Trimming and the COG etc.

[Tim Mackey]

You know this can be interesting - trimming etc!  Another thread asks about setting the COG by adding a lot of nose-weight and so on. I have been flying for many years - and all sorts of models - but I have at times found myself frustrated with certain models. Maiden flight with the COG bang on the suggested mark and many will fly perfectly well. Others however are not so easy, and it can be hard to fathom why. Perhaps a little up trim is used to counteract the models tendency to pitch down off power, maybe she zooms too much under power, or perhaps merely as a result of speed increasing. Obviously any airplane will tend to climb as the speed increases - its kinda the law, but some are just crazy. Under power, this may be simply the thrust line of the motor - more down thrust required, but if it happens on the glide, perhaps from a dive, then is the elevator trim wrong, or maybe the model is tail heavy etc etc?  With gliders, we tend to use the dive test to check the COG - if she tucks her nose under as speed is increased, then the likelihood is that she is tail heavy, and elevator down trim had been applied earlier to compensate...this manifests itself as diving even deeper as speed increases, as the elevator pitch is now raising the tail. If  the opposite applies and the model climbs...then maybe the nose is heavy, and  "up trim" had been applied.

Inverted flight can also help to identify COG issues.  If when inverted a lot of forward stick - down elevator - is needed to maintain level flight (given the variations in wing design of course) then this suggests its nose heavy again and up elevator trim was applied in first flights. Rolling her on her back converts  this "up" elevator trim to "down trim", hence the tendancy to dive inverted.

Of course the problems can start when adjusting two things art once IYSWIM.

If the early flights suggested nose heavy for example, then maybe the COG was moved rearward a tad, but if the previous sub trim remains, things get interesting

Lets assume the COG is actually now correct, but the previous "up trim" was not removed - the model will now tend to sit tail heavy, and you mistakenly think the COG has now moved too far, so change it again !

Should we make a simple assumption here - and that is.....the basis for getting the trimming right should always require that the elevator is set dead level with the tailplane ( excepting flying wings which may well require reflex ).

If, when the elevator IS dead level with the tailplane the model exhibits unsatisfactory "straight and level flight"  then either the COG needs adjusting, or the motor thrust line needs altering. Many people refer to the elevator being overly sensitive when a model is "tail heavy" but again, this could simply be the control throws are set too high, or perhaps you are the type of flier who likes exponential on the sticks?

I know there are lots of pieces about on the whole trimming thing, but many dont discuss this aspect in a simple non techie way that may help beginners.

Worth discussing gents?

Edited By Timbo - Administrator on 19/05/2010 12:50:17

[Ed Darter]

If I get this right Timbo it sounds as though its a case of remembering to reset your trims when making adjustments such as this so you are not chasing your tail continually ? This is indeed something I try and do anyway, reset yourself to a known datum / baseline when making changes. I suspect this wouldn't be news to heli guys as its very easy to end up chasing your settings round the head setup on a heli unless you are quite regimented. Nonetheless a good reminder !

Ed

[Tim Mackey]

Yeh I guess it is Ed... but I am sure theres more to it than that.  If an experienced and not totally dumb flier like me can run into trouble with this stuff, then I am sure there are others in the same boat too

[Tim Kearsley]

I still reckon that this article was one of the most useful I've read on this topic....

 

Cheers,

 

Tim.

 

[Tim Mackey]

I agree, but it never hurts just to go over things again from time to time.

Even Andy E didnt mention the reason that the dive test actually highlights a COG error - because unless I am mistaken in my earlier summary, the actual reason for it to tuck under or climb in this test is because the elevator is out of level trim due to compensating for the wrong COG

Also I think it needed stressing that after altering the COG, control surface trims should be re-set to neutral before further adjustment

Edited By Timbo - Administrator on 19/05/2010 15:03:08

[Biggles' Elder Brother - Moderator]

Not necessarily Timbo! Or at least possibly not the full picture.

 

If you have a cambered wing section this produces a nose down moment in addition to any moment caused by the longitudinal displacement of the centre of lift and the centre of gravity. In a correctly trimmed plane the tailplane should be producing "negative lift" to counter balance this. So that's another factor in the equation!

 

Only a completely symmetrical sector produces no pitching moment due to lift.

 

BEB

[Tim Mackey]

Which bit of my statement are you referring to BEB?

 

1) The actual reason for it to tuck under or climb in this test is because the elevator is out of level trim due to compensating for the wrong COG

2) After altering the COG, control surface trims should be re-set to neutral before further adjustment 

[Biggles' Elder Brother - Moderator]

The bit you underlined  The second point I would totally agree with!

