EAA Chapter 25

A Community of Aviation Enthusiasts in the Twin Cities

Pietenpol – First Flight

Filed under: Member Projects — admin at 10:59 pm on Saturday, June 11, 2005

by Christian Bobka

from On Final June 2005

Pietenpol - First Flight

Greg tends to underestimate the situation…

The first flight report of the Pietenpol is as follows:

2:40 was flown in five flights this morning and afternoon. Wind was 1/2 to full quartering right headwind at a steady 10 kts. Field conditions were dry grass. Location was Stanton Field, near Northfield, Minnesota. The ship has spoked motorcycle wheels and tyres rolling on bronze bushings with no brakes and a tail skid. A straight axle and wrapped bungies provided suspension. The first flight was 45 minutes, second flight was about 20 minutes, third flight was about 25 minutes, fourth flight was 25 minutes and the fifth flight was 45 minutes.

A few years ago I wrote a long dissertation on how to select the proper axle location with the 1929 style wooden gear legs installed on the long “Corvair” fuselage. I was right on the money in the analysis because at the aft CG loading that we had, the ship would perform flawlessly on the grass. Traveling 90 degrees to the 10 knot wind, I could turn into the wind by stick aft, windward rudder, and a burst of power and I could turn away from the wind by stick forward, lee rudder, and a burst of power. I was comfortable taxiing next to buildings and other aircraft with very little practice.

I weigh 220 lbs and Greg computed that we needed 100 lbs of ballast in the forward bag compartment (aft of firewall above passenger’s feet) to get loaded CG at .5″ forward of arbitrary aft CG limit (greg will have to give you the datum and the CG range, etc.) The ship is powered by an A-65 freshly overhauled with a homemade wood prop that was made using a duplicating machine copying an old Sensenich W72C42 blade from about 50 years ago.

The motor mount as originally made had TONS of down thrust and TONS of right thrust welded into it: like 1″ in each direction over the length of the crank. The angle would be arctan(1/24) . This looked so far out that spools were fabricated and used to shim the motor back until it had “a little” right thrust and “a little” down thrust.

On takeoff, a pronounced and uncomfortable tendency to turn left was observed which required a constant input of 1/2 right rudder at cruise settings and 3/4 right rudder at full power and climb speed. To let up on the rudder would invite a rapid yaw-induced roll to the left. This kept me making almost all the turns into the rudder (to the right for those of you in Rio Linda).

The aircraft is equipped with the highly calibrated Johnson wind vane type of airspeed indicator and it showed about 35-40 mph in the climb and about 55-60 flat out. The engine rpm in a moderate climb was 2100 indicated and the flat out rpm in level flight was 2220 rpm. The tach has not been calibrated. The rpms sounded right for 2150 or so in cruise and 2300 rpm (the correct number we want) level flat out. The left turning tendency is mitigated when power is brought back to idle. This fact identifies the problem to be a deficiency of right thrust at the motor mount and/or left offset of the vertical stabilizer. The aircraft flew well in the 1900 rpm range. I did not feel that much was gained by running the power up above 2100 rpm.

Anyway, Greg and Dale’s initial fix for this vicious left turning tendency will be first to offset the vertical stabilizer to the left to the maximum degree we can move it which is about 5/8″ at the leading edge of the vertical stab. This fix will be instituted prior to the next flight. Then the spool spacer on the motor mount will be adjusted to take out the rest of the left turning tendency that we find remaining. Those of you still building, plan to allow for adjustment at the vertical stabilizer leading edge, a little to the right and a whole lot to the left.

The A-65 equipped Piet is said to have increased vertical surface forward of the CG which offsets vertical surface aft of the CG. This is destabilizing in the vertical axis and appears to be present with the aircraft reluctant to return to straight ahead after a yaw is induced. I will investigate this characteristic after the aircraft is trimmed for hands and feet free flight. I would recommend that future Piet builders who plan to use an A-65 increase the size of their vertical stabilizer to help offset the increased vertical area forward of the CG with the A- 65 installations. A little extra way aft has quite an effect…

Another tendency the ship displayed was a severe nose dropping tendency. This required a tiring constant pull on the stick of more than 6 lbs or so. Letting go of the stick would hang me on the straps as the nose pitched over.

This could be attributed to engine downthrust or to aerodynamics and needed further investigation. I found that the pull on the stick was independent of thrust produced. It is an aerodynamic issue that needs to be cured by either lowering the leading edge of the horizontal stabilizer or raising its trailing edge. This is difficult to do in practice as the Vi Kapler rudder hinges are reluctant to move up or down the rudder spar. Again, current builders, allow for the ability to raise or lower the leading edge of the horizontal stab by using shims at the forward attach point only. You may also need a space to exist between the bottom of the vertical stabilizer and the centerline of the horizontal stab to allow for a slot where vertical positioning of the horizontal stab can be made. Just give some thought as to how you will allow the leading edge to be raised or lowered 1/2″ or maybe even more after the ship is assembled.

Knowing that we had 100 lbs of ballast in the forward bag compartment, we removed 40 lbs of it and that relieved maybe 1/4 of the 6 lb pull on the stick. Greg was concerned that would put the ship aft of the arbitrary CG aft limit. However, power-off stalls were performed both with 100 lbs and 60 lbs of ballast and in both cases the ship had no difficulty in lowering the nose to unstalled flight upon the slightest easing of aft stick pressure. When the stick was held full aft, gingerly use of the rudder could hold the ship in a falling leaf but you had to stay right on it with a good horizon. There was also a left wing heaviness that was mostly mitigated by shortening the left rear strut by 1-1/2 turns and lengthening the right rear strut by the same amount.

As test flights go, the ship was moderately difficult to fly as it needed continuous substantial input in all three axes, gobs of right rudder, a lot of aft stick, and a bit of right stick. I tried taking pictures but gave up after three because I could not take pictures and fly at the same time.

We will work through each item until the ship flies properly. Ideas, comments, and insights are welcome.

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