109 out turning a spit

[nrshida]

Yes, but you are missing the essential point: Lowering flaps (or actuating any other control surface for that matter) does not in of itself create an aerodynamic force that counteracts it. All you are doing is increasing the curvature of the wing, and from that increased curvature you are increasing the angle of attack of the wing as well since the wing's chord line shifts with the lowering of the trailing edge. In the 109 this induces a nose down movement in the aircraft as the wing, now with its increased curvature, centers itself against the airflow.

(Image removed from quote.)


The controls of many aircraft stiffen up at high speed because of shock waves forming over the control surfaces, usually bounced off of other parts of the aircraft. The P-38 suffered greatly from this due to its twin-boom tail. By design or luck, unlike many other WWII fighters the Spitfire was blessed with very light control forces even at extreme dive speeds. They were in fact considered dangerously light since pilots could easily over stress the aircraft.

But you still have to push the flaps against the initial resistance of the airstream (which wants to continue as it was doing) and / or overcome the inertia as a consequence of changing camber (plus the friction of the system) I think that's in accordance with what Newton said in his famous analysis combat aircraft: that that takes force. I was mainly responding to your comments about it being 'easy'. Either it's easy in terms of leverage which means it takes an awfully long time (confirmed by WMaker's comments), or it's quick and takes more force (often supplied in other aircraft by a power system).

I don't think what you are saying can be accurate otherwise the 109 wouldn't need the reduction box attached to the geared wheel but could have a simple handrake lever instead. Furthermore the N1K uses hydraulic pumps, the 190 reduction boxes driven by electric motors and the Spitfires a compressed air system. None of those would be fitted if they weren't needed. Right?

[nrshida]

The controls of many aircraft stiffen up at high speed because of shock waves forming over the control surfaces, usually bounced off of other parts of the aircraft. The P-38 suffered greatly from this due to its twin-boom tail. By design or luck, unlike many other WWII fighters the Spitfire was blessed with very light control forces even at extreme dive speeds. They were in fact considered dangerously light since pilots could easily over stress the aircraft.

What about the Zero's ailerons which were proportionally large? I thought they got stiff because of that and not shock waves? Also didn't the 109 have a short stick and needed a lot of force at speeds approaching 400 m.p.h? Are we in shock wave territory already at that speed?

[PJ_Godzilla]

I don't think we are, generally, though I forget typical ratios of free stream to local peak velocities for some of these airfoils. Typically, when you look at the wing of something flying nearly Mach 1, you can actually see the pressure discontinuity at something like the peak local velocity point on t he upper wing surface. You get a pressure and density delta there and it causes a visual distortion.

Often, what will cause control in effectiveness is a wake/wash that takes the surface more or less out of the free stream. some of the high t-tail early DC jets had problems with high alpha because the wake of the separated flow off the wing would trap the tail in wash.

As for high loads at high speeds, I think a lot of it depends on the proportion of surface area ahead of and behind the axis of rotation. The former wants to cause divergence, the latter wants to align with free stream. In stability derivative terms, for those who remember, that last was called "weathercock effect".

[BnZs]

The controls of many aircraft stiffen up at high speed because of shock waves forming over the control surfaces, usually bounced off of other parts of the aircraft. The P-38 suffered greatly from this due to its twin-boom tail. By design or luck, unlike many other WWII fighters the Spitfire was blessed with very light control forces even at extreme dive speeds. They were in fact considered dangerously light since pilots could easily over stress the aircraft.

Minor point, the P-38 did *not* suffer from the elevator becoming "stiff". Pilots could still move the P-38's elevator when compressed, it just didn't do much, as it was in turbulent air from above-mentioned shockwaves, and this phenomenon was tied to Mach, not IAS. By comparison, the 109's elevator could be very stiff at high IAS, but whatever deflection the pilot physically could put on the surface still worked at higher Mach than for the 38. A logical ramification of this is that high TAS becomes more problematic for the 38 and less problematic for the 109 as altitude increases.

[GScholz]

The P-38 stick would be like set in concrete at first, then as the speed increased and the shock wave moved forward, away from the control surface, the controls would free up, but would still be useless since the airflow over them was disrupted. Same for the P-47, but its stick would first give the pilot a thorough beating as the buffeting over the control surfaces would fling the stick around violently.

