prop thrust line

[GT280flyer]

I am installing a Hirth 3503 engine in my model III and need to fabricate a new engine mount. There is no specs in the build manual on prop thrust line or prop offset if there is any offset. Can anyone help with info on thrust line or offset?

[jtpitkin06]

GT,

There are several threads if you do a search, but none of them give a definitive answer. There is nothing in the manual about installing any engine other than the factory supported models.

Some of the previous postings blindly tell the reader to measure perpendicular to the firewall. That is not a reliable method as you have no way of knowing if the firewall is designed to be 90 degrees to the thrust line. For that matter, you have no way of knowing if the firewall is relative to anything other than foot room.

I am building a model 7 with an alternative engine. I will fabricate the engine mount myself.

To accomplish this I will use the cowling as a height reference so the spinner and prop will line up with the cowl. Vertical placement of the engine is not critical for control. If the thrust axis is located up or down as much as a foot, you probably won’t notice much in control forces. There might be very slight pitching moments with power on or off, but the center of thrust compared to the center of drag on a low horsepower aircraft is not much of an issue. The stick forces are not that great. (If you want to experience some major stick force change between power off and power on, get some time in a B-757)

To get the thrust line yaw I can use the longitudinal axis of the aircraft. There is not a lot of p-factor with 100 hp engines. In keeping with normal design practices I will set the engine thrust offset initially to 1 or 2 degrees. My engine rotates CCW so I will use left offset. If your engine rotates CW then use a small amount of right offset. Most of the Rotax installations have zero offset but many also have a trim tab on the rudder to keep the aircraft straight in cruise. The yaw offset should improve cruise performance by reducing rudder drag as well as reducing rudder forces in climb.

To get the thrust pitch axis I can set the horizontal stab to cruise and average the angle of incidence between the wing and the stab. The engine pitch axis should be set midway between the two minus 1 or 2 degrees down thrust to help pitching moments because the propwash hits the horizontal stab on this aircraft. Propwash also gets deflected with the flaperons causing a small pitching moment.

The above should give me a good starting point. I will build the mount so that it can be shimmed and adjusted for pitch and yaw.

When using unsupported engines, you are on your own. I hope this helps.

John Pitkin
Greenville, Texas

[HighWing]

I agree with John, however my approach was less on the technical side. I took some carefully positioned pictures of several Model IV airplanes and using Photoshop, Leveled the pictures using the Rotate Image function until the prop blades were precisely vertical. I then found that the prop was exactly 90 degrees from the line of the lower (below the cockpit) longeron and door sill. I am using the 912, but was trying to get the oil tank in a more favorable location than on the factory mount.

To make the engine mount, I then leveled the fuselage on a stand and made a fixture that could allow for engine alignment. Lots of plumb lines later, I built to close the gap. I chose to use the zero offset of the original mount. but could shim at either the firewall or dampener donuts.

Lowell

[Geowitz]

Good luck on this one. I've spent many frustrating hours trying to get an answer. If I had a 912 I could tell you in a few minutes. All someone has to do is level the plane, run a level string line from the tip of the prop and measure to some fixed point on the firewall. This would give you the original vertical placement of the thrust line. You'd think there'd be a drawing somewhere. I'm starting to think it's proprietary information or something.

The best I could find from pictures of kitfoxes and pictures in my manual is that the vertical center of the engine thrust line relative to the bottom of the bottom tube is about 16 to 16.5 inches. So bascially, if you place a tape measure on the bottom of the bottom tube and run it up the firewall about 16.5 inches then come out perpendicular to the firewall that is the centerline of the thrust using the 912.

Like others have said. On a Kitfox this measurement is not critical to be exact, but it's nice to have a starting point that is known to work well.

The angles are all set at 0 I believe(left/right and up/down). You could add in a little to overcome torque, p-factor, etc, but on a Kitfox 0 seems to work fine.