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Old 05-17-2011, 07:10 PM   #1
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Question Trim Tabs

Can anyone answer a question for me? Why do we not use trim tabs on RC aircraft, (no pilot on board to fight the control surfaces, right, just servos, if forces are to much, get a better servo)? I know theres a trade off with drag,.. just thinking out loud and I began to wonder is all!



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Old 05-17-2011, 07:34 PM   #2
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Well, for one thing, trim tabs on full scale aircraft are to relieve pilot workload. You wouldn't want to be pushing or pulling the joystick or yoke, or pushing one rudder pedal the whole flight to keep it flying straight.

It's also too complicated for model planes. A servo moves to a position and holds it. It won't get tired. Even if you had a trim tab (Which I guess would need another servo, double the weight, double the space, double the power consumption.) the servo moving the main flight control moves to that position and holds it anyways. Way too much complexity.

I guess on some models, you could use a trimming stabiliser for more aerodynamic efficiency, but what would the feedback mechanism be like to know, while your model is flying, that the elevator is deflected up or down and the stab needs trimming? I guess you could just rig the elevator straight, then mod your radio so the elevator trim switch just moves the stab up and down.
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Old 05-17-2011, 07:53 PM   #3
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Default Yepper as I thought

Originally Posted by xmech2k View Post
Well, for one thing, trim tabs on full scale aircraft are to relieve pilot workload. You wouldn't want to be pushing or pulling the joystick or yoke, or pushing one rudder pedal the whole flight to keep it flying straight.

It's also too complicated for model planes. A servo moves to a position and holds it. It won't get tired. Even if you had a trim tab (Which I guess would need another servo, double the weight, double the space, double the power consumption.) the servo moving the main flight control moves to that position and holds it anyways. Way too much complexity.

I guess on some models, you could use a trimming stabiliser for more aerodynamic efficiency, but what would the feedback mechanism be like to know, while your model is flying, that the elevator is deflected up or down and the stab needs trimming? I guess you could just rig the elevator straight, then mod your radio so the elevator trim switch just moves the stab up and down.
Thats is what I was thinking my uncle says I might need them and I said it was not the way to go, not needed no pilot, servos dont get tired,...

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Old 05-17-2011, 09:40 PM   #4
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Something else that may be interesting for the true geeks out there. Some planes, like the 727, use balance tabs which reduce aerodynamic load on the surface. However, if hydraulic power is lost, the balance tabs will now work as boost tabs, since it would be very difficult for the pilots to manually move the control surface, they just move the tab, which aerodynamically moves the flight control surface.
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Old 05-18-2011, 12:26 AM   #5
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Default ???

Originally Posted by xmech2k View Post
Something else that may be interesting for the true geeks out there. Some planes, like the 727, use balance tabs which reduce aerodynamic load on the surface. However, if hydraulic power is lost, the balance tabs will now work as boost tabs, since it would be very difficult for the pilots to manually move the control surface, they just move the tab, which aerodynamically moves the flight control surface.
That's the answers I need to see, some planes, like the 727, use balance tabs which reduce aerodynamic load on the surface Hey I just saying,....?

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Old 05-20-2011, 12:44 AM   #6
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Just for an experiment, I put these servo tabs on my old beater Corby Starlett. I had to be careful how much throw they have, as if you overdrive it, it will force the surface hard to the end. I went one hole at a time, test flying it at each one till finally the roll rate went way up like I put the next size bigger servo in it. After researching them, I found the tab had to be no bigger than 10% of the area of the control surface. Also said it took about 75% of the load off the servo.


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Old 05-20-2011, 01:31 AM   #7
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That's cool. I could see how balance tabs could be helpfull on some models.
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Old 05-20-2011, 03:15 PM   #8
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Flypaper2, that is not a trim tab (the topic of this particular bit) but is a boost tab, two different items. Your comments are very useful however. I have done similar test with similar results and am a big advocate of boost tabs to use with weaker servos. As for trim tabs, not at all practicable for models.
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Old 05-20-2011, 03:18 PM   #9
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Originally Posted by xmech2k View Post
That's cool. I could see how balance tabs could be helpfull on some models.
Those are not balance tabs. A balance tab is where you add weight in front of a hinge line to equal or compensate for the weight of the surface aft of the hinge line. Again useful items in many cases but definitely not trim tabs.
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Old 05-20-2011, 03:58 PM   #10
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Of course we're lucky enough to have trimmer switches on our transmitters. My old reed trans. never had any. Had to mechanically trim them. Guess I'm dating myself.
By the way, the old Corsair had a trim tab on one elev. and a servo tab on the other.
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Old 05-23-2011, 10:43 AM   #11
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There are also different types of tab.
A trim tab is usually directly controlled by the pilot to set the neutral load joystick position. The Auster had a bit of aluminium sheet fixed to the rudder which you bent after test flights until you were happy it was flying straight!
A servo tab, as done by Flypaper 2, reduces the load to move the control surface. Essential on larger aircraft before power assisted controls. Can also act as the trim tab.
Anti servo tab. Increases the control load. May be used on an 'all flying' surface where there is little or no natural aerodynamic load. Can also act as the trim tab.
A good example was used on the Slingsby Dart glider all flying tail plane.

