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Apr 21, 2005, 10:46 PM
Registered User
Thread OP

Home-brewed DLG launches funny, help!


Here's my latest creation. 1.2m Wings off a Zuni-V. The pod is 3/32 ply with wood blocks at front, middle, rear.
(ala KaiWorks DLGs http://kanoh.web.infoseek.co.jp/original%20glider.html). An Avia UL spar for the boom.

Wingspan - 47"
Length - 31.5"

Vert Stab - 8.5" x 3"
Horz Stab - 11.5" x 3"

Weight - A little on the heavy side I'd guess. Basically a Zuni-V with a heavier tail.

When I DL it, it immediately rolls over on it's back. When throwing with my left hand, if I push the stick about 1/2 to the left I can get an ok launch. Not as great as I would like...

My question: What's wrong? If I change the tail can I get a straight launch?
Or should just use a preset at launch?

It flies like a dream once it is in the air. Getting in the air is the problem.
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Apr 21, 2005, 10:59 PM
Registered User
Short tail moment, perhaps an aft cg,
My first sal home brew did the same thing, it was wing flex. Try a slower side arm launchand see what happens.
Apr 21, 2005, 11:02 PM
When you say it rolls on to its back, maybe it's an aileron trim issue? It's hard to tell if there are ailerons because of the light color wing, but it looks like it does.

I'm also in FL and frequently near Jax. We should fly sometime.
Apr 21, 2005, 11:21 PM
Registered User
Thread OP
When I launch it slower just rolls over a little slower. It is a nasty launch unless I lean on the stick. And I think the trim is ok, flies fine once it is up.

Could the tail be too tall on top, too short on the bottom? Maybe the Vert Stab isn't big enough. More experiments tomorrow...
Apr 21, 2005, 11:27 PM
Registered User
yes it could be too tall on top. The rule of thumb is 1/3 bottom to 2/3 top(looks 1/4 to 3/4). If it is twisting in flight it will roll. The faster the launch speed, the more the twist, the faster the roll. You may have enough area, but might want toincrease it, due to the short coupling. Look at the gambler and note the vary large tail area, due to it being short coupled
Apr 21, 2005, 11:30 PM
Registered User
Try stiffening the upper portion of the tail, may be the quick fix
Apr 21, 2005, 11:32 PM
Registered User
SloppyT nailed it, tail moment is too short. After you have been flying these things for a long enough time you can spot problems just by looking at a plane.
Apr 21, 2005, 11:36 PM
I think the top part of the tail may be long, yeah. The area of the tail surfaces looks more than sufficient - I think most people tend to overdo the size of the tail, even though your boom is relatively short. I would try taking some material off of the tail's top.
Apr 21, 2005, 11:43 PM
Registered User
can you extend the boom? If not, the easiest would be to add more rudder area and leave the top alone. just cut a straight line off the back of the rudder and glue on an additional section. trim to taste.
Apr 22, 2005, 02:38 AM
Slopeaholic
Hutch's Avatar
I noticed that you don't have a launch peg installed. A problem I had when learning to launch with my little DLG was that I was releasing to late. I was also using to much wrist almost giving it a frisbee type of release. I had the same exact symptoms you are talking about. I would release and it would slow immediately as it rolled onto it's back. Pay attention to what your arm is doing when you launch. I found that when launching with a grip on the wingtip I needed to release earlier than it felt right to. It was my fist attempt at DLG so I had no experience in what was right.

Check out your technique before you do a major rebuild. I'm building a new fuse for mine with the intention of shaving an ounce or two of body fat, but before I took her down for repairs I found that by launching differently I could get a straight launch without fighting a roll at 20 feet with almost no airspeed.

These are the answers I got when I asked almost the exact same question...
https://www.rcgroups.com/forums/show...ighlight=hutch

Good luck!!!

-hutch

P.S. FWIW, just to jump on the bandwagon the tail moment does look a little short too.... I had the same comments on mine. That's what I love about scratch building though, It doesn't cost 400 bucks to find out that something isn't quite right. But I suppose that for that much cash it oughta be right already. I'm going to bed now before I start babbling incoherently.
Last edited by Hutch; Apr 22, 2005 at 02:59 AM.
Apr 22, 2005, 06:54 AM
Dark Side of the Red Merle
Curtis Suter's Avatar
I have an MS Excel spreadsheet that will tell you what your tail moments should be and your tail sizes. It uses Mark Drela's formulas on tail sizing.

Email me at [email protected] and I'll send it to ya. If you don't have a spreadsheet program and would like to know the measurements I can have you send me some simple measurements and I'll get you an answer.

Curtis
Montana
Apr 22, 2005, 07:56 AM
Registered User
Thread OP
I'm surprised that everyone thinks the tail moment is too short. The Vert Stab moment is an inch longer than both the Zuni and Zuni-V (Planes with the same wing). If I scale the size down on the ArtHoby Hybrid DLG, the numbers are close to my DLG. (I know... scaling down isn't such a good idea)

Here are the numbers from the spreadsheet
Distance from Wing Trailing Edge to Stab Leading Edge - 12"
Distance from Wing Trailing Edge to Fin Leading Edge - 15"
Vh=.42
Vv=.046 (needs to be .05-.06)

My easiest fix would be to make the Vert Stab 2/3 top, 1/3 bottom and make it a little wider. Hopefully that will help. Otherwise I would have to do some major surgery to remove the boom and replace it with a longer one. (It flies great once it is in the air, so I am reluctant to change things around too much.)

