Ben Buckle Fokker DVIII electric conversion

[robk]

Well its been a while - work and life getting in the way as usual ... plus some flying and the consequent running repairs which always happens with me once I start flying. Anyway I have got going again on the Fokker and been working on the control surfaces. Here is the aileron fixing - took ages due to me making the slits with a knife. On the second one did with my dremel and took a tenth of the time.

Here is the servo mounting. After a lot of worry I decided the easiest way was just to epoxy it in. I might regret this later if the servo goes down - but I figured I will have to take off the covering at that point anyway and it wont be much more trouble to take it out than a screwed-in one.

Here is the wing mounting top and bottom. I don't like the idea of rubber bands ruining the scale look so I am going to fix some washers in a plate to the top of the cabane struts and then have bolts fixing to the wings into captive nuts.

Finally here are the assembled tailplane parts now with hinged control surfaces. On to the fuselage next...

[Stevo]

All looking remarkably familiar!

 

I recommend using nylon bolts, around 6mm... but the imperial version. The fine metric thread maens that some times the nylon bolts get stuck...

 

I thoroughly recommend creating a 'D' box section around the Leading Edge - it really does strengthen it!!!

 

PS - is it me ot do your wing mounted servos not quite line up with the elevators?

 -  one last thing - make sure you have enough elevator movement - it does look quite tight - but if you stick to the recommended settings (with differential) you wont go far wrong!

 

Edited By Stevo on 06/06/2015 19:15:19

[robk]

Hi Stevo - yep I got the wing mounting plan from you. I've put vertical balsa bracing between the spars across the whole wing and it's quite stiff. The other build I've found (Nigel Hawes) didn't make a D section or do the vertical bracing and there is plenty of pictures of it flying... I'm thinking about the D section though. With the servos - once you put the arms on they line up fine with where the horns will be. There is 27mm down and 30mm up movement - sound ok to you?

[robk]

Now onto the fuselage - first laying out the sides. I built the port one over the starboard with a layer of thin acetate between.

Oh and the wheel wing is there too. I built it complete but didn't glue the end rib in place so it can be easily removed to fit the undercarriage legs in when they are complete. Here are some pictures of the fuselage going together.

I made a simple home made brace with elastic bands and some balsa offcuts to hold the sides firmly together while the glue sets. The tail end is brought together in the photo below. I also put in a light ply 'floor' at this end to support the tail skid - although I put it in the top first by mistake!

Finally here are the curved sides in place. Not very happy with the Ben Buckle formers which apart from the ones around the front of the fuselage were the wrong size on the plan - I had to remake them when I had the stringers in place. I am also deciding the motor, battery setup now. Stevo - what was the weight of your machine when complete? I have some left over motor and bits suitable for a 2.5kg all-up model I am thinking of using - will that be enough?

[kc]

I have to say the front fuselage looks different to my part built model..... the side stringers on mine form an almost round shape. I reckon I built mine to the plan! The stringers ( in my opinion ) are notched into the side formers and are therefore on the outside making a rounded shape. Mine look as though they match the cowl. I hate to say mine is right and yours is wrong but.............

[robk]

Thats interesting... Ive just checked to see if I did something wrong but I don't think so. In the meagre instructions it says to cut down the full size former blanks to the sizes on the plan. I did this and then discovered these formers were too small so the stringers were hovering in space unless I pushed them in to make the fuselage sides concave. I checked the pictures of the real aircraft and it shows the sides gently curved but with a distinctive line down the centre so I have increased the dimension of the formers so the stringers meet them to give an appearance like that. I have held some cloth over it and it looks ok but I guess I wont know for sure until its covered...

[Charles Smitheman]

A friend of mine built one of these many years ago in Cape Town, powered by an Irvine 20 Diesel. I flew it for him, and it behaved perfectly. It looked the part in the air too.Sadly it did not last long, due to radio malfunction. I have always wanted to build one myself, and that little engine was an absolute gem.

Charles

[Erfolg]

Looking at what i can see of the plans, there appears to be a ply fairing missing, which was a narrow base isosceles triangle which was centred on the central longeron from the firewall, to approx a line coincident with the cockpit area.

The firewall is certainly a circle, which seems, to fit with Robs model, then the cross section morphs progressively to the rectangular cross section, where the ply fairing/triangle finishes.

That is in my opinion.