 

It might not be the CoG that is "wrong" - the tailplane may be incapable of being trimmed (or the CoG moved sufficiently) to compensate for an excessive negative pitch moment caused by the wing section camber at the very high speeds encounted in the dive.

 

I am not really disagreeing with your basic statement I'm just saying is that in the dive test there is another possible factor in the equation that is often overlooked.

 

I think it is overlooked because we assume that the model's designer has done his job correctly, i.e that the pitch couple can be matched by the tailplane's negative lift in some achievable trim position - even at the extremems of performance (i.e. speed) represented by the dive test. Thus the CoG is left as the only remaining degree of freedom - and the one we always blame.

 

The assumption is often true. But not always, particularly in the case of scale models, and so I don't think we should forget that it is an assumption. If its not true then the plane will be untrimmable at such high speeds until you change the angle of incidence of the tailplane.

 

 

 

BEB

[Tim Mackey]

OK, thanks. I accept that some designs /builds may not be as good as they could be, but in most cases where I have had issues such as described then the model has been a proven and worthy flyer. I think what I was trying to say ( and obviously failing ) was that in the majority of cases where "dive testing" is used to assess the COG, what is really happening is as follows.

 

1) The dive test is merely a means of increasing the models speed without adding power, as adding power could then introduce motor thrustline issues.

2) Where a model then appreciably alters its trajectory upon the elevator being neutralised, the reason for it so doing is that pitch trim is in force...which was implemented to compensate for an incorrrect COG, rather than the cause being the COG itself.

Does that make sense?  LOL

[Tim Mackey]

I have amassed a few articles and scribblings from various sources over the years on the matter of trimming. I confess to having amalgamated much of it into one larger document, and an extract from this is shown below. Apologies for not knowing the original author, but this litttle snippet does I think follow my thinking

it also refers to your point re: decalage

C OF G - The way to trim a plane is first of all to get the CG right.

 That means using the trims to get it to cruise level, and dive testing it. If it pulls out of the dive really smartly with zero stick, its CG is too far forward. If it stays in the dive forever, or even starts to tuck under, the CG needs to come forward + nose weight. Once you have done all that, examine how the plane 'sits' in the air and especially the glide. It should probably look almost level in flight, and a little tail down in the glide. If its too tail down increase the incidence of wing AND tail (or use down trim on the tail) and vice versa. Once you have got it sitting right, look at the power on/off. If it’s really wanting to climb under power AND you don’t like it, add down thrust. If it wants to turn under power but it’s straight on the glide, add side thrust.

Suspect nose heavy? Try this….

In crude terms - if the model's nose heavy it will need to be “up trimmed” so that the wing has an increased Angle of Attack sufficient to lift the nose & maintain level flight in the "cruise". When you then apply more power, this increased AoA will cause the wing to tend to pitch up, hence the steep climb.

The WORST thing you can do to a ‘plane that wants to climb under power is add weight to the nose. That simply means it needs MORE up trim in level flight, and will want to point its nose MORE up when you open the throttle.
The most irrelevant thing you can do is change stabiliser incidence. That merely adds permanent up or down trim, but affects nothing else at all. The most usual cause of a model to climb under power is a forward cg. The reason for this is that at low speed you have to apply up trim to keep the nose up and then when you increase speed the wing produces more positive lift but the tail produces more negative lift, thus pushing the tail down. If the tail is heavy, at low speed you need to apply down trim to push the nose down. However, when you increase speed the wing's lift will increase but the tailplane, which is already producing lift, may be less efficient at increasing that lift, particularly if the amount of trim is large. It could, in the extreme, even stall.
Other things that can cause problems are the size of the tailplane or the distance between the wing and the tail. Excessive nose heaviness would result in a strong tendency to climb when power is applied, but would be accompanied either by some up trim - or a tailplane installed at a negative angle compared to the wing ( decalage)

[Tony Smith 7]

This is interesting/useful.   My electric glider used to need quite a bit of up elevator trim, so I moved the CofG back just a little.   Now it glides nicely with the same amount of up trim, but tends to "mush up" under power.     I had been thinking that trim for min sink while gliding would also be correct for best rate of climb, but its obviously not that simple.

[Erfolg]

I have followed this thread in all its incarnations on this forum for some time.

 

In the past I have disagreed with it.

