effect of changed thrust line.

On Fri, 14 Nov 2008 07:25:03 -0800, Alan Baker
wrote:

In article
,
Alan Baker wrote:

In article ,
Stealth Pilot wrote:

On Thu, 13 Nov 2008 22:20:25 -0500, wrote:



How does a person determine what the proper height of an engine should
be when building an airplane? If a particular engine design mandates
the prop is 4 inches, say, lower than where it would be with the
----------------------------------------^^^^
engine originally installed, what effect will it have on handling, and
what changes in downthrust might be advised?


Draw few diagrams of the situation and you'll see what I mean. I'm not
even going to try to draw that situation here. Basically, if the thrust
line was already above the CoM and you move it up, then the change is
smaller than the arctangent of h/l and if it was below the CoM the
change is a little greater than the arctangent.

Did a quick little check:

As an example, a Cessna 150 is about 25 feet long and from looking at
wikipedia's little jpeg, the centre of mass should be about 5 feet
behind the propellor disc.

So if you raise the thrust line 4 inches, you need to angle the engine
up an additional 3.8 degrees; arctan(4/60).

alan his corvair engine has the thrustline lower than the original
O-200.
so that would be down an extra 3 degrees.

Stealth Pilot