Angle of incidence

[Steve Houghton 1]

I have a PMP Synergy slope soarer which I purchased 2nd hand. I've tried to trim it, using the dive test, but am still struggling with it.

 

The problem is that even with full down trim applied on the tx, the model still wants to climb all the time. I've altered the COG by adding tail weight, and the trim mechanically, allowing enough movement so as to be able to apply down elevator but I find myself still having to apply that down elevator during level flight.

 

So my question is:

 

If I were to reduce the angle of incidence by adding say, a thin piece of card towards the TE of the wing seat, would this have any beneficial effect?

[Tony Bennett]

i would have thought that you needed nose weight .

[Stefan Hafner]

Altering the CoG will mainly affect the static margin, which affects how sensitive the elevator control is, reducing the angle of attack as you suggest I think will have better results. Clinbing will be caused by the wing producing too much lift, and this changed will reduce the amount of lift by reducing the angle of attack. Just my 2 pennies worth

[Steve Houghton 1]

Well when I did the dive test, with a neutral elevator, the model pulled out of the dive, meaning it was nose heavy, hence adding tail weight.

[Steve Houghton 1]

Stefan. Well those were my thoughts exactly, but I wanted to check with others to see if my theory of a reduced angle of incidence equaled reduced lift was correct.

[Tim Mackey]

If a model climbs in a dive test, then it usually means its nose heavy.

The reason is this ( as I understand it anyhow )

It applies equally ( especially so in fact ) to gliders, as they are not affected by errors in motor thrustline

 

The model has been previously trimmed for straight and level flight at normal cruise speed.

In the dive, speed increases, and if the model climbs, its because the previous trimming exercise had applied a little "up" elevator to compensate for her sitting a nose down. The increased speed increases the effect of this up trim, and therefore the model climbs.

 

The opposite is of course also true, if it steepens its dive, or "tucks under" then you have a bit of down trim in, because...... it was tail heavy earlier

[Steve Hargreaves - Moderator]

My understanding of the "dive test" was that you did it after you had trimmed the model for s&l flight....thus if it was slightly nose heavy you would add up trim to counter this & this up trim would result in the model pulling out of a dive.

 

In your case Steve it appears you would need lots of down trim to achieve s&l flight & this would result in the model possibly "tucking under" during a dive indicating a rearwards cofg....

 

Thats how I understood it anyway............

[Steve Hargreaves - Moderator]

Cross posted Tim......seems we agree anyway...

[Steve Houghton 1]

Ok guys, so, my dive test was conducted with a neutral elevator, cancelling out any previous trimming, and the plane pulled out of its dive , indicating it was nose heavy, which as stated previously, was why I added tail weight until this was corrected, although I have to say not entirely so as I ran out of space to add more weight.

 

So I guess what I will have to try is adding thin pieces of card, a little at a time, at the trailing edge of the wing to change the angle of incidence and reduce the amount of lift so it will fly straight and level, (more or less) before using the elevator trim to make those final small adjustments.

[Evan Pimm]

Seems you are a little confused Steve, yes the dive test will indicate a stable balance position, and this can be adjusted by adding weight fore or aft to produce a gentle pull-out from a dive. But this has nothing to do with the model climbing from 'S&L'. Balance only affects pitch stability. The climbing issue you have is a result of the 'trimmed speed' being lower than the speed you are trying to fly it at. So, of course, the model tries to return to 'trimmed speed' (the speed at which it will fly S&L at the angle of attack the tailplane is trying to hold the main wing at) by climbing. You have two choices, fly it slower so it maintains 'trimmed speed' and 'S&L', or re-trim by reducing the angular difference between the wing and tailplane, and fly faster to maintain 'S&L'. The consequence of changing the wing to fuselage angle, as you are doing, is that you also change the apparent angle of the fuselage to the line of flight, so long as you do not change the weight of the model, the wing will always fly at the same AOA so that, at whatever speed it is flying, Lift equals Weight, in S&L flight. Align the fuselage at the nicest looking angle to flight, set the main wing at 1/2 degree? positive to the line of flight, then shim the tailplane to set a comfortable cruise S&L speed (trimmed airspeed). Most likely this will be a degree or two less than the main wing angle. You won't need to change the balance once it is set per the dive test.

