very positive. Quick stick movements of full aileron left to right without waiting for a bank to increase more than about 5 degrees showed definite and positive aileron response. This latest was a good demonstration of aileron control one can expect when at a three point landing is made in strong and turbulent cross winds are encountered.

Prior to feathers VGs at 35 mph there would be very minimal aileron authority.

Now at 35mph there is strong aileron authority.

Next test was to set rpms at 1550 rpm and continue with slow deceleration to minimum airspeed or stall which ever occurs first.

Very surprisingly the airspeed bottomed at 27 mph at full back stick. 27 miles per hour!!! The rate of climb meter showed a steady 200 fpm down but the plane would not stall. Keeping the ball centered continue controlled flight. There was still some aileron control remaining, but no longer strong. Using coordinated rudder and aileron I was still able to make turns left and right to any heading I wished.

On the upper end of speed at cruise I wished to determine if increased drag there would be a penalty. Using routine 4 way gps testing techniques the speed at 2350 rpm (my cross country cruise rpm) at 4,000 msl and 43 degrees F the following:

102 average mph. This very closely matched the speeds of the first tests after VGs, where the speed was 102.5 mph. It therefore looks like a drag penalty of 1.5 to 2 mph at cruise speeds.

On the taxi strip coming back in to the hangar area I was taxiiing at about 10 mph into a 10 mph wind. The airspeed indicator was bouncing between 20 and 22mph. I decided to look for the aileron effect at this speed. Oscillating full left and full right with the stick the wing tips could be rocked an estimated 4 inches up and down (total excursion of 8 to 12 inches, the more significant when it is understood that my gear is quite stiff and wide. It has the Fisher spring kit. No doubt this is added evidence that there is considerable force available to the pilot to counter wing lift situation (wing lifts in a cross winds, is a major reason for experienced tail dragger pilots to still have a loss of control on landings. In that case a crosswind gust can lift a wing and full aileron will not be enough force to bring it down. If this continues for just a very short time loss of directional control is almost sure to follow.

I had in previous times worked this same exercise and could just barely see a response at the wing tip.

................

Today I flew in calm weather shooting several takeoffs and landings most on asphalt and the last on grass.

While at altitude I tried a near full power (2400 rpm ) stall straight ahead. Slow decelleration until stall, indicated airspeed 24mph and plane at a vertical pitch attitude so extreme as to be disconcerting. I felt it was near 60 degrees. The break when it finally came was very sharp, ( the bottom just dropped out). It did drop straight ahead.

I have been flying for a long time and and a cfi for a good part of that time. I was not greatly surprised by the quite dramatic downward pitch as I expected it, however it was the most complete and full break I have ever experienced. The Mfgr of Feathers vgs told me that when a full stall occurs with vgs it will result in a "sudden break". He is correct.

I guess this is not surprising as vgs keep the air attached to the entire wing more than is ever possible with out them. So when finally the break occurs it is complete and total across the entire wing.

The recovery was without incident as the pitch down was straight ahead. I did not observe the altitude loss. In due time I will perform it again with more care to watch rate of climb or loss just prior to stall break or the altitude loss.

Later I practiced stall approaches at 1500 rpm and it again repeated to be 27 mph indicated air speed. and 1000 rpm and again it repeated to be 31.5 rpm. In these last two I had full back stick with constant rpm would not get a stall break.

Tom Marsom, Wisconsin, USA.

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