Advice on Mods for Gentle Lady Kit from 1980's? - RC Groups
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Mar 11, 2011, 02:53 PM
bmwman91's Avatar

Advice on Mods for Gentle Lady Kit from 1980's?

I have been preparing to build my first sailplane, which I thought would be a Super Soarwatt from Aerocraft. The idea of a stick-plane really appeals to me since it is almost completely built from scratch & I enjoy that sort of thing. However, they seem to have a serious backlog due to a move & I have no idea when my order will ship.

When I was visiting my parents last week, I mentioned this to my father & he pulled a Goldberg Gentle Lady kit out of the closet. I guess it sat in a neighbor's attic for 25 years, was given to my dad during some cleaning, & sat in HIS closet for 5 years.

I have taken stock of the kit, and it is missing a few items. Some can easily be cut from 1/16" balsa sheet & replaced. One of the 3/8"x1/4" basswood pieces for a main spar is missing, as well as the aluminum spar joiner for the middle. I happen to have a good amount of pre-cured unidirectional CF strips on-hand from ACP, so I was thinking of doing things a little differently. One of the #1 ribs is broken as well, but that seems like something I can either cut from sheet & replace, or just glue back together & patch with CF.

What are your thoughts on:

1) Composite CF/balsa main spar. The original main spar was longitudinally-grained basswood. I am thinking of getting some 1/4"x4" balsa sheet & cutting it across the grain into strips so that I can make the spar web from vertically oriented balsa, and the flanges (caps) from longitudinally oriented pre-cured CF. Ideally I would like to skip all the plywood dihedral/polyhedral spar joiners & just use CF caps. Thoughts?

Based on the rolled drawing for this plane, it looks like the center of lift is about 1.5" behind the main spar (assuming the arrow directing where the CG is supposed to be coincides with the CL). Because of this, I am thinking it would be OK to cap both the main spar & the rear spar with CF (and replace the rear one with balsa in a similar fashion to the main one). If the CL was in front of, or in the main spar, I wouldn't consider doing the rear spar since that would shift the shear center further back, which sounds like a recipe for instability/flutter. But since it seems to be back there, it might be OK. I'll avoid capping the TE since it is very wide & that would shift the shear center way back. The LE might get some CF, I am still thinking about that.

2) Fuselage reinforcement. The original kit calls for fore/aft grained 1/16" balsa covering the fuselage. I am thinking of using some 1/32" balsa sheet, with the grain running left/right & up/down, and then covering it with fore/aft oriented 0.007" CF. How would this compare to just left/right oriented 1/16" balsa in terms of durability? The bottom of the nose area will probably have a couple layers of CF in different orientations.

3) Wing mounting. I would like to screw the wing onto the fuselage. I need a little input here. With the dihedral angle in the middle, I assume I need to build a saddle into the fuselage & then use 4 screws (2 left, 2 right fore & aft).

4) Capping ribs with CF. The ribs do seem to have some pretty thin sections, even when glued to spars. Would there be much value to capping them with CF, top & bottom? I'd trim them so that the CF on the main spar would be flush with the bottom of the rib, & the cap on there would mate neatly to the rip & spar (reducing the chances of the spar cut-out breaking open in torsion).

5) Sheeting the LE all the way to the tip. It seems popular to run 1/16" or 1/32" balsa sheet from the LE to the main spar from root to tip. Pointers there? Is this to improve wing bending stiffness or torsion stiffness (or both)? It seems like it would do a little of both, make the coating sit more cleanly (aesthetics), and maybe help in rough landings. It sounds like adding a little weight to the wing (particularly toward the tip) helps to reduce torsional resonances in typical frequency ranges, which appeals to me since the idea of wing flutter sounds pretty irritating.

Any advice is valued. This is my first airplane building experience. I have built loudspeaker enclosures, lots of electronic gizmos & furniture in the past, so this is (luckily) far from my first precision-building experience. I don't want to over-complicate this or anything, but it seems like one of the fun virtues of this hobby to be able to go a little overboard.

I would like to make this plane as light as possible, since with the motor & everything it will likely weigh enough to not need ballast under good conditions. I am hoping to keep the finished weight under 40oz. For the covering, I am looking at translucent Ultracote in the stock colors. Wood planes with translucent covering are beautiful (from what I have seen here)!

One other question...