[kc]

Erfolg is certainly right about the firewall being circular. This means ( on my model) the rounded front former F4 is 24mm wide at the middle, the next F5 15mm and the third F6  12mm. The stringers are notched into the front former and are on the surface of each subsequent former until the middle stringer meets the upright flush ( rear end of cockpit) The stringers acquired a slight curvature to fit these formers which might be correct or might be wrong!  This gives a smooth transition from circular at firewall to flat behind cockpit. 

there is nothing on my model so far that resembles the iscoceles triangle shaped ply fairing.Erfolg talks about!

Edited By kc on 25/06/2015 19:06:43

[kc]

Of course all this might be an optical illusion and/or my view of the plan might be wrong. But I feel that my model has a rounded shape which the covering will fair in nicely. But it's possible Robk is right and I am wrong! Judge for yourselves when comparing my quick photo with Robk's photo.

I have to admit my model was built years ago and as I lacked a suitable engine that could be hidden in the cowl  it was left uncompleted and set aside.  Seeing the electrifed version in RCME made me want to finish it with electric power.

Edited By kc on 25/06/2015 19:22:42

[robk]

That's a useful picture kc. Actually I don't think we are that far apart - the dimensions you mention are about right. My moan was about the profiles shown on the plan which were too small so I had to remake them bigger so the stringers follow a profile something like yours. My stringers are notched at the firewall and rest on top of the other formers too. The RCME article by Nigel Hawes is useful but didn't eh building it light? I've strengthened mine a bit which I might regret later... I'm going to put my servos in the bay in front of that shown on the plan and use snakes to get past the cockpit. The batteries will be installed vertically, probably from a hatch on top - I don't like turning planes upside down to fit batteries as my wings get damaged.

[Erfolg]

I have taken this from Google. Somewhere I have a Profile mag, which shows the same thing.

One other aspect which many plans seem to ignore is that the D8 wing was completely covered in plywood. In my opinion it is much easier to build a wing which is sheet covered, rather than open, particularly when coming to cover.

[Daithi O Buitigh]

There's a complete collection of the old Profile pamphlets, including the D VIII, over on hippockets download site

[Erfolg]

This link may be of interest, as it shows the building of a reproduction D8, being built, complete with the fairings and giving a feel of the general features.

[kc]

According to a book which seems to give the definitive version of the fullsize original D8 wing failure solution the wing was fabric covered. The solution to the failures was to not to strengthen the wing , but to reduce the thickness and stiffness of the rear spar to prevent twisting.

The book is " Structures or Why Things Don't Fall Down " by Professor J.E. Gordon. Penguin ISBN0140136282 Page 260 to 267.

[Daithi O Buitigh]

Any version I've read pinned the blame on faulty workmanship in fitting the spars incorrectly, and planed down to 'fit'. Additionally, the nails attaching the skin to the ribs weren't lined up and consequently splintered the internal structure.

It was all covered in 'The Fokker Years' and also in the Profile pamphlet

[Erfolg]

Daith

I am pretty sure that the wing was ply wood skinned, rather than an open structure.

I totally agree that the ply wood was covered with fabric.

Again my principle comment was directed towards that I prefer to sheet a wing, onto which i can then cover with what ever I like. My reasons are that you do not need to over dope if using an open weave material. Then there is no sagging between ribs. Also it is generally easier to make a stable and torsionally stiff wing.

However in this case I believe the method also replicates the full size practise.

I have been looking for my pretty poor book "Fokker the Planemaker to the World" if I remember its title properly, it seems to have gone the same way as my rather better Putnam book, which I loaned to a modeller, that is lost. So i am relying on memory and the Internet.

Edited By Erfolg on 26/06/2015 10:07:52

[Erfolg]

Perhaps a picture is helpful

[Stevo]

Yes some good and valid points on here. I also read that the glue was weak, plus some poor workmanship led to wing failures.

I'll nod towards the Blue Max here, which of course used some Dewotine type with a 'wing failure'

A good option for this particular model would be a foam wing, veneered of course.

Once I added the D section at the front of the wing, the torsional stiffness went from 0% to 90% (guess!) and it handles a 52FS well. The strengthening added weight and it does sink when you back off that throttle.

[Erfolg]

I have found these links which will be of interest

Vol 1

Vol 2

This is shows the book that has been compiled, for me difficult to read, although after a lot of eye strain I have concluded that it describes how the spars had been planed down. I am sure that there is much more, if only my eyes were up to it.

