RES set up help

[ozmo01]

Hey all,
I have been flying my Gentle Lady as much as possible and have it close to dialed in. But I am unsure what to change at this point. Upon release from the high start I have little forward movement. One flight was like a long float over the release point. I get some better forward speed with a couple of clicks of down. I am using the airfoils from the Alegro Lite. I have approx 1/3 inch of CG range and I am right in the middle of it. My launches are nice and steep and release is fairly smooth. I understand to try different approaches but I am hoping for a little guidance.

Should I try a more forward CG? Or keep using the elevator trim for increased penatration.
I have the ability to add ballast but this is not the issue as the wind speeds have been under 10 MPH. My all up weight for the glider without ballast is around 26 ounces on a cheap kitchen scale.

[RBMartin]

When you put down in, can you see it as down on the elevator? Or does the elev look nuet after you put down in it. If you can see down in it relative to the fixed stab then you need one of two things. Either move the cg forward. Or the fixed stab incidence is off and has built in up. If you built according to planes that is prob not the case.

I know you said the wind was low, but if you are flying into a headwind the gentle lady will fly very slow. I know for that type of plane when I need to move or penetrate it take a decent amount of down to keep the plane on step.

You might also try a dive test for the cg. See if it pulls it self out of a dive or tucks under.

Where is the cg in relation to the planes?

Bruce M

[FrogChief]

I'm betting he has too much positive incidence on the wing. Shim the back of the wing with a 1-2mm shim and see if the behavior improves. Since he's using the Drela AG-series airfoils, penetration and / or induced drag from a crap airfoil shouldn't be the problem.

[SUNSQUINT]

I'm with frog chief on this one...on all points he made.

Your new foil (or at least different from the original) may be sitting at a slightly differnt angle of attack than it should....

Try an incidence gage to set the incidence to zero degrees between the wing and stab. You will have to fly it a little more, but it will improve speed range, respond to trim, and go where you point it. As with most kits (and ARFs) the designers usually put in too much incidence in order to help the newbies have a more stable plane at the cost of induced drag and limited speed range.

Shimming is also still a good way if you don't have the gage.

[CloudyIFR]

Decalage is way over rated. Don't worry if you're flying around with a little up or down elevator. The main reason you'd want to fix a decalage issue is if you wish to adjust the angle the entire fuselage is flying at. Have you seen B52 fly, it has a little bit of a decalage issue down low but it needed that so the flight deck angle was proper at maximum altitude with a heavy bomb load.

Anway, read this article from Dr. Drela:

A more precise and technical answer is reprinted below from Mark Drela in the Allegro-Lite YAHOO group, message #6005;

The importance of decalage is way overblown.

First of all, if you have an all-moving tail, then obviously "setting
the decalage" has no meaning. You just set the elevator trim to
whatever it has to be for your CG position and desired trimmed speed.

It turns out that for a hinged elevator, decalage should also have
little effect. You can change pitch trim either by moving the
elevator or moving the entire tail. There should be little or no
difference in pitch stability or elevator response between the two
types of changes. This assumes that the tail airfoil is reasonably
good, and the elevator deflection is modest so that the tail's
cl(alpha) and cl(elevator) curves are still linear. The only reason
to change the decalage (angle of the front stabilizer) in this case is
to null out the elevator trim deflection. This will give more
symmetric elevator range, reduce elevator servo load at high speed,
and perhaps reduce the tail's profile drag very slightly.

If there IS a difference in handling from a change of decalage, then
you surely have poor airfoil flow over the tail, with some combination
of dead band or hysteresis. This also indicates the presence of large
separation bubbles and high profile drag. A simple slab airfoil with
a semi-circular LE is very common, and is prone to these types of
problems. Making the LE more elliptical rather than round should give
a significant improvement.
End. quote.

Now the way to really get her dialed in is balance.

Trim your model for straight and level flight, then push her over in a 30-40 degree dive and then center the elevator, note what she does.

If the model climbs, she's nose heavy.
If the model flys straight ahead or tucks she's tail heavy.

Make sure that each and every time you adjust weight you retrim for straight and level flight prior to performing the dive test. Also perform the test in little to no wind and/or lift.

See this chart by Dr. Drela as well.
http://www.polecataero.com/handlaunchu/cg-location


All the best.
Curtis
Montana

[Jack Hyde]

You said "I have approx 1/3 inch of CG range and I am right in the middle of it." What does that mean? If you shift weight such that the cg moves ahead or behind this 1/3" range what do you think happens.?

[FrogChief]

Now the way to really get her dialed in is balance.

Trim your model for straight and level flight, then push her over in a 30-40 degree dive and then center the elevator, note what she does.

If the model climbs, she's nose heavy.
If the model flys straight ahead or tucks she's tail heavy.

Make sure that each and every time you adjust weight you retrim for straight and level flight prior to performing the dive test. Also perform the test in little to no wind and/or lift.

