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Old Jun 03, 2014, 04:31 PM
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Build Log
Foka4 - 2.2m (DIY prototypes for the slope)

Hi ensemble,

the first prototype of my Foka4 went in flight.

Foka4 P1 (3 min 57 sec)


It is 2.2m wingspan
Wing charge is 65g/dm (light).
Fuselage is a first, light one in fiberglass/kevlar.
Wings are obeche covered foam.

Wings build log is here and will be updated to the finished wing.
I will continue here then about the complete build and next prototypes.

Best regards,
Ric
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Last edited by chiloschista; Jun 12, 2014 at 05:28 AM.
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Old Jun 07, 2014, 08:58 AM
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Nice plane bro.. enjoyed da choice of music also flying wing vid
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Old Jun 09, 2014, 03:53 PM
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Quote:
Originally Posted by guamflyer View Post
Nice plane bro.. enjoyed da choice of music also flying wing vid
Thanks a lot !
Was on slope recently to do some more test flights, but no chance to fly: my RC tx did not work
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Old Jun 11, 2014, 08:44 PM
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Well done Ric

The Foka looks great in flight and nicely flown too. Looks like you have great lift on the site. It was clearly worth building those new wings.

Allan
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Old Jun 12, 2014, 04:51 AM
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Quote:
Originally Posted by AllanK1 View Post
The Foka looks great in flight and nicely flown too. Looks like you have great lift on the site. It was clearly worth building those new wings.

Allan
Hi Allan,

thanks a lot for your compliments: very kind!
No, conditions were light and I was surprised how well the Foka4 flew, despite the relatively high weight (1.950g), but wing charge is still on light side with 65g/dm.
It is set for low CL, low incidence then also.
This was the maiden flight, so everything was to check: wing incidence, CoG, throws etc.
I was on Swiss french slopes for the weekend again, to do some more testing, but unfortunately my RC tx went broke, display problems, so no chance to fly.
At least my son did
Btw the fuselage is a new one also. I did the mould.
I tried to upload pictures on mould build, but something failed.
I will try again.

Ric

P.s.: ... and thank you very much for your suggestions and tips about elastoflaps =
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Last edited by chiloschista; Jun 12, 2014 at 05:07 AM. Reason: P.s. added
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Old Jun 12, 2014, 05:22 AM
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Hi ensemble,
as this will probably be another testing platform I did the mould to do several fuselages.
The mould is done covering the plug with a layer of R&G mould epoxy resin P mixed with EPH573 hardener.
Two layers of 163g/m fiberglass with epoxy resin L and hardener L to follow.
Two layers of glass fabric M1, to reach about 10mm of thickness, with Ludur XL ceramic powder.
I like this method because it is easy to do, cheap and no toxic (ceramic is mixed with water). It is very stiff also (and heavy ...).
Best regards,
Ric
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Old Jun 15, 2014, 06:04 PM
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A big one?

Way more professional than my lost foam efforts Ric. How about making one twice the size?
I remember a really big one in Australia back about 1980. The guy who built it broke the national RC sailplane duration record with it as I recall. 14 hours slope flying or something similar. The airplane had an early vario in it too. I wonder what happened to that model?
I believe it's builder died around that time too. A heart attack I think.
Anyway, I'm rambling. What I do remember was that it was big and looked fantastic in flight.

Allan
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Old Jun 16, 2014, 07:34 AM
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What !? Your SHK is at least amazing. I am going to study carefully your analysis about it.

A bigger one would be great, but actually this is the size I fly the most. I have a 3.80m Salto which I fly 2-4 times a year
I'm searching for the most performance I can get from this glider size for speed, DS and aerobatics. The stress on a bigger one will require a great increase in technology, materials and especially know how (not to mention costs).

Plans are for a 3.20-3.50m glider next and probably some Lunak, M28 and Macka.
And maybe I finally get finished the 3.88m Habicht I have between my feet from years now (all wood contruction ...).

I just am a little bit worried about the heart attack: is this the expected end of planes developpers?

Btw I unmoulded yesterday the third fuselage, a heavier one.
It is the best one until now, but there still are some problems with gelcoat.
It comes off. Is it a problem between polyester gelcoat not happy to be bonded to epoxy? I used polyester gelcoat because it will not yellowish, how does the epoxy one instead. But seen I paint the fuselage, this will probably not be an issue.

