I have been doing some static thrust testing. I strapped my main gear to two steel stakes, and measured thrust using an electronic livestock hoist scale in line with the strap. I did a stepped test and recorded the scale readings with a video camera for the entire experiment. I did the test in a tie down area with thick grass, where I measured an actual 75 pounds of “break-away” pull to move the plane by hand with a strap on the nose gear leg. Here is graph of the RPM history (green) for the experiment. Blue is my header tank fuel level. Time in minutes at the bottom of the graph
I am using a “generic” thrust computation equation to compute the expected thrust from my prop (54X62) for comparison to my actual measurements. My max static RPM is 2760 RPM, at a density altitude of 6000’ when I did these tests. After adding the 75 pound “break away” into the measured thrust, I get 295 pounds of thrust at 2760 static RPM. Plotting thrust measurements for all RPM steps in the experiment I get the blue curve in the right hand graph below. The orange curve is the computed static thrust vs. RPM (zero forward velocity) using the equation. A very good match between computed and measured. The left hand graph (single blue curve) is CAS as a function of ground roll, factoring in 50 pounds of landing gear friction up to 80 MPH (take off). This calculation includes airframe and induced drag increasing with speed as well as airspeed corrections applied to the calculated thrust.
The curve shows that I will require about 1300 feet of ground roll at 1300 GW to reach 80 mph CAS and then should achieve 160 CAS about 3 miles down range after takeoff if at full throttle and neglecting climb. At 1000 GW about 1000 feet of ground roll should be required to reach 80 mph CAS.
The Jabiru engine manual lists recommended static RPM range of 2600-2800 for my engine. I am in the upper end of that range, so I feel pretty good about the prop that I have for test flying.
Oh, by the way, I’m getting better with taxiing now. Partially because of new master cylinders and partially because I am learning technique. Still going slow, but getting better and faster.
Jay Scheevel, Tri-Q, prepping aircraft and pilot for flight.