Combat Trim?

[SectorNine50]

Just to take this one step further.  Given that trim tabs can add to AOA, if the elevators froze in a neutral position and could not move for some mechanical breakdown reason, then dialing elevator trim tabs down (deflecting them up) would be a way to get your nose up, although it would only do so slowly due to the small size of the tabs.

Correct?

Realistically, given their size, they probably wouldn't do anything remotely noticeable.  They have no chance of overpowering the elevators themselves, so it would be safe to say that they would be ineffective in controlling the aircraft, but I guess in theory, they would generate some sort of lift if deflected...

Your best bet if your elevators are frozen in controlling the aircraft would be through throttle control.

Side note:
This actually happened to an Airbus carrying cargo over Iraq or Afghanistan a year or two ago.  They got tagged by a SAM, and lost authority on all of their control surfaces.  They were trimmed out for a climb at 250 or 300 knots (can't remember) and flew the plane to the nearest airport and crash landed safely using just engine controls.

http://www.spacedaily.com/news/iraq-03a.html

[Traveler]

Just to take this one step further.  Given that trim tabs can add to AOA, if the elevators froze in a neutral position and could not move for some mechanical breakdown reason, then dialing elevator trim tabs down (deflecting them up) would be a way to get your nose up, although it would only do so slowly due to the small size of the tabs.

Correct?

I take it that in your hypothetical the elevator failure took place with the aircraft trim already set for straight and level flight with airspeed at a constant.

In any other flight condition you might only be able to slow a decent by trimming up or increase the decent by trimming down, in the case where the aircraft was trimmed for a decent.  Like wise the same issues for an aircraft trimmed for a climb.

[hitech]

In any other flight condition you might only be able to slow a decent by trimming up or increase the decent by trimming down, in the case where the aircraft was trimmed for a decent.  Like wise the same issues for an aircraft trimmed for a climb.

He is asking if it is opposite of what you state. I.E. trim down to slow the decent.

HiTech

[Traveler]

He is asking if it is opposite of what you state. I.E. trim down to slow the decent.

HiTech

my bad, I assumed he was talking in terms of triming for the positon of the nose of the aircraft, up or down.

[EskimoJoe]

Descent*

Sorry, it was bugging me.

[hitech]

my bad, I assumed he was talking in terms of triming for the positon of the nose of the aircraft, up or down.

He is stating that normal trimming, when  adjusting the trim for up will raise the nose. If the elevator is frozen turning the trim tab for up ,will now lower the nose unlike normal trimming conditions.

HiTech

[Traveler]

He is stating that normal trimming, when  adjusting the trim for up will raise the nose. If the elevator is frozen turning the trim tab for up ,will now lower the nose unlike normal trimming conditions.

HiTech

Yes, I got you, as long as the elevator was failed set for level flight,  yes, it’s the reverse, trim nose down , the trim tab acts like a mini elevator and the nose would rise.

thanks

[Traveler]

Descent*

Sorry, it was bugging me.

yes, thanks

[Buckaroo]

In almost all cases having it on is easier. Even if it is on , you simply press any trim key and it is off.

In dives with planes like the 109,if the plane is trimmed for high speed of the dive, pulling out will be difficult vs trimming for a slower speed and holding the stick forward in the dive, then more elevator is available for the pull out. But this can be accomplish by trimming before the dive or pull out with combat trim on. But it can also be simpler to simply have the trim set at say 300, then not having to move it during a fight.

Buckaroo stating it causes drag in situations is incorrect, it makes no difference if the plane is trimmed, or the controls are moved with the stick / rudder. The drag will be the same as long as the plane is in the same state.

HiTech

Thank you for the clarification.

My bad.

[mtnman]

Just to take this one step further.  Given that trim tabs can add to AOA, if the elevators froze in a neutral position and could not move for some mechanical breakdown reason, then dialing elevator trim tabs down (deflecting them up) would be a way to get your nose up, although it would only do so slowly due to the small size of the tabs.

