Model characteristics.

[Basil]

Hi, I am a novice here but am somewhat puzzled at a particular situation. I have more than one version of a particular electric model known for its stability etc.they have the same motor/Lipo combination, but they all 'Trim up ' differently.  The dimensions seem the same.The only difference is one is a tail dragger the other a tricke u/c.The CG is the same but the tail dragger needs large amount of lead in order to fly trim up.( By a senior club member). I am in the throws of weighing  the individual  components and checking wing balance etc. Has anybody have any comments.

Bas

[Brian Cooper]

They are all different.  . That's what makes it fun. 

[Simon Chaddock]

Basil

The physical difference in the planes might be very small. Just a one degree difference in the incidence of the wing or tail plane can make a big difference in the trim required.

Without the approrpiate measuring jigs it is hard to spot so unless you are into some experimental trial and error adjustment of the wing or tail plane incidence I would suggest you leave everything alone.

 

Do you have to fly with up trim? 

It could be argued that for maximum efficiency the plane shouuld fly hands off at a required speed with the elevator exactly inline with the tail plane.

Many full size, especially airliners, can adjust the incidence of tail plane inflight to achieve exactly that condition.

[Cuban8]

Even the very slightest variation in construction and rigging that you wouldn't visually notice, and even would be difficult to measure accurately will give variations in two models that are essentially identical. Not a huge difference - but maybe a few mm difference in cg position or control surface neutrals. Scratch or kit built models may even show wider discrepancies between models but will perform quite similarly.

Control surfaces that are well out of line with the main surface to fly correctly will require investigation. 

Edited September 24, 2023 by Cuban8

[Basil]

22 hours ago, Simon Chaddock said:

Basil

The physical difference in the planes might be very small. Just a one degree difference in the incidence of the wing or tail plane can make a big difference in the trim required.

Without the approrpiate measuring jigs it is hard to spot so unless you are into some experimental trial and error adjustment of the wing or tail plane incidence I would suggest you leave everything alone.

 

Do you have to fly with up trim? 

It could be argued that for maximum efficiency the plane shouuld fly hands off at a required speed with the elevator exactly inline with the tail plane.

Many full size, especially airliners, can adjust the incidence of tail plane inflight to achieve exactly that condition.

Simon, a misunderstanding; re the 'Up trim' I did not mean it to sound like that.I mean the act of trimming not to apply up!!!. Yes they should when trimed corrctly fly straight and level etc, but these wont fly the same.

Bas

[Robin Colbourne]

If the centre of gravity is in the right place, you should be making wing or tail incidence changes to sort the glide out, not adding weight.

The model with the nosewheel is going to have more parasitic drag below the thrust line and centre of gravity, so it is would be natural that it will have a tendency to dip its nose more as it accelerates.

Very slight differences in the incidence of the wing and tailplane mountings and any wash in/wash out (twist) in the wings and tail, whether intentional or not would also affect this.  If they are essentially the same design, can you try swapping the wings (and tail if possible) between the two fuselages and see what happens.  Make one change at a time and record the results.

If the wing is held by rubber bands, you can add more incidence with bits of balsa or card under the leading or trailing edge to see its effect.  If the wing is bolted on at the trailing edge, you can reduce incidence on a high wing model or increase it on a low wing model by adding packing between the wing and fuselage.

Control linkages can have differing amounts of slop in snakes, hinges and clevises, so these could also be behaving differently between the two models.

By testing the models in the glide as opposed to under power, you should be able to rule out motor power and thrust line differences.  Make sure you use the same prop for both models so its unpowered drag is the same in each case.

 

[GrumpyGnome]

Or.... just accept that they are different and enjoy the variety 🙂

[Nigel Heather]

For starters the total weight of the two aircraft may be different.  One has the nose wheel removed, tail wheel and nose weight added.

 

Next there are are things like wing and tailplane incidence - simply, this is the angle of the wing and tailplane as they sit on the fuselage.  Yes they should be the same but the components are never precisely identical and even tiny differences can make a difference.

 

Then there is wing and tailplane level - this is the how level they are port to starboard - again even the slightest difference will result in different trim.

 

Then is there is the fin - are they both precisely vertical - even the slightest difference will result in different trims.

 

Then there is aerodynamics - the air resistance of the trike and the tail dragger will be different.  The trike will have much more drag on the nose.

 

The there is the motor thrust angle - the direction the motor points relative to the fuselage - often a bit down and a bit right.  But if it is not precisely the same on the two aircraft then trim will be different.

 

Basically, pretty impossible to build two identical aircraft that need precisely the same trim.

 

Loads of reasons why two apparently identical plane willbtrim differently.

[Learner]

2 supposedly identical cars can drive different to each other even in a accurately manufactured and set up F1 car 

 

[Basil]

Thank you all for your contribution, lots to think about.

Bas