Stick Force and Control Surface question!

[gomwolf]

Hi. First, I am live in aisa, so my english in broken one. Please understand it.


I search some kinds of documents, but I can't understand about relation between Stick Force and Control Surface.

For example,
Bf109K-4's stick force problem is very famous. It is hard to control in higher speed. some interview told can full control in 700kph but isn't easy.
F4U-1 Corsair(no spring tab) has very light stick force. It will be harder to control in higher speed, but easier than Bf109.

F4U corsairs Control surface is very big. It is bigger than Bf109K-4.

I think bigger control surface be more pressure in high speed, it make stick more stiff. But F4U had lower stick force.

I can't understand why it can be done. Please someone advise me!

 

[GregP]

A lot of the control FORCE is due to the leverage the control system has over the connection to the control surface.

If the control rod or cable is connected, say, 250 cm from the control surface hinge line for one aircraft and 350 cm from the hinge line of another aircraft, then if the two control sticks are the same length, the one with the longer arm will be less difficult to move, but the same travel will also give less surface movement. Basically you need a balance between speed of control surface movement and force required to move it fully at high speeds.

Also, some surfaces have no tabs, some have trim tabs, and some have boost tabs. A trim tab will move opposite to the surface and relieve the force on the pilot's arm so the plane will fy stright and level "hands off" the control stick. A boost tab will move up if the surface moves up and down when the surface moves down. It makes movement harder near the end of travel but give better aircraft response if you are strong enough to use it fully.

The Bf 109 has a very narrow cockpit and the pilot cannot apply as much force to the stick sideways (in roll) as in planes with a wider cockpit.

That was correctable by the designer, but was never corrected in the real world. That may be just Willy Messerschmitt being stubborn and it may be the Nazi leadership could not tolerate the production interruption that would be caused by the changes in their new aircraft delivery schedule.

In the case of the USA, we COULD have made the F6F ellcat roll a LOT better, but the production interruption was not worth the improvement ... according to the guys who made the decision. I bet the Navy fliers would have made the change if given the chance! They don't usually consult pilots before making decisions about airplanes ... I find that odd.

Welcome to the forum!

 

[OldSkeptic]

There is another factor for ailerons (not for rudders and elevators); the (complex and nonlinear) interrelationship with the wing itself.

The airflow over the wing tries to hold it stable (if it well designed of course), therefore resisting change.

Take the Spitfire. With the later universal wing and the metal ailerons the roll rate at lower speeds was pretty good.
Note I am not picking on the Spit, every wing then or today will show this effect, just that the Spit's is one of the best known.

As speed builds up the forces caused by the altered airflow from the ailerons causes a corresponding force against the trailing edge of the wing, causing it too distort (twist) in the opposite direction.
This causes an opposite aerodynamic force, reducing the effectiveness of the aileron.

Therefore, to get the same deflection of the air flow (and hence roll of the aeroplane) you have to apply even more force to move the aileron even further, which causes the wing to twist more ... and so on.

Eventually you get to a speed where the forces balance, no matter how much the aileron is moved (and no matter how much force is put into the controls) the corresponding wing twist nullifies it and nothing happens.

In more basic terms, without this effect the effort on the controls goes up as a (basically) liner factor of the square of the speed. The faster you go the more effort you have to put into the control to move the control surface (simple test, put your hand out of a car at 60kph, then 100kph and then, of you can do, it 200kph).

The plane tries to remain stable, therefore all forces are in balance. Changing this balance requires work.
For ailerons twisting of the wings (in very simple terms) means you have to do far more work to get the same effects at high speed than at lower speeds.

Elevators can be affected also, both positively and negatively, depending on the fuselage and wing/'body design.

Taking the Spitfire again. It (in contrast to its ailerons) managed to maintain very high elevator authority even at extreme speeds and this was with, by the standards of the day, a small sized elevator.

This was a deliberate part of the design, where the join between the wing and fuselage was carefully designed to 'funnel' air across the elevators, thus enabling Mitchel and his designers to have a smaller elevator than a more conventional design would need (this reduced overall drag and increased speed).

It also enabled the Spit to maintain, for the same stick force, a higher level of elevator movement, even at very high speeds, than many other aircraft of the time.
Look at the wing/fuselage join on the Spit (top and bottom) vs the Me-109 and you can see the difference.