So in theory at least you could use a micro servo on even a large model, after all only one man arm power is available to move the controls on a 60,000lb Lancaster!
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Old 08-01-2011, 02:08 PM   #12
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Originally Posted by Rodneh View Post
Flypaper2, that is not a trim tab (the topic of this particular bit) but is a boost tab, two different items. Your comments are very useful however. I have done similar test with similar results and am a big advocate of boost tabs to use with weaker servos. As for trim tabs, not at all practicable for models.
I agree with you, larger scale aircraft maybe? Just asking because my uncle insists it would solve some issues which I say are non-issues

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Old 08-01-2011, 07:14 PM   #13
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Trim tabs dont work on RC models because for them to work the surface needs to be free to float against aerodynamic forces, as is the case in 'real planes'.. It's not the case in RC models where the surface is held rigidly by the servo.. A trim tab on an RC model would actually work opposite (but to a lesser extent) to it's effect on a full size plane.

And PS..Rodneh; balance tabs are not the weights on control surfaces that protrude forward.. they are called 'mass balances'... a totally different concept.
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Old 08-01-2011, 10:12 PM   #14
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Originally Posted by JetPlaneFlyer View Post
Trim tabs dont work on RC models because for them to work the surface needs to be free to float against aerodynamic forces, as is the case in 'real planes'.. It's not the case in RC models where the surface is held rigidly by the servo.. A trim tab on an RC model would actually work opposite (but to a lesser extent) to it's effect on a full size plane.

And PS..Rodneh; balance tabs are not the weights on control surfaces that protrude forward.. they are called 'mass balances'... a totally different concept.
It is a matter of symantics, balance tabs can take many forms, often on models just weights that are forward of the hinge line to counteract or balance the weight of the surfaces or can be aerodynamically shaped structures that provide both weight forward of the hinge line and aerodynamic functions such as boost tabs accomplish.
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Old 08-01-2011, 10:50 PM   #15
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Originally Posted by Rodneh View Post
It is a matter of symantics, balance tabs can take many forms, often on models just weights that are forward of the hinge line to counteract or balance the weight of the surfaces or can be aerodynamically shaped structures that provide both weight forward of the hinge line and aerodynamic functions such as boost tabs accomplish.
I think you will find a balance tab is a specific thing, it's not a 'generic' name for any type of control surface balancing device. Specifically a 'balance tab' is a hinged tab on the TE of a control surface which is set up to move in the opposite direction to the main control surface movement. They are used to reduce stick forces, as illustrated here on this diagram from the NASA web site: http://history.nasa.gov/SP-367/f154.htm. A trim tab is similar in appearance but different in operation and purpose.

Aerodynamic balances and mass balances are different things altogether. Two types of aerodynamic balance (the horn type and the inset hinge type) along with a mass balance are illustrated here, also from NASA site: http://history.nasa.gov/SP-367/f153.htm

The main NASA page the diagrams come from: http://history.nasa.gov/SP-367/chapt9.htm (refer to sections 173-175)

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Old 08-01-2011, 11:33 PM   #16
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I had those aerodynamic balances on the elevators of a 1/4 scale Cap 231EX. I unhooked the servo and added enough finishing nails to the forward end of it to balance the weight of the elev. itself. Also helps stop elev. flutter. I don't know why these are not used more on the bigger models. I've seen on some of the bigger 3D planes, 3 servos to drive the ails. when a servo tab would do it on one servo. Look at the weight and wiring it would save.

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Old 08-14-2011, 10:10 PM   #17
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Mass balance (as much weight forward of the hinge line as aft) can be very useful on larger models as it take a big shock load off the servos if it get a high g load like on a poor bouncy landing as it prevents the servo gears from being stripped.
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Old 09-05-2011, 10:09 PM   #18
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Cool question

Well,

I have some answers, just forgot the question,,,......aaaaaaahhh

Thanks Everyone

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Old 06-26-2016, 05:03 PM   #19
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Default Booster tabs.

4 yrs later I'm still using these booster tabs on many planes including the 48 in. Sbach and the Visionair. Big boost in performance on these 3D planes with lots of ail throw with the little HK , Hextronic 9 gram servos.


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Old 06-26-2016, 06:37 PM   #20
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Trim tabs CAN work on models to reduce servo load in level flight.

You'd have to put a logging current meter on the servo and experiment to find the ideal trim tab position and it might apply to only one airspeed, since trim can change with speed.

There have been occasional RC model experiments with having the servos operate boost tabs and the control surface free to flop around. Most of those worked.

Every control system that works full scale can be made to work on the models.

One guy built a model with hydraulically operated controls. The servos moved the plungers of syringes. That one had issues due to leaking the water (his hydraulic fluid) out and the controls not holding any trim setting.
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Old 06-26-2016, 07:33 PM   #21
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Default Booster tabs.

To find the magic spot, I move in one hole at a time with the pushrod to the horn on the tab. When you hit the spot the ail response goes up drastically. The airspeed shouldn't make much difference as the airspeed/ thrust over the tab is also increased. These are not fast planes, 3D, but very large throws. But even at speed is where they really show up, as without them, the servo thrust effect is diminished.
If anyone wants to try it, I did a lot of research on them before I started and the area of the tab was 10 to 15 % of the control surface area.
I also use them on the rudder and elev on some planes.

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