Float attachment loads
 

I'm building a 1/4 scale PZL Wilga kitted by Frisch Modellbau. One thing that I may add to it later are floats. The fuselage is epoxy-fiberglass and requires a number of ply/hardwood/balsa formers and reinforcements. It will have a scale interior and this would be the best time to add reinforcement.

There is a ply landing gear plate at the main gear and a ply former where the conical tail section joins the main fuselage, but there is not any reinforcement where the rear float struts will attach to the fuse. This is an epoxy-glas fuse and I think it will need some reinforcement at the attachment points. The landing gear plate is more than strong enough to take landing loads of the 25+ pound plane, but I'm not sure what sort of loads will come from the rear float struts and how strong to make the added rear attachment points. Can anyone give me some feedback on the landing loads of the rear float attachment?

--Bill

RE: Float attachment loads
 

You'll need about the same strength attachment for the rear as the front. A 1/4 lite ply piece with enough aattachment to bulkheads and to the front mount would be desireable.

RE: Float attachment loads
 

With an "N" strut arrangement like that, the landing loads get tranlated to compression loads on the rear struts. The loads on the rear struts can easily be higher than the loads on the front struts. I'm sure the full scale has some kind of tube structure at that point inside. Your best bet would be to duplicate it with hardwood or carbon fiber dowels.

Jim

RE: Float attachment loads
 

That was my concern. The N-strut will transfer loads to that un-reinforced area of the 'glas fuse. Fiberglas is very strong, but unsupported like this it can be like an eggshell. My tendency is to overbuild and add too much weight.

The "problem" with the Wilga is that it has a lot of windows and the (scale) interior will be very visible. So what I will plan to do is duplicate the main landing gear attachment plate (with a 1/4" plate and former across the fuse floor) and add a rear fuse former to distribute the load and transfer it to the wing plates. I've attached a photo of the full-scale fuse in that area and a drawing of the reinforcements I'm adding.

--Bill

RE: Float attachment loads
 

I agree with jrf in that the loads are greater on the rear struts. Be wise and do not skimp in that area. Take your time and do it right now --- you will kick yourself if you need to repair it later. Would love to see pics of your project.

RE: Float attachment loads
 

It shouldn't be too tough. Beef up from the bottom of the fuse to the interior floor with a 1/4" ply bulkhead. Securely epoxy the Bulkhead to the skin. Then you can feed pretty darned substantial loads into it.

RE: Float attachment loads
 

I've got the ply former in and I'm working in the other "scale interior detail" pieces.

Another scale plane "in the works" that will also be fitted with floats is a Curtiss Condor:

Thanks, all...

--Bill

RE: Float attachment loads
 

epoxy a strap of carbon fiber tape (say 1" wide) in front and behind your new bulkhead. You can also run cf fore and / or aft to the next structural bulkhead to distribute the loads. It adds a huge amount of strength to the fiberglass, and widens the load footprint so the stresses are disappated over a wider area of the fuselage............

RE: Float attachment loads
 

Good points, Wayne. In the orig, there is a rib encircling the fuse at the area where the bulkhead attached. There are also longerons attached to the skin. These are the aluminum equivalents of the same reinforcement you suggest.

In the larger sense, what is important is to feed the loads into the structure without distorting the structure. The semi-bulkhead accepts those loads and the loop of CF you propose will certainly help. Even eggshells are pretty strong until they begin to distort...then it's Omelet City!