Fokker Stabilizer Rib Construction

[AchimEngels]

Hello Folks,

The Fokker type stabilizer rib construction appears to be a very interesting thing.

As you know Fokker introduced a standard pattern of stabilizer Design with the triplane prototype, the V.4. This pattern was continued through out the war.

The way the ribs have been made puzzled me for a long time and I would very much apreciate to hear any comment on it by anybody who might have dealt about the subject on his own.

The ribs have been made of steel tubes wich are attached to a sleeve that is pulled onto the basic frame work of the stabilizer. The reason for this whithout any doubt was to minimize distortion and shrinking during welding by avoiding to add heat to the basic frame. This methode of course also allows an easy adjustment of the structure after welding is completed since the ribs now can slide over the basic frame (to some degree).

My point now is the way the ribs have been shaped with late production aircraft and if there was a change in the way they have been done. There is no question that it was a symetrical profile, but the question is whether those ribs formed a uniform curved surface or if they have been sligtly bent over the sleeves to leave straight sides in front of the basic structure and at its rear.

I know about the image showing Stapenhorst´s 144/17 in the enemy view rooms at Islington. I also agree about the fact they are defenitely curved in this image, but this triplane is a early production piece.

I know about the Images of the National Aviation Museums original Fokker D.VII,which shows straight ribs and Fokker Factory image 1583 showing the V.37 fuselage with stabilizer attached and streight ribs as well.

My guess is that they first did the curved ones and later shifted to the much more easy to do streight ones.

As a matter of fact the question came up again when we started to construct our D.VII stabilizer which you can see images of in the "Replicas" section. We decided to go for the streight ones. The ribs simply fall that way when building it. It would require exact pre forming when doing it curved. This in turn would complicate the whole construction and assembly process. Since almost no aerodynamical advantage is achieved by using curved ribs instead of straight ones, I do personally come to the conclusion that they changed it to streight ribs during construction of the later aircraft.

I would appreciate any comment or photograph or drawing that would yield more light on this subject.

Best

Achim

Achim,

I have also wondered about the stab ribs for some time. I have a theory. First after seeing pictures of inside the factory they had more than one team of people building the different components, fuselage, stab, rudder, wings. This would mean that there would be more than one fixture for each part and a different person fitting and welding (three people to a team) the tubes on different stabilizers. I have seen at least four or five different rudder outlines and they are all on different production numbered aircraft. You probably could shed some light on this too; didn’t they build the rudder and stab in a batch and just assign them numbers after they were put on the aircraft? So the workers would just get a stabilizer from a stack instead of them being build specially for each aircraft like the wings were. This would allow one type on low number aircraft the same one appearing on later ones too.
So what if team 1 built the stabs with curved profiles and team 2 took the short cut. Really it’s only a difference of welding the leading edge first or the trailing edge and welding the rib tubes parallel on the sleeve or at some angle so as when the rib tubes are bent together front and rear the form an arc or our straight. As you know when you weld the tubes to that sleeve the welding distorted the ribs into an arc anyway. I believe each rib unit would have been welded separately in it’s own jig away from the main spar or the advantage of deforming the spar tube would have been lost.
This is my theory and I welcome any ideas you have on this. When you have some time let’s talk about the rudder main spar I could spend days on this alone!


Mark

Achim,

these are from the Canadian Aviation Musuem, Ottawa

and this one,

and finaly this one, hope they help

cheers,
jbs

[Dan_San_Abbott]

Achim:
* I had read an interview of Reinhold Platz, where he stated that tubes were straightened between welded joints to relieve stress in the joint. for what it is worth.
* * * * * * * * * * *
Indycar:
* All welded fuselages, struts, ailerons, fins, rudders, stabilizers and elevators were assembled and welded on jigs and fixtures, and all were identical and interchangeable. *Think of the cabane struts or the myriad of struts that made up the engine mount assembly, nothing would fit together, if this was not so. *
* Not only were the parts interchangeable within Fokker Flugzeugwerke, they were interchangeable with those made by the Albatros and OAW.
* The proof of the pudding the the Fokker D.VII (Alb) in Paris, it is a combination of wings tails, struts, undercarriage and cowlings made by all three manufacturers.
* * * * * * * * * * * * Blue skies,
* * * * * * * * * * * * * * *Dan-San
*

I agree they were all built on a fixtures and jigs and that all will inter change at the mounting points to some degree. How the tubes get there has much room for variance. Not all fixtures would have or were identical and I'm sure they were not as I said I have mapped more than one DR 1 rudder outline. If all were identical there would be one only. This aircraft were built very speedily and with not much more than hand tools so some slight differences between parts would be expected. I think it would be fair to say that one stabilizer could have the rib tubes welded tapered together at the rear and the next one to the front as this would have little effect to the final product.

Mark

[AchimEngels]

Hey Mark!

Nice to hear from you! It has been a ong time.

I absolutely have to agree with you. There is no way I can see reason for all this discrepancies.

Of course, he is right when he sais that things are interchangable and I am sure that curved rib stabilizer would fit to the same aircraft that had a straight one attached previously.

I agree to the idea of pre manufacturing parts on jigs. I have done this with mine as you have doubtless seen on the "Replica" section!

Getting rounded or curved ribs would require much more than just welding them streight onto the sleeves. Even if you do this you will note that they just bent at the sleve and do not generate a curve at all and put together at the front or the rear.

While building our stabilizer we noted that it is best to pre shape the ribs and to weld them together whithout any structural force while welding. The way you suggest the curve was generated by welding the tubes straight to the sleeve and putting them together under force at the tip and the rear would not work that way. It would require a very carefull bending of the curved shape before welding.

I have a theory close to yours. I think there was a shop at fokker that build all the jigs and handed tehem out to the working stations. In general they have all been identical but left enough space for differenced of the final product when used by the workers. Most important for sure have been the interchangability of all joint points.

These jigs have been used th the workers in different ways. As you can see in the image below, I can use this jig to do a straight or a curved rib. It will fit to any basic frame no matter which way I do it.



The point just is this that the curved ribs would look smoother and require a lot more time to get pre shaped, while the straight ones are done in a minute!!!! This is difference. I think workers did it the curved way because the foreman told so first until an engineer - or even Fokker himself - saw it and said "Hey boys just do it that way, it`ll do!" I say so because I have not seen a late production aircraft having these curved ribs. I would change my mind if I would see one in an image.

The rudder topic is another thing I really would love to discuss with you at another point!

Glad to hear from you again!

Best

Achim

Hello all,

I'm going to jump in with a bit of experience we found while building two Ryan M series tail surfaces, which have very similar constrution methods to that of the Fokkers. After much frustration in trying to pre- form the tube ribs pryor to welding to spars, etc., we found that welding them "straight" was much easier. Uprights and diagonals are brazed in. When all was installed, the symetrical airfoil was/is simply "induced" with a body hammer!
Interesting note about the sleeves. Ryan typically used sleeved joints in its welded construction (1020 steel) of fuselages but not in the empanage. We can only figure they were used to reduce distortion, but why not in the tail where its really needed with the lighter structural pieces?

[AchimEngels]

Visaliaaviation,

Your participation is welcome!

I absolutely agree. They might have made the same experience at Fokker as we did *

Sometimes it is hard to tell what the original intention of a designer 80 years ago was. I also think that the Fokker sleeves are there just to reduce distortion and avoid shrinkage of the steel during welding.

Achim