Edited By Erfolg on 26/06/2015 12:06:11

[kc]

Prof Gordon was at RAE Farnborough throuout WW2 and later became head of plastics structures at RAE. He worked on lots of problem aircraft during WW2 and his comments on Hurricanes , Mosquitos etc makes interesting reading. Problems with them in tropical conditions causing rot etc. A very clued up person I suggest.

I tend to believe his explanation of the D8 rather than others because he explains it in detail and shows how it twisted due to centre of flexture and centre of pressure. The explanation is about 7 pages long and difficult to summarize here. But basically he says if you strengthen the rear spar more than the main spar instead of just bending the wing twists..Then whatever the pilot does to correct tends to make it worse. Fatalities got much worse after they strengthened the spar. Very applicable to our models too so worth reading about it. I would suggest reading this section of his book if you think other explanations are more likely. It's a basic textbook and therefore should be in most libraries and is still in print. Plenty of info about other aircraft and much else in the book. A very readable textbook.

This is of course a genuine vintage design ( 1942) so foam wings would not please vintage people! But it would be fine by me if they could be made light enough.

I rather think Erfolg is right and they might have been ply covered on the original fullsize.   However the photo of the wing loaded with sandbags conflicts with that.  Read Prof Gordon's comments about that test and how Anthony Fokker did his own tests later with a different spread of the load & proved that the wings were twisting. ( maybe that's what the photo shows?)

 

Edited By kc on 26/06/2015 12:25:03

[Daithi O Buitigh]

IdFlieg and Flugzeugmeisterei both mentioned that the plywood skinning was cut, letting out a stream of foetid water (glue rotting) when they cut it. As there are no original D VIIIs left, we have to go by the original reports from Adlershof on what they did and they do say the wings were plywood skinned. The tests were done on the original E V and the D VIII was the production model (with the defects corrected). The recommendation was either to ad external wire bracing to the E V or to retrofit D VIII wings

[Erfolg]

Although the reproduction aircraft appear to have been built in some ones old shed, reading what I can about the build, it was a faithful reproduction, sourcing original drawings and documentation, as a Museum sponsored project.

I have already forgotten which one, although i am pretty sure that one is based in Germany and another in Australia. The image i included in my previous post, also has some text attached to the image on yet another web page of "Finewoodworking.cm in the USA. below:

"I had to include this project. This is a wing I helped build with Michael DeSanti. The airplane people didn't realize how beautiful a 40 foot varnished baltic birch wing really was. The wing is for a replica Fokker DVIII airplane and I've included some more pictures with it. The plywood is 1/16 aircraft grade. The ribs are plywood and steam bent centimeter square sitka spruce slats and the spars are just plywood boxes with cross pieces every so often. Fokker had 32 men stand on one of these wings to demonstrate it's strength. Design or Plan used: Copies of original Fokker factory drawings"

I am now 99% certain that the wing was skinned.

I would go as far as to say that a foam Veneer wing would be closer to replicating the wing than an open structure built up wing. Although I would enjoy building a built up, sheeted wing far more!

[Stevo]

I agree Erfolg...I loved the challenge of taking that free flight wing and strengthening it!!

Yes the foam version would be easier, cleaner and lighter when converting this to RC. I think if I had to do this again, I would sheet instead of strengthening. Can't think of how I would construct up a foam wing though, but I'm sure someone on here will!

[Erfolg]

KC

I agree that Prof Gordon knew what he was talking about. Yet I also do not dismiss all the other suggestions as to the causes of failure.

I guess Prof Gordons argument runs along the line, that when diving the AoA reduces and the CP moves back, increasing the load on the rear spar. I am also guessing that he was one of the early proponents of aero elasticity.

I cannot dismiss the image that a lot of fetid water pouring out of the wing did not indicate a probable issue. From what i have  gleaned from writings, that most WW1 aircraft were stored in the open. If this is true, i can only imagine that this practise in conjunction with the adhesives of the time and the general porosity of doped wings did not help in maintaining structural integrity. It seems that the QA on Fokkers perhaps would be far lower than would be deemed acceptable today.

With respect to the planing down the spars, and the impact on structural integrity, I can also see this being a potential issue. From what I have read, Fokker was a brilliant business man, also a brilliant showman, even better salesman. But Quality Control was not great, Rheinhold Platz may have or may not have controlled the factory quality effectively, it seems that the sub contracted control was appalling.

I would not be surprised that all the issues played there part, when combined. At the modest speed that the the D8 flew, the design failings were possibly less significant than all the other issues.

Edited By Erfolg on 26/06/2015 17:33:34

Edited By Erfolg on 26/06/2015 17:35:10