HPA

Wing trailing edges

I have a clear memory of using too soft a strip for one of the wing trailing edges on the original model. It bend after doping the tissue. This was my introduction to wood selection.

Now, I weigh every sheet of balsa. It is rather overkill for a sports model, but keeping a record of what I used and seeing how it fares on the completed  model, gives me an insite.

I cut a 5mm strip from the edge of a sheet of #7lb/ft3 balsa. I was lucky that I could use the Jim Jones balsa stripper at its maximum width setting.

I had cut from the sheet where it was very opaque when held up to the light. This strip turned out to be  #10lb/ft3 density! The variation within a single sheet is supprising.

I dislike sanding trailing edges on the completed wings as I often damage the ribs and spend much effort repairing the damage.

So, I made a sanding jig consisting of a  length of 1/16" diameter piano wire cyano'd near the edge of a glass shelf from a fridge. It took several attempts to determine the nessesary inset from the edge. An 8mm inset, the width of a Lego brick, yielded the correct chamfer for the strip to match the rear profile of the ribs. The trailing edge of the strip is quite thick: about 1mm. I don't care about achieving a thin trailing edge, but don't want a change in slope at the trailing edge.

Using fresh sandpaper on a hard sanding block made short work of the job. A horrid scratching noise indicated that the sand paper was touching the piano wire and the edge of the glass. Thus, the strip was abraded to the correct profile. The strip was then inverted and a sanding sponge cleaned up the other side of the strip. The yielding surface allowed a little more material to be removed.

I did not induce any noticable bending of the strips due to sanding stresses. If they appear over the next few days then I will soak them in boiling water and weigh them down flat.

I was a bit worried about the stiffness of the sanded sections. So, I performed a simple buckling test by standing each strip vertically on a digital scale and pressing down with my thumb. The reading at which they buckle gave a measure of its stiffness. For indoor duration models; it is critical to employ the combination of the lightest and stiffest woid. But, for this application I only wanted to eliminate the outliers. I found a couple of strips of half the average stiffness and one of double!

So, I cut a strip from a different sheet and and repeated the process untill I found four trailing edges of similar stiffness.

Nice work Stunthenk! Your use of bamboo is interesting.

John

I plan on anchoring the motor peg at Former D.
ian

Also following with interest, Ian.

John

Waiting with bated breath to see where you put the motor peg

About 0,6 mm. I started with a 1 mm sheet that weighed 9 grams (standard size, 1 meter by 10 cm. so a density of 0,9). After applying the sheeting I did some more carefull sanding at the spar and the LE so now it is anybody's guess!

OK the Jet Hustler. I cleaned up and built the tail end, here it is all stuck together for a dry fit. I made a saddle for it which I do for all my round fuse models. This one I am going to use the thrust I used on my Mig-15 (Srull) which was slightly up a degree or two. Im going to cover this with mylar and Gampi.
The dihedral. The plan calls for flat. It looks better and I feel better with a few increments of dihedral, so going with the gut. I am going to test out the idea that swept wing requires no dihedral, but I confess to be a doubting Thomas
because it might only work in prime conditions. I work on a sliding scale of prime conditions, therefore I err on the safe side.

What is a thickness of the LE sheeting?

Marcin

   Nice work , very neat.

It's nice to hear from you again, Crabby. We've missed you. Mike.