Evan.

[Steve Houghton 1]

Thanks Evan. Even straight after launch, when the plane is flying slowly, it wants to climb at such a steep angle that eventually it stalls, which is why I have so much down elevator mechanically dialled in, to try and stop it from doing that, but even that isn't enough at times and I have to input down elevator on the stick. I can't alter the incidence of the tailplane as it is a corex affair pushed into a slot in the EPP.

[Steve Hargreaves - Moderator]

Where does it actually balance Steve....compared with the recommended cofg position?

 

What happens if you trim it for s&l (loads of down trim as you say) & then do the dive test?

 

It sounds like it is tail heavy to me......but then I'm not a glider guider...

[Steve Houghton 1]

I sent an email off to Stan Yeo at Pheonix MP regarding this and he tells me that the COG should be on the main spar and is non negotiable, so I guess I'll remove my added tail weight and start again.

 

He goes on to say that the ailerons require careful setting up as it is easy to set them up with up or down flap which leads to a gross deterioration of performance. So with that in mind, what if I happen to have a slight amount of camber on the ailerons? This is going to increase the amount of lift I'm getting, which is the problem I'm experiencing.

 

Thoughts please.

[Steve Hargreaves - Moderator]

Well certainly if you have a rearwards cofg then this will make it more sensitive in pitch & more likely to climb I would have thought.

 

Conventional wisdom would suggest that if the ailerons are below where they should be they will act as flaps & increase the lift.....if they are raised then they would act as spoilerons & reduce the lift.....

 

Where are your ailerons set? if you look at the wing side on would you say they are in line with a line drawn through the middle of the wing section? Thats where I would set them.....

 

Maybe go back to the beginning.....set the cofg on the spar....elevator at neutral & ailerons as above & then chuck it off a big hill & see what happens.....

[Steve Houghton 1]

Thats what I'm goint to do Steve, go back to the beginning and start again. Stan says to make a female template of the wing section and slide it over the wing and then adjust the control rods accordingly.

[Simon Chaddock]

Yes Steve H - start again.

I am sure you are tail heavy.

With the CofG in the correct place try a simple strait glide.

When you have got it trimmed for a constant descent and travelling as far as possible, have a look at the position of the elevator. It should be more or less in line with the tailplane.

If is 'down' a bit add some nose weight, if it is 'up' take some off (or add tail weight) and keep testing until it is virtually neutral.

Then and only then you might like to try a dive test to see how the trim changes with speed.

A slight nose up tendency at speed is better than nose down!

 

If the ailerons are not rigged neutral it is more likely effect the handling than the total lift.

Drooping ailerons can promote a tip stall which is followed by an uncontrolled spiral dive, usually on your final turn as you come in to land!

Raised ailerons will reduce this possibility but will effect the gliding performance.

 

I hope this helps.

Edited By Simon Chaddock on 03/11/2011 16:55:11

[Evan Pimm]

Again, for Simon, once the balance is set, you must not move it. This model seems to have excessive 'Longitudinal Dihedral', modelspeak for too much difference between the wing and tailplane. Check your ailerons, they should, more or less, line up with the wing chord line. Now check the difference between the wing chord line and the tailplane chord line. They should be more or less the same, with the wing a degree or two more (positive) to the tailplane. Balance per instructions. Then fly. Now you can do the 'dive test' to fine tune the balance. Most rearward is when the model will just slowly pull out from a 30 deg dive. The elevator trim at this point will not necessarily be in line with the tailplane, but will be at a position (trim) for the model at its 'S&L' cruise. Changing the elevator angle with the 'TRIM' lever on the tranny will allow the model to fly slightly faster, or slower, on command. You will probably have to do several flights to sort out a comfortable trim, but do not be confused with those who tell you you can adjust the elevator trim (speed trim) by adding or removing noseweight. As stated, balance is pitch stability. Wing to tail angle is speed trim. the two are quite different, but one can affect the other, which is where the confusion arises.

Evan.