Thrust lines.. do they matter

[Adam Chambers 2]

Im currently replacing an engine on my big old flair hannibal and can use a prefered engine if I lower the trust line by 10mm.

Is this a problem or am I going to create a whole heap of problems?

I would think it wont have any impact at all?

[Mark Lubbock]

I can't see it making that much difference-there ia a school of thought that says the thrust line should pass slightly above the CofG, so in theory, lowering the thrustline should need a little more downthrust, but 10mm on a Hannibal shouldn't be a problem

[The real Ron Truth]

jetsome,

thats in line with advice so far. Cheers. Strangely falir recommend no side or down thrust , which may be due to the size of the damn thing !!!

[Mike Rolls]

To be a tad pedantic - there is only one CG in an aircraft because it is a point. It's location can actually be described in relation to the three axes of the a/c - fore and aft, lateral, and vertical.
In the example under discussion, lowering or raising the engine will affect the vertical location of the cg, but not its lateral nor fore and aft locations.

On a model the size of a Hannibal, moving the engine up or down 10mm isn't going to have much effect upon the vertical location of the CG. If you are concerned, it would be worth while - if you can without too much difficulty - locating the CG's vertical location and arranging for the thrustline to assume the same relationship to the new location as the original thrustline did to the old CG - but personally I wouldn't bother.

Mike

[Richard Wicker]

Offset rudder helps too.

[Myron Beaumont]

Usually about 2 degrees right thrust are built into tne fin on full-size 1 seater A/C ie the home built type !

[Erfolg]

I have read this thread with interest.

I thought that how much down, none or up thrust required, was to do with moments.

Taking a forces diagram approach there are a number of forces at work.

a) Drag force from wing
b) drag force from the body
c) Drag force from tail plane
d) Drag force from undercarriage
e) Drag force from any other element of note
f) Rotational forces from the wing and tail plane (due to lift forces)
f) Force from propeller

If we take moments about a point, we should be able to establish equilibrium conditions. Although many of these forces will vary with speed. I thought that offsetting the thrust line was a practical measure to establish equilibrium and minimise trim changes with varying airspeed.

If the engine is positioned at the ideal position, with the other major forces (i.e. wing) it is possible to have the tail plane rigged so that no effective up or down elevator is required or down thrust is necessary irrespective of engine power output (i.e. no trim change with speed).

As most of these forces are unknown to us modellers and most are not into mechanics/applied mathematics, a practical approach has been generally adopted. I understood that some rule of thumbs have been established (which ensure all forces/moments are in an acceptable equilibrium)

1) High winger, with engine as part of cabin, down thrust required
2) Low winger, with engine as part of cabin, zero to very possibly slight up thrust required.
3) Mid winger, engine part of cabin, zero up/down thrust required
4) Engine set high, wings in a low type position, lots of up thrust.

As with most things, a little understanding of the science helps us to be able to apply the practical solution in a predictive way. Also interpret our observations in some context, to plan the way forward (if what happens is not what we expected).

I am sure that one of these German flight groups will have program that that predicts what will happen, for any given configuration and wing section etc. I picked an old magazine up recently to find that 10 years ago one group was using FE for stress analysis and performance prediction in F3b.

Regards

Erfolg

[Simon Reid]

How do you measure 2 dgrees of side thrust?

[Mike Rolls]

2 degrees is a slope (accurate enough for our purposes ) of 1/8" in 3", so appropriate packing under a radial mount or similar skewng of a motor mounted between bearers will do the job

HTH

Mike

[flytilbroke]

Most of the main Model dealers will have thrust wedges in stock, they are designed to give the percentage degree of angle you wish to add.