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Keil Kraft Caprice CoG

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John Roberts 913/07/2019 18:32:20
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Hello,

I have just completed a KK Caprice for my next door neighbour and my final task was to check the CoG. Putting it on the balance showed it to be surprisingly tail heavy and it will require 75-80 grams of noseweight to get it balance as per the plan.

The model only weighs 125g and so another 75g seems like a (relatively) huge amount to have to add.

The model is built exactly as per the instructions, using the supplied materials in the kit and is covered in tissue & dope.

Has anyone else had a similar experience with this model?

paul d13/07/2019 18:36:35
164 forum posts
11 photos

I recently built one for 2 channel rc, my best glide is with the C/G just on the second spar, despite it being burdened with radio it flys a treat, for free flight you will defently need a d/t, it'll thermal off a lit match!

I built mine from the plan rather than a kit so I was able to select light balsa for the tail, think mine is around 225 grams?

20190515_082106.jpg

Edited By paul d on 13/07/2019 18:40:30

Edited By paul d on 13/07/2019 18:43:05

Levanter13/07/2019 18:40:01
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I built one of those in my teens and I doubt I had the faintest idea what a CoG was for. I think it had a lifting tailplane and maybe it tolerated a CoG quite well aft.

I tested it in long grass just in case but remember it flew very well just floating around. I certainly can't remember adding any more weight but it was a long time ago.

Levanter

John Roberts 913/07/2019 18:50:21
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194 forum posts
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The wing chord is 16cm and the recommended balance point is 9cm behind the leading edge. That seemed a long way back but I assumed it was required due to the lifting tailplane.

The plan does make reference to the 2 compartments in the nose being used to accommodate balance weights so perhaps adding 75g is the expected norm. It looks as though I am going to have to do some surgery!

paul d13/07/2019 18:56:52
164 forum posts
11 photos

Try a test glide, it's quite a benign model.

Don Fry13/07/2019 19:04:47
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In long grass. But's that is a plan.

Edited By Don Fry on 13/07/2019 19:09:12

brokenenglish13/07/2019 21:18:25
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542 forum posts
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All free flight gliders fly with weight in the nose!

If there's no engine, electric motor or batteries, then ballast would be needed in any plane, and all gliders fly that way!

A total weight of 200g may be slightly on the heavy side, but it's not ridiculous. It won't prevent the plane from flying OK!

Edited By brokenenglish on 13/07/2019 21:20:16

PatMc13/07/2019 23:19:00
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Posted by John Roberts 9 on 13/07/2019 18:50:21:

The wing chord is 16cm and the recommended balance point is 9cm behind the leading edge. That seemed a long way back but I assumed it was required due to the lifting tailplane.

 

It's actually the other way round. The rearward cg was meant to take advantage of a "lifting" tailplane. In fact it's a concept that doesn't really add up, at least for rc flight.

Edited By PatMc on 13/07/2019 23:20:34

Piers Bowlan14/07/2019 06:21:22
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My dad built the similar but smaller KK Conquest FF glider many moons ago. It had a very large lump of plasticine in it's nose but flew quite well. Putting a battery in the nose for radio control is, at least, useful weight with the added benefit of avoiding a flyaway (purists look away now!).

I don't think too much surgery is required John apart from just drilling a hole in the nose block, add a slug of lead and fill hole!

I am not too sure where the logic of a lifting tailplane came from with these vintage models. Perhaps the thinking was that both mainplane and tailplane contributing to lift was more efficient than the wing providing lift whilst the tailplane producing negative lift (and stability). Perhaps the lifting tailplane might also produce less drag although most vintage models were pretty draggy.

Edited By Piers Bowlan on 14/07/2019 06:33:26

brokenenglish14/07/2019 07:18:09
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542 forum posts
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I flew FF competition for years, but not gliders. Anyway Piers has mentioned part of the reasons.

The wing and t/p both lifting generate more lift than the wing alone. As the objective is maximum duration and stability with constant aerodynamics (i.e. without waggling control surfaces), and not manoeuvrability for example, the rearward CG and positive t/p incidence work well.
This setup also makes stable flight largely independent of airspeed, i.e. increasing the airspeed (on a power model) doesn't put the model into a loop!

John Roberts 914/07/2019 08:36:52
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194 forum posts
169 photos

Thanks everyone for the interesting replies. Although I have flown R/C powered models for quite a few years this is my first real foray into free flight.

Although I expected to have to add a bit of lead to the nose (perhaps 5g), what surprises me is just how much is going to be needed. 75g will increase the weight of the Caprice by 60%!

My plan now is to open up the 2 forwardmost compartments in the nose and add the necessary weight. These compartments are pretty slim so I was thinking of pouring in a 'slurry' mix of lead shot and epoxy...….or is there a better way to do it?

Thanks once again.

 

 

Edited By John Roberts 9 on 14/07/2019 08:46:12

brokenenglish14/07/2019 08:43:39
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542 forum posts
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Sounds OK to me...
In really restricted spaces, it's not rare to actually pour molten lead into the nose cavity, but I think I prefer your suggestion!

Simon Chaddock14/07/2019 09:35:14
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A "slurry" of lead short and epoxy is significantly less dense (60%?) than solid lead.

Much better to fabricate a block of lead that exactly fits the nose compartment. Lead is soft and cut be cut and filed fairly easily Hopefully you will get fairly close to the 75 g required in the first compartment so only a small bit will be needed in the second.

The very fact the plan says add weight as necessary in the first two compartments to achieve the balance point shown suggests it was always going to be required?wink 2

Edited By Simon Chaddock on 14/07/2019 09:35:34

Barry Ainsworth14/07/2019 20:49:34
2 forum posts

Hi, I used "coffin" fishing ledger weights, they are flat and fitted nicely in the front compartment, available from all good tackle shops! and denser than a load of lead shot. The CoG on the plan is 89mm behind the front of the former F4, my current model I built about 10 years ago and it weighs 190g all in and floats beautifully across the local playing field. My original Caprice I built in 1970 and we launched it standing on a stile in a field and we watched it disappear down the fields and over the woods, never to be seen again .... have fun

John Roberts 915/07/2019 15:48:35
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194 forum posts
169 photos

Due to the very narrow fuselage I have opted to use a Deluxe Materials product called 'Liquid Gravity'. This makes it easy to drill a small access hole into the top of each compartment and slowly add enough weight until I get perfect balance. Add a few drops of cyano, plug the hole(s) and top it off with a small patch of tissue. Job done!

In truth, simple lead shot would probably work just as well and be cheaper too but Liquid Gravity offers a very convenient solution.

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