What sort of wattage should I look into in a motor? I am going to use a GoBrushless DIY GBx kit. I want to make a plane that soars well & can climb up maybe 1000ft. It certainly does not need to be able to do a vertical climb (not the first one anyway). From discussions here, it seems that 100W/lb can give a pretty hot plane, and that 50W/lb is perfectly fine for the patient ascender (me). I'd probably be looking to run a 1400-2200mAh 3S LiPo battery to keep weight down. If a 40oz target weight is something that I should worry about instability with, I'd probably look for a higher capacity battery (maybe 3S 3200mAh).

Thanks guys!
Last edited by bmwman91; Mar 11, 2011 at 03:00 PM.
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Mar 17, 2011, 08:41 AM
Registered User
Firstly on the GL the wing is one piece, so you are not missing a wing joiner. It doesn't exist.
Secondly if you are going electric the plane as it is is strong enough. No need to beef up any parts.
The GL was designed in the day when radios and batteries weighed a ton and were HUGE in comparison to modern stuff.
Find a thread for an electric conversion of the GL. Usually the mod is to just attach the motor to the front (exisiting bulkhead). Don't mount the servos and receiver and battery till the plane is completed. You can then move these around to get the CG in the right place without adding ballast.
The original GL used a very heavy battery in the nose (four AA cells) and then still needed an ounce or two of extra nose weight to balance)
If this is your first plane build, just follow the plans. Get some experience before modifying an already good plane. Maybe on your second build you can you can make changes.
Mar 17, 2011, 12:44 PM
High Lift Coefficient
Sailhigh's Avatar

I've built and modified a number of GL over the years, though not for electric motors. My answer may not be what you're looking for, but your extensive list of modification is telling me you are trying to change the GL (basically a trainer) to another type of plane. It will be a lot less time consuming and cheaper if you did buy another plane that will satisfy your requirements. I suggest to build the GL as is and enjoy it for what it was meant for. You won't be disappointed.

The GL was in fact originally designed to be built with an optional three piece wing. That option is still shown on the plans. The vast majority of builders opted for the one piece wing.
Mar 17, 2011, 02:02 PM
B for Bruce
BMatthews's Avatar
Your plan for the spars would be superb if somewhat of an overkill solution. But the wing would sure be bullet proof!

On any model the center of lift FOR THE OVERALL MODEL will always coincide with the location of the balance point which we often call the center of gravity. So yes, you would want to do both spars in the manner you suggested for two reasons. First the lift center is located at the balace point which is between the two spars and second because doing both spars would make the wing more torsionally stiff to aid in reducing the risk of flutter and to make it more warp resistant. And if you wanted to do the spars with no joint doublers it could be done with angled and beveled caps just as you suggested as long as the glue joints would be long enough.

The plan you suggested for the fuselage is overkill plain and simple and quite over the top and just not required. If you want to do something different instead of the wood box I'd suggest a stick built fuselage with a Warren truss pattern of upright diagonals that is done in the same pattern as lift crane booms. For the main corner longerons go with 3/16 sq basswood and just plain 1/8x3/16 balsa for the diagonals. While you're at it you could thin down the fuselage a bit in width since the modern radio gear sure doesn't need all the room that is built in. The resulting fuselage would be light, stiff and strong. But best of all if covered in a transparent or translucent film it would have the same sort of old time'y free flight look as the wing and tail surfaces.

By the way, the original airfoil and wing design is at it's best when flown with a light wing loading and at lower speeds. The Gentle Lady is one of the models that defined performance back in the AoF or Age of Floaters. If you try to make it into a zoomy higher speed model without a total redesign then you're just not going to get there from here. So it's best to do any mods in light of the idea that you're going to end up with a pleasant to fly calm to moderate wind model that takes its own sweet time getting from one spot of the sky to the other. It's the sort of model that when you find a big lazy thermal you set the trims to put the model into a nice circle then set the Tx on the ground or let it hang idly from one hand while you just look up and watch the sun shine through the covering while the model "free flights".
Last edited by BMatthews; Mar 17, 2011 at 02:08 PM.
Mar 17, 2011, 02:14 PM
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rdwoebke's Avatar
Originally Posted by kaindub
Firstly on the GL the wing is one piece, so you are not missing a wing joiner. It doesn't exist.
The plans or maybe the instructions I think include ideas for making the wing 3 piece. I made my GL wing 3 piece with music wire joiners slipping inside of brass tubing.


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