 

I now have re-evaluated my position. If the CG is not to far out, it seems to work. I have tried it a number of times now on differing models. To the extent quite recently, it was remarked, "did you do that on purpose"?, it looked that you were trying to rearrange it!

 

I guess a massively rearward CG, results in a crash. It is the massively forward CG that does not seem to  easily fit with the description. In normal flight,  a very forward CG  is not easy to fly, in some ways similar to a rearward CG, Taking an age to respond, by which time you have probably inputted another command and the model is doing something else unwanted. But this was massively forward. Possibly over 1" on a 9" chord. Ironically this was a result in trying to get some stability in pitch.

 

I do now accept if you have located the CG somewhere near right it seems to work.

 

Although coincidentally, a check I was only undertaking today suggested that a model that I thought was well trimmed regarding CG, is still tail heavy, by a tad. But all in all, seems to work, within limits.

 

Erfolg  

[Biggles' Elder Brother - Moderator]

One question I have about the dive test is this...

 

I assume we can all agree that while there are definitely "wrong" CoG positions there isn't in reality a single "right" position. That being the case any acceptable CoG position is a compromise and is only really "right" for one particular speed - at all other speeds (because of the pitching moment of the wing section varying with speed) it must to some extent be "wrong" - albeit hopefully only a bit "wrong" and managable using either control inputs if the performance point your at is just short term temporary (like being inverted) or by inflight trim adjustment (if its a longer term situation, for example on approach to landing)

 

Given this is the case, surely the potential problem with the dive test is that it optimises the CoG for high speed flight - i.e. one extreme of the performance envelope. Now that's fine if that's how you intend to fly the model most of the time - but if your actually going to spend most of the flight at, or around, the cruise surely it makes sense to optimise the CoG position for that point in the performance envelope and make extreme high or low speed conditions points at which you may need to put in a bit of extra control/trim?

 

Just a thought - as its always bothered me about the dive test.

 

BEB

[Tim Mackey]

Its an interesting thought BEB.

In my oversimplified way, I just took it that the only function of the dive was to increase speed enough to highlight any elevator trim that may be present, and hadnt really considered your point.

I must say I hadnt considered the COG to be a major factor oin the speed variation scenario, unlike surface trims which obviously will show up as much more pronounced?

[Andy Freeman]

I have had cause to think about this recently.

 

For me the key to getting my head around it was slow in coming but i think i have a decent handle on it now. 

You are looking for a state of equilibrium . That equilibrium needs to maintained across a varying airspeed for optimum flight characteristics. It therefore makes sense to me now that the elevator should be adjusted for least drag and if the model behaves differently as you increase the airspeed then you need to adjust the other side of the equation ie the CofG.

 

Some of the long term flyers might think this was obvious but for me as a relative new starter it was a revelation. Apart from "just doing as Enrico says" and moving the CofG back i now can see why and can figure it out for myself. My gliders now go faster, hold the speed better and actually accelerate out of a turn.  

I suppose it is more apparent in gliders and electric as the CofG does not vary over the course of a light like it will with a fuel powered model but it must still be relevant?

[Biggles' Elder Brother - Moderator]

An interesting example of variable CoG to optimise flight at different speeds is the fact that a few years ago some full-size competition gliders were fitted with two mercury reservoirs one in the front and one aft. The pilot could pump mercury from one to the other thus moving the plane's CoG backward and forward. The idea was he could set up a more rearward CoG when he was going for speed, on say a cross country leg, then switch to a more forward CoG when flying slowly, say circling for lift.

 

Unfortunately we can't do that! Unless Timbo can devise a battery pack connected to a servo

 

BEB 

[Tim Mackey]

Ooooh thats an idea!

Isn't the reason for a more rearward COG on the speed runs simply to eliminate the need for any up trim on the elevator at all....thereby reducing drag?

Edited By Timbo - Administrator on 19/05/2010 23:00:05

[Biggles' Elder Brother - Moderator]

Yes Timbo, what is happening is as the speed increases the nose-down wing induced pitch moment (that stems from the tip vortices ultimately) increases. And as this is a nose down moment you need either up elevator trim or a shifted back CoG to counteract it. As you say, shifted CoG is preferable if possible because it causes less drag.

 

If you had an "perfect aeroplane" you wouldn't need to do either of course, because the increased downward force on the tailplane with increasing speed would exactly neutralise the effect of the nose-down wing induced pitch moment at every speed in the plane's performance envelope - if you ever find a plane that can do that treasure it, and please tell me

 

BEB

Edited By Biggles' Elder Brother on 20/05/2010 00:05:14