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Jun 13, 2003, 04:04 PM
Registered User

I need your input on this plane

Hi All,
I bought two of this kit in the 70's in Japan. I like the design so much that I have built only and keeping other kit as a collection.
The kit was first built rudder/elevator per plan. Here are some stats:
-Wingspan 98"
-Clark Y flat bottom
-7 degree dihedral under each wing
-Double taper wing. About 700sq in.
-No twist in structure. About 1/4" washout. CG per plan

The ship was the best floater I have even owned. In dead calm, it consistanly stayed up 6-7 min off a highstart launch. However, it has two major handling problems. Problem#1, it will roll the opposite direction when small rudder input is given. It will only turn correctly with a big rudder command. Secondly, it would tip stall when turned tightly.
After flying it for several years, I added ailerons. Following the original plans, 1:3 differential was used. The plane gain about 6oz but still retained the low sink rate. With ailerons, the "adverse roll" disappeared. However, I still have to fight the tip stall problem.
I am in the process of rebuilding this ship and would appreciate your input to correct these bad habits. Attached is a picture of the kit. Can anyone tell me if this has ever been imported into North America?
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Jun 16, 2003, 11:37 PM
Gambler-AG DLG Designer
Allan Wright's Avatar
I don't know anything about the kit, but if you build in 3/8" of washout in each wingtip that should correct any tip-stall problems without adversely effecting the wing's nice lifting ability.

The rudder adverse yaw is strange - possibly the area on the control surface is too small? You could try enlarging it some. 15-20? maybe. This is just a guess though - the washout suggestion is not - I've used it to correct the same problem on another kit built sailplane (skimmer 400).
Jun 16, 2003, 11:45 PM
Registered User
Kez I cannot understand why the 'rudder/elevator' version would turn the plane in the opposite direction when you applied small changes in rudder if the wing was built and adjusted correctly (with proper washout) and you were not flying near 'stall' speed. I assume what you mean by 'turn' is that it 'gracefully turned' in the opposite direction. If the rudder moves toward the right side of the plane then the plane should bank right. A properly set up plane traveling at an acceptable speed will show the most minute changes in rudder on a calm evening by producing a very small turn. The plane can be flown properly with even small changes in the rudder 'trim control'. If this was not possible on your sailplane when it was rudder/elevator then obviously something was not correct about the setup or flying conditions.

I hope by 'washout' you mean there was a slight twist to the wingtips such that the trailing edge of the wingtips were higher off the table than the leading edge of the wingtips when the wings were placed on a 'flat' table. With proper washout, appropriate dihedral and adequate flying speed it should have turned in the proper direction with extremely small changes in rudder movement. This should have been easily noticed on calm evenings. If the twist in the wing was opposite to what was mentioned above then you had introduced 'washin' and during a turn it most likely would have caused the forward moving wingtip to 'stall' and drop the wing which would have caused the plane to 'flounder' into a turn in the opposite direction you desired. It would probably required a few feet to recover proper flight if that tip stall had taken place. And I would not refer to this as a 'turn' in the opposite diection but rather a 'stall into' the opposite direction.

You mentioned with enough rudder it would turn in the proper direction. This may have been because with excessive, fast rudder movement you may have forced the main section of the wing to 'win out' over the wing tip and hide a wing tip problem.

I can't think of anything else that could have caused the type of flight your are describing. Something was just 'not right' on that plane.

Now with ailerons you have more 'authority' and they could cover up the original problem and introduced another. If the ailerons extend out to the wingtips then they could be causing 'tip stall'. When you turn right the left aileron should go 'down' and that wing should lift 'up'. In this process the aileron actually introduces 'washin' (this is not a good thing) and the tip could stall because of that. This would be most likely if you turn 'sharply' or if you are flying near stall and turn even slowly. One way to limit the ailerons from causing tip stall is to keep them away from the wingtips. Some planes end the ailerons maybe six inches from the wingtip in order to keep the wingtip clean. Also maybe you are not keeping the flying speed fast enough (thus flying near stall). If you are right near 'stall' and you lower an aileron you may cause that tip (and maybe that entire wingpanel) to stall.

If the ailerons extend to the wingtip you could also introduce some 'twist' in the outboard part of the aileron. Twist the outboard ends 'up' and it is like adding 'washout'.

If you think about ailerons they could also be adjusted in such a way thay they act like 'flaps'. That is, they both droop down like flaps. If they do, this may make that section of the wing attempt to 'lift' more than the other section of the wing. This may cause a stall in the area of the ailerons before the main section (the area where there is no aileron). If so, then this is a problem. So the ailerons should follow the shape of the wing but should not introduce a 'flap' like look to that section of the wing.

You mentioned that you placed the CG where the plans suggested. Generally plans state a 'safe' location for the CG. Fine tuning by making small changes to the CG might make the plane fly even better. You can look into the 'dive test' and try that. It may point out that the CG is just right or it might point out it should be changed. A change in tow hook position may be required if you decide to keep a new position for the CG.

