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Hi peeps,

Trim?

I have posted this topic because trim on a wing is very new to me and i won't be on my own. It will also make good reading for up and coming Paramotorists (that's if anyone has the answers)

So, can anyone have a go at explaining what trim is and does, the advantages and disadvantages, the pros and cons, the do's and don'ts, here are a few questions to set you all off.

What is trim?

How does it work?

Why put (manufacture) trim on to a Paramotor wing?

Do Paraglider wings have trims?

What about wind speed, does this effect the choice of settings on landing or take-off?

Can you alter the trim settings whilst in mid-air or even while turning?

What would happen if you did alter it in mid air but only managed to alter one side?

Anyone want to have a bash :D:D

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The exact ins and outs of trim and reflex handling is something I've been particularly interested in recently and made me realise that many of us misunderstand its use. My following comments are pertinent to use of trim in conjunction with a reflex profile.

What is trim?

Trim is a system on our wings that allows us to adjust the angle of attack of the wing.

How does it work?

Adjustable length on D risers which by their design also (usually proportionally) change the length on the C and B risers. If the change of line length on the C and B lines is not proportional then the wing will not maintain the same profile.

Why put (manufacture) trim on to a Paramotor wing?

Speed range achievable.

Do Paraglider wings have trims?

Some tandems do and several manufacturers sell riser sets with trims for paramotoring.

What about wind speed, does this effect the choice of settings on landing or take-off?

Yes, the wing comes up much nicer on fast trim but you would need to run like the clappers... unless you have a good headwind. Personally I never take off or land on slow trim as the wing tends to hang back on take off and doesn't carry as much energy on landing for a nice flare.

Can you alter the trim settings whilst in mid-air or even while turning?

Most definitely yes. Trim has a large effect of fuel economy so if you are just pootling around with nowhere to go then use a slowish trim setting for less power requirement. If you want to cover distance use fast trim for better speed over the ground. If it gets rough then speed up for more pressure in the wing and a more forward c of g.

What would happen if you did alter it in mid air but only managed to alter one side?

Some people do to offset torque but personally I don't. Weightshift is a fine way to counter residual torque and in my opinion I don't like the idea of asymmetry in the wing. Getting close to the stall in this configuration could spoil your day.

Hope that helps. One of the key things I have discovered through conversations with Clive Bunce and Michel Carnet is that the Dudek designs have a riser trim system that apparantly keeps the reflex profile through ALL trim positions (unlike some of the Paramania designs) so they recommend that these wings should not be actively flown like a pure paraglider at any trim setting.

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Trim, trim-tabs, trimmers:

A means of altering the profile of the aerofoil.

How curved a wing is, in cross section can be altered by lengthening the D and C risers. (some trims also act on the B risers but not proportionally like on the C and D).

As the wing flattens, drag is reduced because the chord is effectively narrowed (aspect ratio increases). Paraglider wings of various generations have had trims and some (I'm thinking of Airwave's Triple X) have as much (even more) trim length than a modern reflex wing.

The Centre of Pressure moves forward (more of the weight is supported by the A lines.) This reduces the opportunity for cell closure (front tuck).

The reduced drag, increases forward speed. Sink rate increases but glide angle can remain the same or become shallower. Each trim system alters the wing profile in a particular way. The Dudek system retains some reflex in the profile some do not. On a paraglider some trim systems introduce some reflex at the fastest setting.

The angle of attack will often change on these systems as a consequence of the change in drag and the move forward of the centre of pressure.

Speed systems (seen on most paragliders) act directly to alter the angle of attack by progressively shortening the C,B and A risers but the profile of the wing is not altered. Lowering the angle of attack also reduces drag and the centre of pressure moves forward but often not as far as with trimmers, there is more opportunity for front tuck using the speed system. (the triple X mentioned earlier would let you push out the speed bar until the the wing collapsed.) Trimmers effectively narrow the chord whereas speed system the effective chord is not narrowed.

I feel a drwaing coming on !

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Now we wouldn't be substituting angle of incidence with angle of attack here, would we?