This should be equipped with a laminar airfoil and an increased tip chord.
It will be no more scale, but who cares. I'm only interested in performances.
A scale wing should have a tip chord of 4cm, definitely out of any reasonable dimension. The P1 wing tip chord is 8cm, still short, but it flies not so bad (video).
I am going to increase it to 11cm. Root chord is 19cm.

Best regards,
Ric
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Old Jun 16, 2014, 07:03 PM
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Yes, you are likely to have adhesion problems. Have you considered a polyester laminating reson. It will stick and save you money. It will be down a little on strength but I don't think that will matter for your model. We used pol,yester all the time in the old days. Nice to use, wets out well and cures quickly.

Allan
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Old Jun 18, 2014, 03:51 PM
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Hi,
I spoke today with a technician at composite seller shop.
He told me that the polyester Gelcoat I'm using is not compatible with epoxy.
I seem to recall that it should be, but I have to check it.
I even did seven fuselage and a canopy already and the problem only appeared on the last two.
I think I'm going to switch to epoxy gelcoat, or maybe polyester resin.
Ric
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Old Jun 19, 2014, 03:05 AM
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(Way more professional than my lost foam efforts Ric. How about making one twice the size?
I remember a really big one in Australia back about 1980. The guy who built it broke the national RC sailplane duration record with it as I recall. 14 hours slope flying or something similar. The airplane had an early vario in it too. I wonder what happened to that model?
I believe it's builder died around that time too. A heart attack I think.
Anyway, I'm rambling. What I do remember was that it was big and looked fantastic in flight.)


The chaps name was Brian Laging, the record was 28 hrs and 28 seconds and the model was about 1/3 scale Foka 5 , glass wings and balsa foam wing and tail. Very advanced for 1984. There are still some flying, see the VARMS site. The original models are still owned buy his wife, hanging under a tin roof in his shed.
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Old Jun 19, 2014, 01:24 PM
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Quote:
Originally Posted by Col Collyer View Post
The chaps name was Brian Laging, the record was 28 hrs and 28 seconds and the model was about 1/3 scale Foka 5 , glass wings and balsa foam wing and tail. Very advanced for 1984. There are still some flying, see the VARMS site. The original models are still owned buy his wife, hanging under a tin roof in his shed.
Hey, thanks for that info.
Wow a Foka5!
I found a couple of related links:
- a couple of pictures (please have a look at the cars in the background)
- a mention to the world record (go far bottom of the article)
Ric
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Old Jun 22, 2014, 12:05 PM
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Hi,
yesterday I flew the Foka4 for some more testing.
Maximum speed was of 160km/h. Not so bad considering the weak conditions and that I just pushed it for a short dive and not steep down (see attached picture).

Average speed of the entire 29minutes flight was 68km/h, which is very close the the expected 20m/sec for the general settings.

I still can't sort out the geometry. I finally was able to do a dive test and the plane showed a clear fore CoG position.
That's very strange because the original plan recommends 44-50mm with 1.2-1.5 wing incidence. I have it at 70mm with wing incidence set at 0.7.

Neutral point is not clear, because I have different data from different calculations and programs. It goes from 63mm to 80mm of the root chord.

Flight tests seems to confirm more the last position.
Now I'm wondering if go toward a more aft CoG or increase the elevator incidence, going to a negative wing incidence (-1). Calculations and flight tests seems to confirm that, but I'm a little worried where am I going with such a setting

Ric
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Old Jun 22, 2014, 04:35 PM
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Hi ensemble,

in the search for the optimum settings, I did play a little with XFLR5 to watch what happens to a glider when crow flaps are applied.

All the pictures show a behaviour at Alpha0, except the panel forces one which is taken at Alpha-5. This is the more likely attitude when I land in this flight condition.

To have a concrete example I enclose a video I did with a Proxima II, where the forces applied to a wing are well shown. Wing flexing is shown especially in the second part of the video, where the videocam records wings movements.
Btw Proxima II wings are not stiff as the Foka4 one, but are definitely not weak.

Best regards,
Ric

Stressing the Proxima II (6 min 39 sec)
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