In the 109 it was different. It was a "feature" (™ Microsoft) that became a flaw. The 109 was designed for max level speeds in the 300-350 mph range, and its controls were meticulously balanced and harmonized for that speed range so that no matter the speed the controls would generate the same amount of G-load for a given stick force. However, with the G model it was clear that the 109 had become too fast for its control setup, but nothing was done for various reasons. First the 109 was supposed to be replaced by the 209 and later 309, but these designs failed. Partly because they were overambitious, but also because they would require a complete retooling of the production lines. The German war situation did not allow any such delay in production, so the 109 soldiered on in 1944 with basically the same controls it had in 1939, but with ~100 mph more level speed and much greater dive speeds.

With the G-10 and K-4 they had to do something since the 109 was becoming almost unresponsive at max level speed. They tried to remedy the problem in a way that didn't impact production much; they added Flettner tabs (servo tabs) to the elevators, but in the field they discovered that they produced a force-reversal at certain speeds which was absolutely lethal. (The stick wanted to move to its extremes unless the pilot forced it to stay centered.) In the end the field mechanics just nailed the Flettner tabs rigid to the elevator and the pilots soldiered on with basically 1939 controls in a 450 mph plane.

In the Spitfire it was different. Its designers paid little or no attention to harmonization of the controls. This resulted in a much lamented control setup that had heavy aileron forces but very light elevator forces. It proved dangerously light in some cases. However, later in the war this proved a blessing since the Spitfire was much better able to handle the increases in speeds as engine technology improved without much redesigning. That is not to say the Spitfire didn't have its own vices... It suffered from aileron reversal at high dive speeds, which must have been quite something to experience for the first time... (The ailerons would literally produce the reverse effect of the control input!)

However, the flaps of the 109 did not suffer from excessive speeds. They were still mostly used at the low speeds they were designed for, and if we all can agree on a ~25 second time to deploy full 40 degrees of flaps in that video then 10 degrees should take 6 seconds. I'm sure a pilot could shave a second or two off of that if he put more effort into it. The mechanical advantage of the 109's jack screw system was also enough that some flaps could be deployed at almost any speed. Indeed one Finnish pilot used flaps to pull out of a near fatal 950 km/h (590 mph) dive. Though I would have had my mechanic check the flaps system thoroughly after something like this...

"The story of Valte Estama's 109 G-6 getting shot down by a Yak-6 was also an interesting one. Their flight of nine planes was doing high-altitude CAP at 7,000 meters (23,000'). (snip) So it happened that the devil fired at him. One cannon round hit his engine, spilling out oil that caught fire. Estama noticed that it wasn't fuel that leaked or burned, just oil. He pushed the nose of the plane and throttled up. His feet felt hot, but the fire was extinguished and there was no more smoke. The speedometer went over the top as the speed exceeded 950 km/h. The wings began to shake and Estama feared the fighter would come apart. He pulled the throttle back, but the stick was stiff and couldn't pull the plane out of the dive. Letting the flaps out little by little gradually lifted the nose. The plane leveled at 1,000 meters (3,300'). Clarification of the escape dive: "It didn't stay (vertical) otherwise, it had to be kept with the stabilizer. I trimmed it so the plane was certainly nose down. Once I felt it didn't burn anymore and there was no black smoke in the mirror, then I began to straighten it up, and it wouldn't obey. The stick was so stiff it was useless. So a nudge at a time, (then straightening off with trims). Then the wings came alive with the flutter effect, I was afraid it's coming apart and shut the throttle. Only then I began to level out. To a thousand meters. It was a long time - and the hard pull blacked me out."

Virtualpilots.fi - Edvald Estama, Finnish fighter pilot. Source: Recollections by Eino and Edvald Estama by Finnish Virtual Pilots Association.

[nrshida]

<snip>

if we all can agree on a ~25 second time to deploy full 40 degrees of flaps in that video then 10 degrees should take 6 seconds.

We can't agree on 10° being the maximum you'd want to deploy in combat however.

So what's the conclusion about the force needed to deploy flaps?

[GScholz]

Sure, but that's not what I meant by "combat flaps". I meant the 109 could deploy a similar amount of flaps to those Allied rides that got a "combat flaps" setting. P-51 and P-38 among others. At least in the 51 that's 10 degrees iirc. As for the force needed to deploy the flaps, as you said there's friction and also the initial drag resistance, but it doesn't amount to much given the mechanical advantage of the system. As Wmaker said, Finnish pilot Pekka Kokko noted it took about 25 second to deploy full flaps at approach speeds. That corresponds well to the video I posted.