CogG problem?

[mtbrider]

I have been trying to get a FF Westland Lysander conversion flying. The weight of the model, minus its motor and 3S2P 1200 E-Tec li/pos or 2S 1200Kokams, is 375 grams, wing span 48", approx area 288 square inches. It has been tried with a 5.3:1 gearbox and an Aircraft world VL brushless motor with a GWS orange 10 x 8 prop it pulls 7amps with 2S 1200 Kokam Lipos.
Using the recommendation of someone with a 36" conversion of the same model I have the CofG just in front of the main spar (at the Leading edge at the wing root).
The model has been tried with the main wing at the same angle of attack as the tail plane and +2degrees to it (as per the plans), using the base of the wing as the reference point.
The problem is that the model will not fly without a good amount of up elevator held in. I thought it may be due to the angle the motor is set at 3 degrees down and right, but the model flies the same even in the glide.
Can anyone suggest what is going wrong please.

[HELModels]

That sounds like CG too far forward. I know the Laysander has a nice elliptical shape so when you say CG is on the "root" LE, that is not the same as saying CG on wing LE. With the elliptical shape, does root LE amount to 25% chord? Tried the dive test?

[vintage1]

Well patently its nose heavy.

Try the dive test. If you dial in enough trim to fly level, and then put it into a dive, does it automatically pull out into a zoom? If so its nose heavy.

OTOH if it stays in the dive, its neutrally stable and needs MORE wing incidence to remove the need for elevator up trim.

[AeroRon]

I think your CG is to far forward. Move it back to 30%-40% of the chord. If the plane feels to sensitive at that location move the CG foward. I wouldn't put the CG foward of the quarter chord (25% of the wing's chord). The wing of the Lysander has a wierd planform. Use the chord distance midway between the root of the wing and the maximum chord. Take the above measurements from that point.

[HELModels]

Just wondering if the humungous undercarriage contributes to pitching the nose down?

[BMatthews]

Just wondering if the humungous undercarriage contributes to pitching the nose down?

It would be a speed related issue if it was. The pitch response a pilot experiences will include drag couples, like the landing gear and like the high wing placement, and it's basically all one pitch response.

Putting the wing and tail incidence angles both to the same value is a good way to ensure you need some up trim. But I agree with the others that say your main problem is that the CG is too far forward. It's a very odd shaped wing so I appreciate that it's hard to figure out where the CG should be placed.

First step would be to figure out the Mean Aerodynamic Chord (sort of like the "average" chord but from an air molecule's point of view). There's lots of online MAC calculators but not all of them let you include a wing like the Lysander with all it's odd kinks.

Here's one that will let you enter in the whole Lysander wing as found by Tall Paul....

http://perso.wanadoo.fr/scherrer/matthieu/english/mce.html

Just square off the rounded tip so it's sort of "average" and it'll be close enough.

[mtbrider]

Vinto you get every where :) The model will not fly just using its trim, it needs a lot of up to hold level flight, releasing it just puts it into a dive. There is no noticable trim change in the dive but it is so out of trim it is hard to judge.

BMathews I have downloaded the file, in french :( can you help me with that also. How do you work out the "centre position%" and also how do you input the total sweep is it in degrees or mm and how do you show it is + or- sweep.

My wings root rib is 120mm, the panel is 190mm long with a 16 degrees, 56mm forward sweep. The next change in shape is at a 180mm long rib, 380mm long panel with a 4 degrees rearward sweep, 34mm to a rounded out 100mm tip rib.

[vintage1]

ah. well move the pack back a bit and re-fly it.

Also check for skads of inadvertent washout.

I think that more than 2 degrees of longitudinal dihedral is unnecessary in a RC model

But that is probably what you need for normal power.

If it tends to land hard and hot and is very un-floaty, thats another sign of too much weight in the nose. as is a tendency to need even more up when power is throttled back.

[kartoffel]

I have the CofG just in front of the main spar (at the Leading edge at the wing root).

Well, after eyeballing this picture,
http://www.easybuiltmodels.com/ff78-01.jpg

I'd say that the CG feels pretty close (just gut feeling and a Mk. I Calibrated Eyeball). Before moving it back too much check the incidence of the wing and the incidence of the vertical stab.

If you have enough excess power (and courage) try level inverted flight. A well trimmed and balanced Lysander with that fat asymmetric airfoil will need quite a bit of forward stick to hold level inverted flight. If your stab incidence is the problem, your plane may fly inverted without needing a big forward push, and if that's the case, check wing and stabilizer incidence.

[hul]

My wings root rib is 120mm, the panel is 190mm long with a 16 degrees, 56mm forward sweep. The next change in shape is at a 180mm long rib, 380mm long panel with a 4 degrees rearward sweep, 34mm to a rounded out 100mm tip rib.
that will look like this then. Sweep is in mm, negative for forward sweep. The graphic shows half the wing but cuts the front edge for negative sweep.... flight direction is up.
MAC (by definition at 25%) is 0.2mm behind the root rib's tip. That's usually a pretty safe CG position.
CG position depends on the size and leverage of your stabilizer. Have a look at this:
http://www.rcgroups.com/forums/showthread.php?t=339401
the interesting part starts at post #12

Hans

[mtbrider]

Vinto, It tends to land hard and with a crunching sound so flights have needed repairs in between. The wings have come off so often that I am positive there is no problem with their alignment so began pondering on the CofG. I have cut some strips of lead incase I can not get the weight of the batteries far enough back. The sun is currently shining and there is little wind so hopefully there will be a flying update later :)
Cheers Chaps for the help.