This meant (and this happened many times in real life) that a Spit could dive to very high speeds an pull out when a 109 (and even a 190) would not do it.
There were many recorded cases of Spits, either chasing or being chased, in a dive to low altitudes, pulling out and watching the 109 or 190 go straight into the ground.

Basically this 'funneling' of air acted like a booster to the elevators, reducing the force needed to move it for a desired deflection of the control surface.

(Note for the technical people here, this was almost certainly the real source of the early Spits sensitive CoG issues, which were largely later cured by an aerodynamic fix to the elevators).

 

[Tante Ju]

A lot of the control FORCE is due to the leverage the control system has over the connection to the control surface.

If the control rod or cable is connected, say, 250 cm from the control surface hinge line for one aircraft and 350 cm from the hinge line of another aircraft, then if the two control sticks are the same length, the one with the longer arm will be less difficult to move, but the same travel will also give less surface movement. Basically you need a balance between speed of control surface movement and force required to move it fully at high speeds.

Very much so. It seems to me that without control force aids (hydralic boosters, boost tabs etc.) the designers had to struggle between control surfaces with less deflection (and less "brisk" effectiveness) or using control surfaces with large deflection angles and good response (especially at low-medium speeds) but greater stick forces at high speeds. The Mustang for example had very small aileron deflection angles (+/- 10 degrees iirc), which resulted in low stickforces accross the range and good roll rates at high speeds (where even these low deflections could result in very high roll rates), but the expense of low speed aileron responsiveness. The aircraft may have had a high maximum roll rate even at low speeds, but from what I have read the subjective roll performance left something to be desired. Possibly the low aileron deflection angles took a bit more time to accelerate to maximum rolling velocity.

Also, some surfaces have no tabs, some have trim tabs, and some have boost tabs. A trim tab will move opposite to the surface and relieve the force on the pilot's arm so the plane will fy stright and level "hands off" the control stick. A boost tab will move up if the surface moves up and down when the surface moves down. It makes movement harder near the end of travel but give better aircraft response if you are strong enough to use it fully.

The Bf 109 has a very narrow cockpit and the pilot cannot apply as much force to the stick sideways (in roll) as in planes with a wider cockpit.

Having sit in a 109 imho the problem is not really the "narrow" canopy but rather that your own legs, being fairly high in front of you because of the g-resisting seating position simply getting in the way of the stick, limiting its deflection. The grip is also in a relatively low position, so you could probably have more leverage with a longer stick. OTOH I am not sure if the latter would be good from the human physical standpoint, I believe you can apply more force with your muscles if your arm is in a lower position (ie. arm applying sideways force at stomach height rather than shoulder height).

That was correctable by the designer, but was never corrected in the real world. That may be just Willy Messerschmitt being stubborn and it may be the Nazi leadership could not tolerate the production interruption that would be caused by the changes in their new aircraft delivery schedule.

In fact they did.

109K supposed to have aileron Flettners to decrease stick forces - see manual. There are very few actual examples seen with it (and there are also very few usable photos to identify them), but the aileron Flettners were fitted to production 109G since 1943, especially those examples produced in Austrian and CZ factories, i.e. for some reason you see those on G-6/G-14, and G-10, but seldom on K-4... possibly because the K-4 was only produced in a single factory in Bavaria.

In addition I believe the 109K control surfaces were either metal or wood covered, with the oft-criticized elevator stick forces lightened compared to earlier versions by limiting maximum deflection angle of the elevator and thus inreasing gearing advantage for the pilot, ie. more managable at high speed.

 

[Gixxerman]

I know they did things with an asymmetric rudder for the 109 - and admittedly I have no flying experience myself, this is all based on what I have read - I just don't understand why a rudder trim tab was never fitted to the 109, it must surely just have added to the workload of the pilot.
AFAIK boosted powered flight controls were just on the horizon as the war ended - and would have been needed had the war lasted jets really got going - I guess that would have made all of this moot?

 

[Tante Ju]

I know they did things with an asymmetric rudder for the 109 - and admittedly I have no flying experience myself, this is all based on what I have read - I just don't understand why a rudder trim tab was never fitted to the 109, it must surely just have added to the workload of the pilot.

RLM specification was that aircraft under 5 tons do not require rudder trim. Its a question of national standard - the Fw 190 did not have a rudder trim either, the Bf 110 OTOH did have one.