See this chart by Dr. Drela as well.
http://www.polecataero.com/handlaunchu/cg-location


All the best.
Curtis
Montana

Not to knock the honorable Dr. Drela, but that article doesn't apply to these type of sailplane designs. The Gentle Lady, SIG Riser and Spirit series are notoriously over-decalaged (if that's a term.)

The dive test isn't gonna tell you squat if you've got a ridiculous positive incidence on the horizontal stab and will even indicate 'nose-heavy' with an extremely aft CG.


Get the decalage between the wing and horizontal stab to 1 to 2 degrees first, then proceed with the dive tests.

[ozmo01]

Hi again,
Thanks for the great tips. I had read them before but I think the review was a good idea. And I saved the references to share with other glider guys.
When we modded the GL we used the Drela foils and copied the incidence on the Alegro Lite Plans, this required a little reshaping of the wing seat. Dive tests will tell if I got close. As far as shifting the CG further either way I am not sure what will happen. It will be fun to test out.
The GL cord is 1" longer than an Alegro Lite cord so I did a ratio formula to determine CG. Again dive testing will prove if I came close. It flys "pretty good" now so I doubt I missed it by much, if at all. Hope to pop out tommorrow AM for some flying. Thanks again for the gracious advice. :)
Dive tests are the order of the day!

[aeajr]

How you come off the hi-start will determine how it behaves and you have total control of that.

If you ride the HS up and just float off the end, then you should expect to have little forward speed. That is how you released.

If you take up, then dip the nose and pop up a little, you will have more speed.

If you take a dip at about 60 degrees through the launch arc, then dip the nose for less then a second to gain some speed then pull up you will come off with a lot of speed and gain quite a bit of altitude. Once you get past about 70 degrees, the HS is actually pulling you down.

Even with a light plane with a standard spar, you can do a mild zoom at 60 degrees through the launch arc, gain height and have some airspeed to help you penetrate.

[ozmo01]

Thanks Ed,
I had a chance to fly for a couple of hours in light wind yesterday and tried pinging a little off the smaller of my two highstarts. It was pretty much as you described. I think with practice I will do fine.

[aeajr]

It is like anything else, you have to try stuff. :D

More useful stuff can be found in the Eastern Soaring League's Novice Lounge
45 articles that can help you. ;)
http://forums.flyesl.com/forum.asp?FORUM_ID=14

[dwells]

Not to knock the honorable Dr. Drela, but that article doesn't apply to these type of sailplane designs. The Gentle Lady, SIG Riser and Spirit series are notoriously over-decalaged (if that's a term.)

The dive test isn't gonna tell you squat if you've got a ridiculous positive incidence on the horizontal stab and will even indicate 'nose-heavy' with an extremely aft CG.


Get the decalage between the wing and horizontal stab to 1 to 2 degrees first, then proceed with the dive tests.

I agree Sean, the Spirit has these issues for sure and likes to porpoise because of it. Shimming the wing is good for experimenting but stresses the root because it's not seated on the wing saddle. I removed the stab mount and shaved the stab saddle to bring the stab in line, took about 1/8" off. Now the elev is nuet and the plane flies great. This is discussed on the CRRC site also...
http://www.charlesriverrc.org/articles/kitmods/dickwilliamson_gpspiritmods.htm

[lincoln]

The Spirit porpoises because the aft fuselage is too short and the stab could be larger if it isn't put further back.

The GL does not have this problem. There's a lot of questionable stuff above, considering that, very likely, the problem can be solved with a few clicks of down elevator. If not, probably there's something like slop in the linkages.

Incorrect decalage is not going to screw up dive test results if the glider is first trimmed to a reasonable speed. A flat stab with a bunch of elevator trim still has an angle where the lift is zero, and the lift will change reasonably linearly with angle of attack, just like other airfoils or a flat plate. Some symmetrical airfoils and, I think, flat plates have a little screwy spot right around zero lift, so if there's a little bit of elevator trim perhaps it would actually work better than flat.

Generally, if you find yourself disagreeing with Mark about aerodynamics, it's a good sign that you need to re-evaluate your ideas. I've disagreed with him myself, but mostly just that I like my gliders a bit heavier than he does. Not sure if a mathematical analysis will support this, but a little heavier suits the way I fly.

[dwells]

The Spirit porpoises because the aft fuselage is too short and the stab could be larger if it isn't put further back.

Ok. All I did was leveled my stab. CG is the same, the fuse is still the same length and the plane no longer porpoises, why?

[aeajr]

I likewise removed the stab, adjusted and reset it. Flies great!

[CloudyIFR]

Okay, this one I'm guessing on.

Do you think it's due to something like hysterisis due to an incorrect airfoil and by changing the incidence of the horizontal it cures this?

It'd be interesting to see a different airfoiled horizontal with the original incidence and see if this would be the cause.

Does any of that make sense?

Curtis
Montana

[ozmo01]

Well I made no changes to the GL (Witched Woman), just got more practice. It flys well.
I do have poor luck in windier conditions such as in wind over 12 mph. I can fly it but I get little penetration even with an increase in down trim. With lower wind speeds it does very well. when I added approx 3 ounces it handled a little better but it still didn't head out very well.