Elevator Trim vs Stick Elevator Control

[Captain Orso]

I was playing around with the elevator trim by advancing the trim bit by bit to nose-heavy while trying to maintain level flight with the control stick. I found that at some point I could trim the elevator down so much that the control stick could not maintain level flight--I went into descent with the stick pulled back all the way.

 

The trim tab is only a small section of the elevator, yet it appears to have more influence over pitch than the entire elevator surface. How can this be

[ZaltysZ]

The trim tab is only a small section of the elevator, yet it appears to have more influence over pitch than the entire elevator surface. How can this be

 

Trim tab does not change pitch directly, however it affects the elevator by applying additional force to it. Because stick is connected directly to elevator, additional force is also applied to stick and its hands off position moves off center. At higher speeds this force can become so large, that pilot simply won't have enough strength to move the stick where he needs, if aircraft is trimmed too much in opposite direction.

 

Unless you have FFB stick, there is no intuitive way to feel this force. Simple spring loaded stick will still move full range while tricking you that same happens to elevator. It does not. Check stick in the cockpit or press CTRL+ENTER and check stick position in red square. In high speed flight with nose heavy trim, cockpit stick cannot be pulled much, despite your real stick is pulled fully.

[Dirty Rotten Flieger]

They say that the trim tab has a leverage advantage over the elevator. Pushing the elevator is torque on the hinge but he trim tab uses the elevator as a lever to pivot the hinge.

 

 

I am happy with this explanation but when I first considered the trim tabs I wondered why the trim tab and elevator don't reach an equilibrium where they both exert equal and oposite effects and net result is not affect.

 

It's the leverage advantage of the trim tab.

[Captain Orso]

Thanks for the replies.

 

Trim tab does not change pitch directly, however it affects the elevator by applying additional force to it. Because stick is connected directly to elevator, additional force is also applied to stick and its hands off position moves off center. At higher speeds this force can become so large, that pilot simply won't have enough strength to move the stick where he needs, if aircraft is trimmed too much in opposite direction.

 

That makes some sense, but only to a certain degree. The control stick is directly attached to the ailerons and elevator by cable using the leverage of the stick to pull up or down the elevator control surface. So of course, the more air flowing over the elevator the greater the force necessary to push or pull the elevator into the air-flow.

 

The trim tabs work in the same way, only they are controlled by by the trim dials which I imagine use the leverage ratio of a spool and a resistance mechanism to be able to force the trim tabs into the air flow and hold them in place.

 

But you have to also consider that the air flow over the elevator will hold the elevator in a neutral position, relative to the air flow, and will do so even when the trim tabs are adjusted up or down, because the airflow over the entire elevator is far greater than over just the trim tabs.

 

Unless you have FFB stick, there is no intuitive way to feel this force. Simple spring loaded stick will still move full range while tricking you that same happens to elevator. It does not. Check stick in the cockpit or press CTRL+ENTER and check stick position in red square. In high speed flight with nose heavy trim, cockpit stick cannot be pulled much, despite your real stick is pulled fully.

 

They say that the trim tab has a leverage advantage over the elevator. Pushing the elevator is torque on the hinge but he trim tab uses the elevator as a lever to pivot the hinge.

 

 

I am happy with this explanation but when I first considered the trim tabs I wondered why the trim tab and elevator don't reach an equilibrium where they both exert equal and oposite effects and net result is not affect.

 

It's the leverage advantage of the trim tab.

 

The leverage advantage is not only relative to the amount of force necessary to adjust the trim tabs vs moving the elevator with the stick, but also to the comparison of the sizes of the control surfaces, elevator vs. trim tab. But that only explains why the trim tabs stay in place once adjusted and that the elevator will tend to return to a neutral position once force is removed from the stick--letting go of the stick, for example.

 

Flying at a steady speed of about 200kn to 220kn without the trim tabs adjusted I have a great freedom of motion with the elevator in both directions. So adjusting the elevator trim tabs at the same speed should not change the freedom of motion if the speed remains the same.

 

BTW and anybody can try this out, I did as I described in my first post--gradually adjusted the elevator trim tab to 10% NH (Nose Heavy) while compensating with the stick to remain in level flight. Once I reach nearly the maximum trim adjustment I used <F2> to look at the tail section. The trim tabs were up about as high as they will go and the elevator was in the center neutral position although the stick was pulled back as far as possible and the controls indicator (<RCtrl><Enter>) shows the diamond on the cross. I do have a curve on my elevator settings, but saturation is not adjusted, so with the stick pulled back all the way I should have full extension of the elevator.

 

At 5000' with room to maneuver while watching the elevator from outside I return the stick to its neutral position and the elevator is pushed down to about the full extent.