 

[spicmart]

On what factors do elevator stick forces depend on other than the "funneling" that OldSpectic mentioned.
Apparently its effect was not too well-known, otherwise there would have been some more designs sporting this design.
Afaik both 109 and 190 had relatively heavy ailerons compared to others.
The La-5 FN that Hans-Werner Lerche tested could go into a dive at the height of 800 m and recover from it whereas this was not possible for the german fighters.
Even other fighters apart from the Spit seem to have an advantage there.
Why is that so?
And the Ta 152 wing also seems to have a funneling?

 

[swampyankee]

The control forces depend on a lot of factors, including the airfoil surface, which will dictate the pressure distribution, the design of the control linkages, and, how well balanced, aerodynamically, the surfaces are. You could start by reading this pdf. Also, US manufacturers had a lot of incentive to get it right: the USAAF and USN placed very high priority on control harmonization (having the stick and rudder pedal forces low and well-matched); it's possible that the Bf109, Zero, and Spitfire would not pass pre-war acceptance criteria because of how stick forces changed with speed and how aileron and elevator forces were related. The USAAF and USN also placed very high priority on fighter aircraft roll rate and roll acceleration: aircraft turn with their ailerons.

I'd say wander over to the NASA Technical Report Server, but I don't know what's still up (a lot of "No Foreign National" reports were posted. Apparently, that included my CR, which is no longer available on-line from NASA). There are a lot of books about the design of aircraft control systems (even with fully reversible controls, hinge moments are of concern: higher moments mean bigger, heavier actuators). Dan Raymer, and Bill Mason at Virginia Tech (see Aircraft Design Class) all have relevant information; a lot of Bill Mason's is on his website. iirc, most of Dan Raymer's is in his books, but he also has some links.

 

[Gixxerman]

RLM specification was that aircraft under 5 tons do not require rudder trim. Its a question of national standard - the Fw 190 did not have a rudder trim either, the Bf 110 OTOH did have one.

Thanks for the explanation...although I wonder if the testing of captured allied aircraft with these devices didn't make the Germans think again?

 

[FLYBOYJ]

RLM specification was that aircraft under 5 tons do not require rudder trim. Its a question of national standard - the Fw 190 did not have a rudder trim either, the Bf 110 OTOH did have one.

A twin would need rudder trim for single engine operations.

 

[GregP]

Multiple factors, to be sure:

1) Control surface moment arm and control stick length- any axis.
2) Gap seals or not - any axis.
3) Fabric / metal / wood covering - pitch and roll, not so much in yaw.
4) Dihedral - roll axis. Not only how much. but how effective the dihedral is in adding roll stability. In the case of the Hellcat, toll stability was very high.
5) Span - longer span means slower roll at medium altitudes but can actually help at high altitude.
6) Amount of differential for ailerons - they should travel up more than down to reduce adverse yaw but many had symmetric deflection.
7) Presence or absence of washout - roll, below, at, or near stall.
8 ) Presence or absence of slats and their position - roll and depends on speed and load factor.
9) Stall margin - can affect roll and pitch. If you are near stall and apply full aileron, one wing may stall while the other one won't. Usually results in a spin if you maintain back pressure.
10) Presence or absence of maneuvering flaps - pitch axis.
11) Wing airfoil - pitch axis. Difference wing airoils have differeny characteristics at different speeds and angles of attack. You want the one with the best coefficient of lift at combat speed. All the designers try to make an optimum choice and, as a result, most were quite close to one another. But you COULD get two planes fighting at a speed and angle of attack where one was markedly better than the other. In which case other factors like acceleration and propeller characteristics could come into play. Some airfoils stall at slightly bigher angles of attack than others.

There other intangibles such as how clean the wing and horizontal tail are, the stall wrnining, the experience of the pilot near the limits, the seating position that can help or hurt g-tolerance, the condition in the cockpit. P-38's in Europe were VERY cold and had a poor cockpit heater. Doesn't sound like much but, after several cold hours, the pilots had lost a lot of physical ability to fight and move quickly.

Finally, the overal airframe strength was a factor. All aircraft moving at, say, 250 mph and pulling 5-g's, turn with the same radius. It is a funtion of speed and g-load. If your plane is slightly stronger and if the airfoil stalls benignly enough, you might be able to pull more g's and close a bit on your opponent, assuming you don't overpull and stall.

Lots of factors, all of which contribute to the flying characteristics of the aircraft.

 

[gomwolf]

Thank you guys, My english is not good, so I have to read this answers very carefully.