Thread by @ond144

1/20 This quite lengthy thread will be about Jona J-6, completely unknown and extremely interesting, experimental sesquiplane from Italy. So let us dive into the rocking wing concept of Milan engineer Alberto Jona.

2/20 In Flight International Magazine issue of 10th October 1935 there is an article about a “Novel Two-Control Aeroplane” which gives us beautiful illustrations and talks about an aeroplane which can be flown only using two pilot operated control surfaces – elevator and rudder.

3/20 The plane features a rocking wing mounted on a pin in a parasol configuration. The wing is free to rotate around rolling axis, but it is balanced into neutral position by an automatic deflection of ailerons and two damping struts.

4/20 The aileron deflections are mechanically linked to the wing tilt angle. If the wing tilts to the right, the control mechanism depresses right aileron and lifts the left one. Restoring moment brings the wing into neutral position while sprung struts damp the oscillation.

5/20 So why does the “only rudder and elevator controlled” plane require such a strange feature. The whole wing system together with slots and spoilers is really complex. To understand it better we need to go a bit deeper into flight dynamics and history.

6/20 In 1930s there was quite common belief that coordination of elevator, ailerons and rudder is too complex of a task for human, requires too much training and it would be better to eliminate one of the three. Perhaps most famous successful attempt is the famous Pou-de-Ciel.

7/20 Pou-de-Ciel is small, strange aeroplane flown only by use of rudder and elevator. Beauty of flying this monstrosity lies in the fact that if you are flying a right sustained turn, you do so by a constant deflection of the rudder to the right.

8/20 Why does the Pou-de-Ciel have this desirable feature? Because of its positive spiral stability – natural tendency to decrease the bank angle in an uncontrolled turn. With sufficiently positive spiral stability you can just keep pressing into the direction you want to go.

9/20 Why don’t all aircraft have positive spiral stability then? Because it is not the only dynamic stability issue they face. A normal aeroplane (not Pou-de-Ciel) typically either has problems with spirals or with insufficient damping of a side-to-side motion called Dutch roll.

10/20 Good Dutch roll characteristics stem from low lateral stability and weak directional stability, good spiral characteristics generally require the opposite.

11/20 As spirals are usually way slower in development and less dangerous we just opt for mild spiral instability and let pilots deal with it using ailerons.

12/20 But we don’t want to control ailerons now! Is there any other way how to make the spiral stable with sufficiently damped Dutch roll? Alberto Jona saw one.

13/20 I will try to explain how I understand his aeroplane. I don’t have access to any unpublished material though and from here on, this thread is based just on my opinion.

14/20 Dutch roll and spiral are not only determined by static directional (change in yawing moment w.r.t. sideslip) and lateral (change in rolling moment w.r.t. sideslip) stability.

15/20 Other stability derivatives have significant influence, but they are typically harder to modify as you can’t just increase e.g. dihedral or the size of a vertical fin.

16/20 Most of other relevant derivatives are dominated by the wing shape. But if you put your wing on a pin and equip it with a control system which ensures it always produces near zero rolling moment you suddenly uncouple other crucial stability derivatives.

17/20 We are namely talking about changes in rolling moment w.r.t. roll rate and yaw rate. These stability derivatives are no longer strongly influenced by (upper) wing design.

18/20 You still have to design such surfaces which will ensure that you get what you want (as it is done on this cute sesquiplane). But it no longer requires modifying your main lifting surface and there is much more design freedom.

19/20 I believe this to be the main idea of this airplane. The wing dictates the lateral-directional dynamic stability. If we however (partially) disconnect the wing from the system, we can achieve stability results we want without compromising the wing shape and configuration.

20/20 And this is what Alberto Jona did. And according to the article in the link below the rocking wing concept worked quite well and the J-6 had some excellent flying characteristics. leonardopavese.dev/2013/04/the-ai…

21/20 Now why didn’t it spread? Again we can only speculate. A logical explanation is that because it turned out that pilots are good at operating three control surfaces at the same time. The Flight Magazine also suggests it may be an aeroelastic hell which sounds plausible.

22/20 But we should still pay our respect to the Milan engineer who came with a very original flight mechanics solution. I found the plane by looking at the least viewed Wiki articles in the list of experimental aircraft. It deserves more fame as it is a great engineering story.

23/20 Pictures are from Wikimedia, Flight International Magazine 10-10-1935 and rcgroups.