On your Roadkill Series Curtiss-Wright CW-1 "Junior" is the engine set 0 or up thrust or down thrust?

From : Don Stackhouse

Simple question, as usual not a simple answer.

On that particular airplane it uses a fairly large dose of up thrust (i.e.: the front end of the thrust line is higher than the back end).

On other pushers we've worked on, there is zero in some cases, or considerable down thrust in others.

The usual point of up thrust or down thrust is to eliminate any effects on pitch trim from changes in power setting. Complicating that is the issue of whether you want the plane to maintain a constant flight path, with an increase in airspeed when you increase power, or whether you want it to maintain a constant angle of attack and lift coefficient, so that an increase in power results in a climb, but with no change in airspeed, or some combination of the two. This is a design decision and a judgement call, based on the plane's mission profile and the designer's judgement and preferences.

In the case of the Junior, flight training was one of the primary missions. Since teaching the concept of increasing power to make a climb, and reducing power for a descent is critical for the proper training of new pilots, we decided to go more towards a setting for the thrust angle that gave a constant airspeed and angle of attack.

Normally, if you want to make changes in thrust create no up or down pitching effects, you set the thrust line so that it passes through the aircraft's C/G (note that to do this, you have to know the C/G location in both the horizontal and vertical directions). In the case of the Northrop N9M and XB-35 prototypes, the props are behind the C/G and just a little above it, so downthrust makes the thrust line pass through the C/G. In the case of the Kyushu J7W1 Shinden, the prop was aft of the C/G and about level with it, so a zero-angled thrust line is needed.

However, in the case of the Junior that was a little tricky. On this aircraft the prop is well above the C/G and almost directly over it. If we used enough down thrust to make the thrust line pass through the C/G, the prop would resemble a downward thrusting rotor instead of a forward-thrusting propeller! In this case, we used enough up thrust so that the slipstream of the prop hit the stabilizer and created nose-up aerodynamic lift from the stabilizer that cancelled out the nose-down effect from the high thrust line. In flight this works very well.

On the ground at the beginning of takeoff roll the lack of drag from the wing and the friction of the wheels against the ground worsens the overall nose-down effects, so a little finesse may be needed. Hold full back stick and open the throttle gradually. The problem goes away once the plane gets just a little airspeed and the stabilizer starts to wake up and do its job. A high-mounted tractor prop would have the same problem.

Then there's things like P-factor and slipstream effects. On the Piper J-3 Cub we had an annoying left-turn tendency with power increases. This suggested a need for right thrust, which is rather difficult to do on a profile model. However, the problem was largely due to P-factor, which is what results when the airflow into the prop is not parallel to the propshaft. By adding downthrust, we realigned the thrust line with the inflow and eliminated the left-turning tendency.

On the Junior we do see some slipstream effects from the swirl in the propwash hitting the aft fuselage and fin. This results in a right-turning tendency, because most of the area of the aft fuselage and fin are below the thrust line. However, altering the thrust line from its present setting would mess up the pitch trim. It's easier to just rig in a little bit of left rudder if required, which seems to eliminate the problem quite nicely.

Don Stackhouse
DJ Aerotech