 

This tells me that the elevator trim tab adjustment doesn't actually trim the airframe as simply shift where the elevator's neutral position is relative to the stick's neutral position.

 

Additionally, from my understanding, the trim tabs should not have anywhere near as much influence on flight direction as the main control surfaces, which is demonstrably not the case.

[Crumpp]

I was playing around with the elevator trim by advancing the trim bit by bit to nose-heavy while trying to maintain level flight with the control stick. I found that at some point I could trim the elevator down so much that the control stick could not maintain level flight--I went into descent with the stick pulled back all the way.

 

The trim tab is only a small section of the elevator, yet it appears to have more influence over pitch than the entire elevator surface. How can this be

 

Some airplanes are like that. A Citabra for example the trim is so powerful, pilots have killed themselves doing exactly what you did in the DCS P-51.

 

You can also land many aircraft just with the trim.

[Captain Orso]

I'm ... skeptical. I don't doubt that some pilots have trimmed their aircraft to beyond their control and crashed because of it. But if the trim tabs, as demonstrated, have a greater influence over the aircraft it would basically make the regular control surfaces obsolete.

 

In a steep dive once the aircraft has reach sufficient speed to make the airflow over the elevator so strong that the pilot cannot pull the stick at all, the only solution is to use the trim tab to slowly pull the aircraft out of the dive. Were the influence of the trim tabs the same as the elevator itself then, why have an elevator at all? A better solution would in that case be to invent a mechanism to control the trim tabs through the stick and be able to maintain full control over the aircraft in all situations with one less system to malfunction.

 

Also, the simple comparison between the surface areas of the trim tabs and the elevator tell me that both cannot have the same influence over the aircraft.

[ZaltysZ]

I'm ... skeptical. I don't doubt that some pilots have trimmed their aircraft to beyond their control and crashed because of it. But if the trim tabs, as demonstrated, have a greater influence over the aircraft it would basically make the regular control surfaces obsolete.

 

In a steep dive once the aircraft has reach sufficient speed to make the airflow over the elevator so strong that the pilot cannot pull the stick at all, the only solution is to use the trim tab to slowly pull the aircraft out of the dive. Were the influence of the trim tabs the same as the elevator itself then, why have an elevator at all? A better solution would in that case be to invent a mechanism to control the trim tabs through the stick and be able to maintain full control over the aircraft in all situations with one less system to malfunction.

 

Also, the simple comparison between the surface areas of the trim tabs and the elevator tell me that both cannot have the same influence over the aircraft.

 

No. You sound like you think that trim tab is additional mini elevator. It is not. At least not in context of whole aircraft.

 

If you want to lower the nose through stick, you move your elevator down, however if you want to do the same through trim tab, you move the tab up. Why? Because it will move the elevator down. It is elevator which changes the attitude of nose, and trim tab just helps in changing the angle of elevator exploiting the leverage. Delete the elevator, and aircraft with trim tab only will be hardly controllable.

[Captain Orso]

Thank you ZaltysZ, you've set me off to some more thinking , and some more research .

 

I found this--but I never would have, if you hadn't spurned me on :thumbup:--: JOURNEY IN AERONAUTICAL RESEARCH: A Career at NASA Langley Research Center - Monographs in Aerospace History, Number 12 - CHAPTER 6: Problems Encountered as a Result of Wartime Developments

 

The quintessence is that the trim tab assists the pilot in moving the elevator and--as you stated--controls the trim of the aircraft by actually positioning the control surface.

 

So adjusting the pitch trim to NH moves the trim tab up, which in turn pushes the elevator down--to some degree--which in turn forces the nose down. Of course in a simulation without FFB there is no way for the stick to experience the motion of the elevator's position caused by the disposition of the trim tab. So ED apparently simply shifts the elevator position relative to the stick position, which is actually what I already stated; I just didn't believe it was justified :huh:.... :doh:.

 

I'm still a bit skeptical as to whether the amount of influence the tab has on the elevator is justified, as its design is to actually assist the pilot in moving the elevator, and only to move the elevator independently for trimming purposes, but I think I'll leave it at that.

 

Thanks to all for accompanying me on this journey of discovery :thumbup:

[sydost]

A better solution would in that case be to invent a mechanism to control the trim tabs through the stick and be able to maintain full control over the aircraft in all situations with one less system to malfunction.

 

You are not far off since this kind of system is actually used in the DC-9 series and on some other aircrafts of the same period.

 

http://www.airlinepilotchatter.com/2013/02/why-are-md80-elevators-split.html

[Captain Orso]

Well, I hope that sirs McDonald and Douglas don't find out that I'm stealing their idea :D.