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Motor Falke ¼ scale - Cliff Charlesworth.

Let us all watch your new project progress.
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Peter Balcombe
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Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by Peter Balcombe »

Max,
I don’t know about the ‘crank’ bit as I don’t know the Falke plan, but as you say for washout then as the ribs twist, if the TE goes up the LE must go down, else the spar also be raised.
A trick I have used in the past is to sit the main spar & TE on a strip of 1/16” or 1/8” balsa. The TE strip section can be raised over the the tip washout length from 0mm at start of washout to Xmm at tip - maintaining a straight TE line over the washout section.
When the ribs are put onto the spar & the TE rests on the TE strip & the small vertical spar offset allows the nose of the rib to go down a little (only a degree or so max.) over the tip washout section.
This does however mean that the LE is not perfectly straight as the ribs rotate about the spar, not the LE.

The alternative is for the spar to have to rise over the washout section in order to maintain full contact with the rib slots. I suppose that it would be possible to calculate the spar vertical offset to achieve the same TE washout offset.

Max, the above only applies if building off the lower spar - not if adding ribs to a pre-built spar box assembly :(

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Max Wright
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Location: Clayton Bay South Australia

Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by Max Wright »

Thanks, Cliff.

I have Cliff's book.

I couldn't figure out how to get the taper on the sides of the box spar, so I'm following the "build it on the bottom of the box spar" method, others have used. It will be easier for me as I have no idea how to accurately cut the ribs into three pieces.

I've built a few laser cut kits, but I've never struck one like this.

Apart from spending extra cash to buy Cliff's book; there are very few instructions on the plans.

Now that you have given me the references I'll have a good read (again), and see if I can make some sense of it.

There's an experienced builder in Iceland who's posting his build on line. He cut the kit himself and cut the ribs into three. He's building the rear stabiliser "in mid air." Can't wait to see what he makes of the wings. :lol:

Cheers
Max

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chris williams
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Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by chris williams »

The cranked wing root is common to most of the Motor Falkes, including the SF28, SF33 and C-Falke. Having built all of 'em, I have always assumed that the cranked TE at the root is to allow the rest of the wing to assume the correct A of A relative to the fuselage. Without the crank, the TE at the root would sit below the level of the bottom of the fuselage. This means, of course, that when you attach the incidence gauge to set up the decalage, you attach it outboard of the cranked section...! ;)

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Max Wright
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Location: Clayton Bay South Australia

Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by Max Wright »

Hi Peter

Thanks for your reply.

Cutting the ribs fills me with trepidation. I need to get a mental picture of the completed wing and then I'm sure that I can build it.

I haven't been able to break through with the Air Scouts, but I'm sure that once I've seen the prototype I'll have a better idea of how it should look.

Cheers
Max

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Max Wright
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Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by Max Wright »

Thanks, Chris.

That makes sense.
Max

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Max Wright
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Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by Max Wright »

OK Lots of reading done.

I think my questions have been too general.

Here is how I see the spar and the crank and washout . . .

spar 2.jpg


I believe that the taper of the sides of the spar run from the top of Rib 1 to the top of Rib 32; and that the bottom of the spar sits flat on the building board.

Then for the washout, I will need to make a wedge which is the length of the distance between Rib 16 and Rib 32; and is 5 mm high one end and zero at Rib 16.

Then for the crank, I will need to make a wedge angled 3° and as long as the distance between Rib 1 and Rib 3.

The riblets forward of the spar all sit flat on the building board.

How many out of 10? :D

Cheers
Last edited by Max Wright on 22 Oct 2020, 09:19, edited 1 time in total.
Max

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RobbieB
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Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by RobbieB »

Max,

Just to be clear about one thing, Cliff's method of pre building the box spar first never intended it to be twisted to accommodate any washout just as in full size. The spar should remain square throughout its length. The washout twist is achieved by varying the cut angle on the front/rear of each rib where it is glued to the spar faces. In order to allow the rotation down of the LE section, the spar will have to be supported high enough off the board to allow this

If you have an aerofoil plotting programme this conveniently does it for you but if not you will have to cut each rib progressively along the tapered wedge to the tip from wherever it starts.

Hope this explains it well enough.

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Peter Balcombe
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Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by Peter Balcombe »

Max, I think you are over-thinking this ;)
Iv'e just had a look at the plan (thanks Cliff) and I can't see any real problem.

The Root crank is dealt with by R1 and R2 having a different (less downturned) TE profile which effectively means that the LE should still be straight when their TE is raised - as shown on the sketch on the plan. Thus set the TE of R1 & R2 at the correct height & the LE should be correct.

The tip washout isn't a problem either due to the fact that the front of the ribs curves up forward of the spar.
The 5mm washout offset is at the TE, not the spar ;)
The attached drawing shows that with the TE raised 5mm, there is negligible difference to the point at which the front of the rib touches the building board. The rib angle changes 1.8 degrees due to maximum washout at Rib27.

The plan also shows a box spar assembly having straight top & bottom edges, so no angles required for the reasons identified above.
Peter
Attachments
Falke Rib 27.jpg

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Max Wright
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Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by Max Wright »

Nope. I am as confused as ever.

I have an appointment next Tuesday to visit the SA Scouts Airfield.

Perhaps I will be able to work it out once I see the prototype.

Thanks for trying.
Max

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Cliff Evans
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Re: Motor Falke ¼ scale - Cliff Charlesworth.

Post by Cliff Evans »

Well, even I understood that Peter!
Laser Cut sailplanes

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