Thanks for all the input everyone.
Apr 22, 2005, 11:06 AM
ADDICTED TO LIFT
glider34's Avatar
The aft part of the model looks "flexyish". I have found that a good launch comes from a Ridgid model with a longer Tail Boom - Moment.
Apr 22, 2005, 11:54 AM
Registered User
Thread OP
Yea, it is flexy.
I built it with what I had on hand. I am going to try and make it work, but it may be impossible. If I can't get something acceptable by this end of the weekend, I might end up hacking off this boom and getting a better/longer one. Or just breakdown and built a proper DLG. The Zuni-V wing really doesn't seem like it is strong enough to install a launch peg.
Apr 22, 2005, 12:44 PM
Registered User
You didn't say whether you had any experience with DLG launches or not. If this is your first attempt at doing DLG launches then part/most of the problem may be with your throwing technique rather than with your plane.

I have witnessed the exact behavior that you describe. In the first year of discus launching, back in 2001, I let a few people attempt some discus launches with my plane. These were guys who had never done it before and just wanted to give it a try. There were at least a couple of these guys who did launches exactly as you describe. The plane would immediately flip inverted at a very low altitude, too low to even recover. Obviously, since they were launching my plane, you have to conclude that the problem was with launch technique. I could launch that same plane to about 35 meters.

Here is what I think is happening: Immediately after release the airplane will travel in a straight line tangent to the circle it was being spun around (it will initially travel in a straight line in the same direction it was traveling just prior to release). However, the model will also continue spinning around it's own vertical axis as it moves along that straight line. This spinning motion around the vertical axis quickly leads to a sideslip (air begins hitting the model from some angle to the side of the nose rather than directly on the nose). Any model with dihedral, upon encountering such a sideslip, will begin to roll. The more dihedral or dihedral effect (there are other aspects of model design that add to the dihedral effect) the model has, the more the model will roll as a reaction to the sideslip. A model with the vertical fin entirely above the boom or mostly above the boom will have an additional roll reaction due to a sideslip that adds to the roll due to dihedral effect. This is due to the side force from the vertical fin acting only/mostly above the model's center of gravity.

The key thing to get from the above paragraph is that the model will have a sideslip shortly after release and that sideslip will be converted to a rolling tendency. So how much will the model roll? Will it roll just a little before getting on track and continuing on a straight launch? Or will it roll over completely inverted and crash? There are a number of factors that determine the answer to that question.

1) You have already gotten some good answers on how to improve the model so that it will better dampen out the sideslips before rolling too much. Longer tail moment, bigger fin that extends below as well as above the model's CG, stiffness everywhere, especially the tail boom (both bending and torsional stiffness) and stiff linkages all contribute to a better launch. Improvements in those areas are likely to improve launch performance and reduce the likelyhood of a really bad "inverted launch".

2) More dihedral means more yaw/roll coupling so polyhedral models are far more prone to this inverted behavior than aileron models are.

3) Launch presets. You have already discovered that a rudder preset to the outside of the spinning circle will help to prevent the model from rolling to the inside of the spinning circle shortly after release. Rudder presets are normally used on polyhedral models and sometimes used on aileron models. An aileron model that is well designed and launched with good technique normally does not need a rudder preset.

4) How hard/fast the model is thrown. Naturally, as you have discovered, the faster the model is spinning after release, the greater the tendency to roll after release.

5) Launching technique. Bad launching technique can make this rolling behavior much worse. Bad technique can cause the model to be released in an awkward attitude. Most commonly and the worst situation would be to release the model such that it already has a bad sideslip angle at the moment of release. Also bad would be to release the model such that it is already banking towards the inside of the spinning circle. It is not clear to me whether or not throwing technique can have an effect on the amount of spin that the model has at the moment of release. The amount of spin may be entirely determined by the speed of the throw and the diameter of the spinning circle.

To make a model flip inverted immediately after release requires some combination of the above factors. Certainly a really bad throwing technique with a model that is not well designed for DLG will cause the problem. If you are not an experienced DLG flyer then you might want to get an experienced person to launch your plane for you just to see how much of the problem is from the plane versus the throwing technique.

Elements of good throwing technique:

1) Lock the wrist and elbow. The arm should be thought of as a rigid stick that does not bend. Do not snap the wrist at the end of the throw. This is a tendency that some guys have from years of throwing baseballs, for instance. Snapping the wrist is almost guarantedd to put the plane into some unusual attitude at the point of release. Bending the elbow will also cause the plane to be released in a bad attitude as well as the fact that a bent elbow only reduces the spinning radius and hence the release speed of the model.

2) Release the plane wings level and with the nose pointed at or a bit above the horizon.

There are other factors related to a good (high) launch but those two are the ones that are likely to be related to the problem of flipping inverted after launch. To get these two factors right you should concentrate during the launch on keeping the arm and wrist straight and locked. I remember my early days of discus launching, forcing myself to think about reaching out away from my body as far as I could during the spin, maybe even imagine leaning (either imagined or for real, not sure which) outward in an attempt to make the spinning radius larger. Then, towards the end of the throw, pick a point on the horizon (either literally, visually, or just in your mind) and imagine throwing the plane towards that point. If you focus on these elements until they become second nature you should avoid the flipping inverted behavior. Later on, after these basic elements become second nature, you can begin focusing on speed and power to increase launch height.


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