Mitsubishi A6M Reisen balance tabs

[cherry blossom]

Does anyone know the story of the balance tabs on the Mitsubishi A6M Reisen?

The book by Yoshimura has on page 108:

...The findings had shown that flutter in the Zero occurred at almost the same speed whether the ailerons had balance tabs or not. The advantage of the balance was that it prevented the aileron from becoming too heavy at high speed. That made it easy to pull up from a deep dive using ailerons. Now, however, that was seen as a possible disadvantage. The easy use of the ailerons led to a tendency to maneuver the plane to the point at which wrinkling of the wings' surface plates became noticeable or even loose and their rigidity much lowered. For these reasons, pilots became suspicious of Zeros with balance tabs. Later the tabs were abolished.

An article by Joe Baugher has:

Beginning with the 128th Reisen, the aileron tab balance was linked to the landing gear retraction mechanism to improve high-speed control by reducing stick forces.
In order to correct an aileron flutter problem, a modified aileron tab balance was incorporated on the 192nd and subsequent A6M2.

One of my questions, on reflecting on the thread about designing a better aircraft, is whether the strengthened structure of the A6M5 (and later versions such as the A6M8) should have allowed the addition of tabs to increase the Zero's high speed roll rate. The A6M2 had a maximum diving speed of only 360 mph. Thicker skinning allowed the A6M5 to dive at up to 410 mph and further strengthening allowed the A6M5c to dive at 460 mph. Was the Zero's roll rate limited by the force that the pilot could supply or was the problem that the wing distorted to oppose the aileron deflection?

Yoshimura, Akira, translated by Retsu Kaiho and Michael Gregson. Zero Fighter. Westport, Connecticut: Praeger Publishers, 1996. ISBN 0-275-95355-6

Added as edit: The diving speed of the A6M2 should be higher and was probably 340 knots. See post below.

 

[GregP]

Does anyone know the story of the balance tabs on the Mitsubishi A6M Reisen?

The book by Yoshimura has on page 108:

...The findings had shown that flutter in the Zero occurred at almost the same speed whether the ailerons had balance tabs or not. The advantage of the balance was that it prevented the aileron from becoming too heavy at high speed. That made it easy to pull up from a deep dive using ailerons. Now, however, that was seen as a possible disadvantage. The easy use of the ailerons led to a tendency to maneuver the plane to the point at which wrinkling of the wings' surface plates became noticeable or even loose and their rigidity much lowered. For these reasons, pilots became suspicious of Zeros with balance tabs. Later the tabs were abolished.

An article by Joe Baugher has:

Beginning with the 128th Reisen, the aileron tab balance was linked to the landing gear retraction mechanism to improve high-speed control by reducing stick forces.
In order to correct an aileron flutter problem, a modified aileron tab balance was incorporated on the 192nd and subsequent A6M2.

One of my questions, on reflecting on the thread about designing a better aircraft, is whether the strengthened structure of the A6M5 (and later versions such as the A6M8) should have allowed the addition of tabs to increase the Zero's high speed roll rate. The A6M2 had a maximum diving speed of only 360 mph. Thicker skinning allowed the A6M5 to dive at up to 410 mph and further strengthening allowed the A6M5c to dive at 460 mph. Was the Zero's roll rate limited by the force that the pilot could supply or was the problem that the wing distorted to oppose the aileron deflection?

Yoshimura, Akira, translated by Retsu Kaiho and Michael Gregson. Zero Fighter. Westport, Connecticut: Praeger Publishers, 1996. ISBN 0-275-95355-6

I completely fail to see how an aileron balance tab helps you pull out of a dive. The elevator would do that, sure ... but an aileron balance tab? No way. Perhaps he meant to say the balance tabs made high-speed rolling easier since they decreased the aileron stick forces.

I am with you in what you are saying, the A6M5 and onward SHOULD have had aileron balance tabs as well as an elevator balance tab. In point of fact, the Zero had very good pitch response ... up until high speed. An elevator balance tab would have helped high-speed pitch at the expense of allowing potential over-g at lower speeds.

What the Zero most needed was an engine with 500 more hp. Can you say "Kasei?" But they didn't really ever get that going in production, and the Zero was just out-performed in the late war timeframe by newer, more powerful Allied fighters and suffered with less than veteran pilots for the last year and half or so.

 

[taly01]

The Robert Mikesh book "Zero Combat and Development" by MotorBooks International, is the best English language publication on A6M that explains and has history, drawings of aileron balance tabs and aileron landing gear flaps link mechanism.

It is complicated history and i'm not 100% confident on it, but i'm interested so will read it again, in development early A6M2 got strengthened wings to reduce wing flutter and so failure speed raised.

The landing flap link is to prevent the aileron assist tabs during low speed landings, as its not useful then.

Mar 1940 first high speed incident of A6M1 2nd prototype, plane and pilot lost, elevator mass balance links strengthened.
Feb 1941 A6M2 > pn.127 aileron assist "balance" tabs added for high speed response.
Apr 1941 Second incident, A6M2 pn.140 high speed and G tests both ailerons blown off at 320knots, and wing skin wrinkles but landed safely!
Apr 1941 Third high speed incident A6M2 pn.50, plane and pilot lost, blamed on wing flutter.
Apr 1941 A6M2 > pn.127 with aileron assist tabs officially limited to 250kt/5G.
May 1941 new A6M2 > pn.168? get thicker wing skin, more internal wing girders, external aileron mass balance "horn" weights and no aileron assist "balance" tabs! Combat allowed.
Jun 1941 old A6M2 < pn.167? retro-fitted with external aileron mass balance "horn" weights, but still limited to 250kt/5G. Not for combat!
Sep 1941 A6M2 > pn. 327 get internal aileron mass balance weight system. Finished design. Cleared for 360kt.

Aileron assist tabs not trusted to be used again until A6M3 m.22, and new Nakajima produced A6M2 m.21 around Dec 1942 as they were desperate for high speed combat improvements.

 

[cherry blossom]

I have given the wrong never exceed speeds in my first post. There is an excellent thread A6M Reisen Performance which tried to sort out the correct numbers. A possible primary source, JICPOA Item 5981.pdf based on captured Japanese documents, suggests that the true figures may be 340 knots for the A6M2 (391 mph) and 360 knots for the A6M3 (414 mph). There is general agreement that the maximum diving speed had increased by the A6M5a and increased further with the A6M5c, possibly to 740 km/h, which is 400 knots or 460 mph (I think that the IJN used knots while the IJA used Km/h). Obviously, compressibility is being ignored and it would be very unwise to dive an A6M5c at that indicated airspeed at high altitude.

 

[taly01]

Mikesh says A6M2 permitted max speed increased from 340->360kt just before Pearl Harbour operation. Must use Knots for Navy planes

You would think the A6M3 m.32 would have a higher allowed dive speed as it had no folding wing tips and smaller ailerons, but they seem to have kept it at 360kt, officially. Some US pilot accounts in Solomons say Hamp, (square wing Zero m.32) followed them longer in dives than old Zeros did.

An experienced combat pilot who returned to Japan in late 1944 tested the latest A6M5 for the factory at 400kt and he said it was safe, but he would never do it in combat (I assume he meant there was poor control at 400kt). I need to keep more notes!