Correct?

I'd think that it would only be possible for reversed-trim tabs to increase AoA if the mechanical design of the elevator caused it to be impossible for the elevator to be deflected to the point where it becomes less effective.

Pulling up elevator forces the tail down and increases the AoA of the wing.  However, it's not necessarily a situation where "more is more effective".  Beyond a certain "xx" degrees of deflection, the elevator is just going to increase drag without doing any more to lower the tail (or at least the ratio of "drag induced to performance increase" will worsen).

Imagine if you could move the elevator to 90 degrees straight up deflection (in relation to the stabilizer).  It would cause a huge amount of drag, but wouldn't add any more AoA than "xx" degrees had.  What is the "magic" point where more deflection equals more drag but equal or lesser-than maximum effectiveness?

To see what I mean drive your car with the window open and stick your hand out in the slipstream, palm down.  Rotate your hand so that it "flies".  Continue to increase the angle of rotation until you hit the point where your hand is palm-forward.  At some point in there you'll no longer be "gaining lift" and will instead just be "gaining drag".

I would expect that a designer would give you enough deflection in the initial design to reach "best, most-effective" elevator deflection under "normal" control inputs...  Plus some more deflection, "just in case".  If that's the case, then going full-up deflection and then dialing-in "down" elevator trim would just take you further into the "more drag for less performance" realm.

Keep in mind that the drag induced at extreme elevator deflection is going to lower your speed as well, which makes it tougher (or impossible) to keep your nose up...

[FLS]

Mtnman in the bit you quoted he's talking about the elevator not moving, frozen level so it acts like a stabilizer extension, and the trim tabs acting as tiny little elevators. You seem to be responding to his original question.

[Midway]

I'd think that it would only be possible for reversed-trim tabs to increase AoA if the mechanical design of the elevator caused it to be impossible for the elevator to be deflected to the point where it becomes less effective.

Pulling up elevator forces the tail down and increases the AoA of the wing.  However, it's not necessarily a situation where "more is more effective".  Beyond a certain "xx" degrees of deflection, the elevator is just going to increase drag without doing any more to lower the tail (or at least the ratio of "drag induced to performance increase" will worsen).

Imagine if you could move the elevator to 90 degrees straight up deflection (in relation to the stabilizer).  It would cause a huge amount of drag, but wouldn't add any more AoA than "xx" degrees had.  What is the "magic" point where more deflection equals more drag but equal or lesser-than maximum effectiveness?

To see what I mean drive your car with the window open and stick your hand out in the slipstream, palm down.  Rotate your hand so that it "flies".  Continue to increase the angle of rotation until you hit the point where your hand is palm-forward.  At some point in there you'll no longer be "gaining lift" and will instead just be "gaining drag".

I would expect that a designer would give you enough deflection in the initial design to reach "best, most-effective" elevator deflection under "normal" control inputs...  Plus some more deflection, "just in case".  If that's the case, then going full-up deflection and then dialing-in "down" elevator trim would just take you further into the "more drag for less performance" realm.

Keep in mind that the drag induced at extreme elevator deflection is going to lower your speed as well, which makes it tougher (or impossible) to keep your nose up...

  exactly how I understand it now. 

I got it.  I understand trim now.   

 

[Midway]

Mtnman he's talking about the elevator not moving, frozen level so it acts like a stabilizer extension, and the trim tabs acting as tiny little elevators.

That was my a further question to help me understand.  Hitech answered it. 

[FLS]

I understand Midway. I just thought it might be confusing for people who didn't read the whole thread.

[mtnman]

Mtnman in the bit you quoted he's talking about the elevator not moving, frozen level so it acts like a stabilizer extension, and the trim tabs acting as tiny little elevators. You seem to be responding to his original question.

Yup, you're correct.  I was trying to respond to an earlier question but grabbed the wrong quote.

Thanks for clarifying for me!