If you have a digital camera you might consider taking a piture of one wing panel from the back so that it shows any washout you have and a piture from the top to show the wing and aileron. If they look right to me in the pictures it would help. Hopefully i can see a possible problem just by looking at the wing.

To look for washout on my wings I hold the wing up in front of me and sight along the trailing edge. i align the bottom setion of the wing with the trailing edge of the wing (easy if the wing has a flat bottom) and view the wing from root to tip. If there is any 'washout' at the wingtip then the trailing edge of the wingtip will be higher than the bottom of the wing in that area.
Last edited by Keith Johnson; Jun 17, 2003 at 12:09 AM.
Jun 17, 2003, 04:13 PM
Registered User
Thanks Allan, Keith for your reply.

By 'turning the opposite direction', I do mean a graceful turn. In fact, it turned very well in the opposite direction. I did have 1/4" washout (TE 1/4 higher than LE) although the original plan did not specify one. I came to 1/4" based on the fact that it has a tapered planform, blunt LE and partly from experience.

Yes, the ailerons did extend all the way to the tip and they were 'barndoor' type. I will try to attach one of the few pictures I have of the plane but I am not sure how well it will show.

The rudder has ample area. I know this 'adverse roll' characteristic do exist in some pattern ship design. My initial thought was that if the rudder has a high aspect ratio, it will act as half an aileron 90 degree to the wings. So if one gives left rudder, it will yaw to the left but roll to the right. I believe that is the reason pattern ships usually have some rudder below the fuselage centerline. However, my rudder does not have high aspect ratio. I compared it to the Aquila and the latter has a taller rudder and turned very well; in the correct direction. Further I have plenty of dihedral so it should offset the rolling effect of the rudder.

I agree with your theory that the ailerons extending to the tip can cause tip stall and that's why I want to rebuild the ship as a rudder/elevator. I did set up the ailerons so that at neutral, both ailerons were 1/8" up. This would have given the same effect of more washout but it did not help.

I eventually crashed the plane (tip stall) two seasons ago. The fuselage is in very good shape and I want to build new wings. I need to know how to correct for these bad habits before I start. Thanks for sharing your experience and thoughts.

Jun 17, 2003, 07:24 PM
Registered User
What a puzzle.

When the plane was set up for rudder/elevator and you fed in a small amount of right rudder the the left wing should have rotated forward from the CG and the right wing should have rotated backward from the CG (a yaw). The left wing rotating forward should have produced MORE lift than the right wing rotating backward thus producing a 'bank' to the right.

What you found was that the plane banked left. Since this was a smooth bank in the opposite direction of the rudder then we can rule out a tip stall.

Consider that when the plane rotates around the CG point one wing will go forward. With dihedral that wing will now be at a higher angle of attack and attempt to produce more lift. However, 'lift' produces 'drag'. Could it be that the drag on the wing rotating forward was so great due to considerable dihedral that the wing actually slowed down to the extent that the plane dropped that wing which then banked the plane in the opposite direction? This would be a bank toward the forward wing.

This would be similar to a pattern plane that lowered the aileron on the left wing produceing drag that was not counterbalanced by raising the opposite aileron high enough to match the drag on the left wing. The plane will attempt to rotate toward the wing with the higher drag (a yaw).

Or another way to say this is to picture a pattern plane that lowered ONLY the left aileron and never moved the right aileron. The left wing would try to lift but the drag is great enough the the plane produces a 'yaw' to the left, opposite the intended turn. The correction is to counter the 'yaw' with right rudder but you had already given it rudder to initiate a turn to the right.

In the case of the sailplane the wings are SO large that when the plane starts to rotate around the CG it appears that a huge aileron has been lowered on one side only. That side experiences large drag that slows the wing, drops the wing, yaws the plane and now the plane is 'banked' toward the low wing.

Based on the above could it be that either you have too much dihedral and/or not enough rudder authority?

I believe you indicated that if you introduced enough rudder then the plane WOULD turn in the correct direction. This would be similar to the pattern flier introducing rudder to offset any yaw produced by the wing with the excess aileron drag.

You mentioned the 'Aquila'. I have a half finished scratch built Aquila here. I need to build the wings, mate them to the fuselage and cover the plane. This is a polyhedral wing as you know. I will be scratch building a wing for it using the Eppler 193 airfoil to the same outside dimensions as the original Aquila wings. I wanted to remove the flex in the wing by using a stiffer wing rod and the Eppler 193 airfoil will allow better penetration into the wind. I will try carbon fiber laminated to the top and bottom of the spars. I believe I will also also set it up with flaps on the inner panels only. It will be transparent orange wings and stab with the rest being white.
Last edited by Keith Johnson; Jun 17, 2003 at 10:08 PM.
Jun 20, 2003, 02:06 PM
Registered User
Lots of good input. Two suggestions. Move your cg backa hair and tape a stiff paper tab to the rudder to give it a little more input. This is a temp cure but might narrow down the problem some. Great looking plane!

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