Building a Dynaflight Bobcat

[mhmitchell333]

Hello every one.

After waiting and reading advice given to me from you and my Brother in law (a serioius model sailplane guy and a pipeline engineer) I have just started the Bobcats fuse. I plan to list my troubles and comments and hopefully if anyone sees a better way to do something they will let me know.

The kit is balsa and ply (I would say lite ply, but like evryone said this bird builds heavy). The die cutting while not as good as laser cut parts is fairly clean. They punch out easily and so far I haven't had to help any part along with the Exacto.

After discussing the weight problem with my brother in law and being told I should have gotten a Gentle Lady or even a Sophisticated Lady, the Bobcat is a dog etc. I asked him about structure strength of the sides if I cut holes . He said that the strength would not suffer at all. Hey! he is an egineer I am sure he studied all that structural stuff. Anyway using a piece of 1/2" brass tubing in my drill press and checking that the holes would not bridge any internal part glued to the sides, I taped the two haves together and drilled a series of evenly spaced holes from the wing trailing edge rearward untill I reached the point of taking more than half the depth (?) , top edge to bottom edge. Then grouping the holes in pairs I removed the wood between the pairs making long "slots" . so maybe 10 to 20% of the wood was removed from this area. I also holed all the formers.

Following the instructions I carefully marked the former positions. Glued the first one in (frmr 'A') installed the top balsa crutch (sub decking. When that was dry I glued the top side (left side) to the former and crutch. Then as per plans I fixed the fuse to the plan using their block and pin method. aligned everything to match the plan and glued the rest of the formers(3) in . Double checking that everything was square I left it overnight to dry.

The next day I un-pinned everything and was about to say well that was fun, when I noticed that while everything aft of the leading edge was true and square the left side of the nose was basically straight and the right side was very bent. Bannana time.

I figure that the two sides were cut from different lot numbers of ply and that one was stiffer (the straight one) and was actually pulling the nose crooked.

Ok what to do? I removed the first former in the nose, way too easy with the ca I was using. Clamped a strong straight board to the weak side. epoxied the first former to its position on the weak side and pulled the other side down to meet the former and clamped every thing. My hope is that since the strong side is now bent too much it will pull the weak to aprox the right position.
We will see tommorrow.

Any comments. Next time I build a flat side ply fuse I will test the flex by clamping each side to the table at the wing leading edge add equal amouts of weight to each nose to see if the bend the same amount.

ok that was fun.:rolleyes:

[Ollie]

If I was building a Bobcat or other model with 1/8 lite ply fuselage sides, I would substitute hard balsa. I would take my postal scale to the hobby shop and select balsa sheets with densities of about 14 to 16 pounds per cubic foot. The selected sheets would have a nice straight grain pattern. I would test the uniformity of each sheet by gently bending it across the grain. The stiffness should be the same at each end of the sheet. If I could, I would get a sheet large enough to cut both fuselage sides from the same piece to insure uniformity. When I got home I would use one of the lite ply fuselage sides as a pattern to cut out the balsa fuselage sides. Then I would weigh the cut out balsa fuselage sides to check for uniformity.

BTW, I always begin by cutting a fuselage side about 1/16 inch oversize to relieve any internal stresses before doing the final outline. If you don't, you may find that the sides don't match because they have changed shape as they were being cut out.

Building from scratch is more work but you gain control of the materials instead of leaving it to the kit manufacturer. If you buy a kit, it pays to buy a high quality kit with accurately cut parts and good material selection.

[mhmitchell333]

You know, I would have used hard balsa also if it wasnt a kit. The only reason I bought the kit was there are almost no plans available for sailplanes that look like sailplanes. I am not a total scale guy but I don't loke polyhedral and long pod and booms.

Wish I could find some good scale 100"model plans that are priced reasonably.

[eborys]

I have a Bobcat that I built more years ago than I can remember. I still use it as the first sailplane up on the winch in the spring, to get my eyes and thumbs working together, after the winter layoff.

Overal weight is not a problem. The model builds TAIL HEAVY. The problem is T-tails, which usually build out heavier and the short nose moment. If you want to concentrate on building light, the tail is the place to put your efforts, or go to a V-tail.

You will end up stuffing lead in the nose. Put some of the wieght in extra battery capacity... if you wire up your own pack, you can shape it fit farther forward in the nose.

There is a weak point in the fuse at the fuse - vert fin leading edge junction. Some CF helps a great deal. Make sure you taper the reinforcements so that you don't end up with the same problem, only in a different place.

Probably the most important thing about the Bobcat is to build the wing with a servo at each aileron. The method show on the plans has tons of slop and the differential is not easy to adjust.

Mine flys great... yes it is a dog compared to my Laser MC or Psyko M, but there's well over a 10 to one price difference.

[mhmitchell333]

Price difference? You can say that again I got mine on Ebay for $8.00.

Oh but I like T tails so much. I have taken 15 to 20% of the ply out of the aft end of the fuse.I plan to use a micro in each wing provided it can be totally enclosed. Otherrwise I will but them in the fuse(just enough room.) my Bro in law seems to like the flexable pushrods says there should be no slop if I glue them down in severl places but if servos fit that is a mute question anyway.

Thanks for your help!

Also plan to put holes in the ribs to make them lighter.

Dog or not I am not a contest flyer just wanted to try airlons. Why have a 6 ch computer radio if you only use two channels.

The plans say put cross grain balsa over the hard balsa crutch. I wonder if it should be cross grain as it make several splices necessary which means more weight. Also was thinking about holing the crutch under this balsa.But will wait your advice.

[Vince inTX.]

I would not leave out the cross grain balsa. I personaly made a similar mod to my old gentle lady a year or two ago. I removed the top aft deck and replaced it with cross grain decking. This may have added some weight but it added TONS of torsional rigidity to the plane as well. Not that is really a major cosideration in a floater like a GL but I though it was a good idea. If you are going to go ahead with the T-tail then remember that all that weight at the end of the verticle stab will be exerting lots of torsional loads on the aft fuse.

I'm no engineer but it seems pretty basic to me

Vince

[mhmitchell333]

Thanks,

All of you people persons are great!

Mike