From: kaiser@elec.canterbury.ac.nz (Chris Kaiser)
Subject: Re: Suggestion on prop for ASP .75 ?
Organization: Electrical Engineering, University of Canterbury, New Zealand
Date: Mon, 16 Nov 1992 00:43:27 GMT

In article <2484@rnivh.rni.sub.org>, torsten@rnivh.rni.sub.org (Torsten Leibold) writes:
> elec140@csc.canterbury.ac.nz writes:
> 
> >I've built and modified a Dynaflight 40 sized Spitfire, so thought I'd
> >throw in a few comments.
> 
> Great, I'm interested in that kit also.
> 
> >It's a good kit to modify, since you don't feel bad about cutting
> >into or discarding a beautifully shaped kit part...
> 
> ;-) That's the only positive point on that topic ...
>
> >The Spitfire also has quite a thick wing, and this does make it fly
> >nicely. It's also not particularly scale (e.g. flat rear fuse bottom),
> 
> How about the wing outline ?? Is it really as round as the original ??
> How has that been done ? By shaping large balsa trailing edges ?

The wing outline is definately elliptical - I'm not sure how scale it is
but it looks right. What it's missing is the fuselage fairing at the rear
of the wing, but this could be easily added. As to how it's done...
The main spar is straight - no problem there. The curved LE is formed from
two laminations of 1/8" balsa. The LE sheeting is 1/16" balsa on the top
only, and is supposed to just glue straight on - I had to cut a shallow V
at the tips to get it to conform nicely, but this was really no problem (and
may have been due to me moving the spar back which increased the compound
curvature...). There is no LE sheeting on the bottom surface apart from
the center-section sheeting. I actually added sheeting here to about 1/2
way up the wing to give me something to cut the wheel wells into. The TE
is also 1/16" sheet - a 1/2" wide strip goes straight from root to tip on
the top and bottom (the aileron spar goes between these), and the curved
rear shape is die-cut sheet glued to the back of this 1/2" strip. The bottom
sheeting is built flat on the board, and the rear edge must be bevelled so
the top sheeting overlaps and sits properly. It actually goes together
quite easily and builds a nicely elliptical wing without creating large
amounts of scrap balsa...

> >into the larger Mustang alright. Having said that, to get the retracts
> >in I had to move the wing spar back 3/4", which meant recutting all
> >the spar notches in the ribs. I added extra ribs and ply reinforcing
> 
> Oh, recutting would have been no problem, cause I had to do that
> anyway. The notches were much too small to fit on the spar ...
> It's a pity that I'm just finished with covering the wing (with
> nylon), so I'm not quite keen on doing it again to add the retracts.
> I'll give it a try next time...
> 
> >interfere with the aileron servo. To take the extra weight of the
> >retract setup I replaced the balsa wing spars with spruce and fibreglassed
> >the centre section.
> 
> I had really bad experiences with balsa spars previously, so I chose
> to install pine spars in every model from then on. So I did with the
> Mustang, but I'm still wondering whether the center section will stand
> the forces - I did not glass it, the brace seems to be ok for that
> light ship.

I glassed my centre section since I'd added some weight with the retracts.
This model is my "good" one, so I haven't stress tested the wing yet :-)

> >I noticed that the cutaway diagram on the Spit instruction sheet also
> >showed the wing attached with rubber bands. Thankfully the kit provided
> >dowel and nylon bolt attachment.
> 
> Hmm, maybe I got an old kit and they added that in later kits ...
> 
> >I mounted my engine upside down, and it's totally hidden within the nose
> >outline (except for the silencer). The plans called for mounting the
> >engine on hardwood bearers - I added extra ply laminations to the
> >firewall and used a commercial radial mount.
> 
> I thought about that also, and added two ply laminations on the fire
> wall, which a bit weak anyway. But when I thought about installing a
> radial mount I realized that the nose would be quite weak without the
> maple spars, and so I decided to install the engine like the plans
> suggested. Did you strengthen the nose in your setup ?

The Spit firewall was 1/8" lightply - totally unsuitable for radial mounting.
I added 4 laminations of 1/16" ply (that's the only thickness I had!) to the
front of this, and reinforced behind it with balsa triangle epoxied in place.
There is a ply tank tray that sits behind the firewall which adds some support,
so I didn't add any more strengthening. Oh yeah, I glued the cutout bits of ply
back into the slots in the doublers that the bearers were supposed to sit.

> >Over all, the kit's seem to be what you pay for - quite cheap. However
> >if you don't mind a bit of work in the building stage they produce good
> >models - the Spitfire flies very nicely indeed, even slows right down
> >for 3-point landings without tip-stalling.
> 
> :-) I hope so. Would be real bad if all that work would be wasted on
> the first flight...

Yeah, took me a while to relax with this model since it had a lot more work invested
in it than my previous models. The worst part of the flight is the takeoff, since we
fly from grass and the retracts meant that the gear had to further back than specified...
Also I had a brand new engine and it liked to cut just after liftoff...

I should also say that the model does drop a tip fairly abruptly when it stalls,
however it's going **real** slow by that stage and unless you want to try tail-wheel
first landings it's not a problem.

> Bye,
>   Torsten.

I just heard from our local Hobby Store guy that Dynaflight has taken over
Bob Martin models (their factory and everything apparently) so hopefully the
quality of the Dynaflight kits will improve a bit.

Chris Kaiser

Postgrad, Elec. Eng. Dept.
University of Canterbury
Christchurch, NEW ZEALAND

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