Angle of incidence being what you set with the trimmers ~ angle of attack being what you achieve for a given flight regime and being 'fluidly variable'.

Reduce angle of incidence in level flight and drag reduces but so does lift - so acceleration required to replace it.

This requires power to effect and fly level. Ergo - Speed system means faster flight but greater fuel consumption as thrust increase is required to overcome (all types of) drag.

The relaationship of these two angles is intricate as they interact - can be very confusing...

Have I got this right - trimmers affect the chord and angle of incidence.

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ahhh yes, I deliberately avoided using "angle of incidence" as I understand there are two meanings for this term: (maybe more?)

]1. the angle the incident air makes with the chord line which I call "angle of attack"

]2. The angle of the chord incident to the geo-horizon.

I would say that what you set with trimmers is the profile of the aerofoil section, its curvedness. (Norman you would lower the flaps i guess and/or push out the slots and whatever other section changing devices you have these days).

the result may be changes to either or both of 1 and 2 above.

I call the incident air direction the trajectory of the craft.

As with all these dicussions we need to agree the definitions first, perhaps? I have made a start on the Isle of Wight site pages with some glossaries but am happy to change those definitions if we feel they need it.

Norman is quite right to point out that more thrust is required to maintain the same trajectory when the drag (and therefore lift) is reduced by changing the section of the wing; So to maintain level flight after letting the trimmers off you will have to increase the thrust (throttle up). The craft will accelerate until drag is the same as the new thrust and the increased speed generates enough lift to equal the weight. The net effect is that the wing goes further behind you but retains the same angle of incidence (with the geo-horizon) and the same angle of attack (angle with incident air)

there will be a "most efficient" trim setting for a craft for a particular pilot+motor weight. A cruising speed that maximises distance/litre. Well worth finding this out for each Pilot and their kit in the tip to tip. It will not be the same speed for all so staggering the start might be needed for all to arrive at similar times without running short of fuel! :o:?:oops: .

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Good point well made......

am I correct in thinking that the T 2 T pilots will have appropriate sized wing:weight ratio rather than all flying the same size wing with than running the risk of some of the more gravity challenged pilots from "ballooning about"?

If so surely this will help compensate for differant sink rates:ground speed:economy etc..

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Simon,

We will all have wings that 'fit' mate. :D (Hence the question about your weight)

Francis, you said,

"(I'm thinking of Airwave's Triple X) have as much (even more) trim length than a modern reflex wing."

Prouty, has said this to me before, he said that he had flown a 'Hill' Reflex wing WAY before the likes of Dudek and Paramania and the like.

SW :D

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Thats a very interesting thought..... I had not considered whether the triplex introduced reflex into the section at full extent of the trim. I have one (several) in stock (they are unsaleable in the current safety conscious market!

I will look at the riser set with a new eye and check to see exactly how its section changes on trimmers.

And report in due course.

Re tip tp tip pilots and weight, there is only one speed at which the wing is most efficient for a particular weight loading. If you are the plated "naked pilot weight" then it should be the plated trim speed (although the manufacturers play with these numbers for marketing purposes). If you are one third up the weight range and the next pilot is two thirds up the weight range the pilots will not have an identical "most efficient" speed. I do not know how much these will affect actual economy; may be hardly at all but may be enough to make the difference between an extra stop for fuel and not. There is plenty of time for each pilot to ascertain their own most efficient speed during the training trips and the run up to the event.

A big factor in efficiency is drag, so slippy tight clothing and as much smoothing as can be acheived makes big big differences.

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"slippy tight clothing and as much smoothing as can be acheived makes big big differences."

Francis,

I dont want to know about your fetish mate! You feel free to bring your spiderman costume, I will be wearing a flying suit (a Tip to Tip one I hope!) :lol:

SW :D

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I am just such a "lighter" pilot and I struggle to get to my feet with just the motor on!!!!!!!!! Now you want me to put on a weight belt? Ooooof my poor old back. :oops:

Good point though, paraglider competition pilots take off with up to 20kg of water to get off the hill then shed it as they find lightre lift.

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