What are boost tabs???
 

I have read about boost tabs, but do not know what they are and exactly what they do. Please enlighten me . Thanks.

What are boost tabs???
 

A boost tab relieves the load the pilot would feel in a pure mechanical control system.
Look at this one on an Edge 540..
It's hinged to the elevator, but attached rigidly to the stabilizer. As the elevator moves (in this case) down, the tab moves up. As it's located on the trailing edge, it possesses a good "hinge moment" for its size, and permits the pilot to deflect the control stick (and therefore the surface) further than he would be able to unassisted.

What are boost tabs???
 

I use these quite frequently on large models, especially on elevator and rudder. They let a less powerful servo move a big surface. Just don't let the boost tab exceed 10% of the total moveable area or you will most probably get hunting or oscillation; the tab overpowers the servo.

Origin of boost tabs...
 

Big airplanes like the China Clipper that came before the days of hydraulic assist had control surfaces too large for the pilot to be able to move them when in flight. To solve this, the ailerons were actually free floating, with a small tab at the trailing edge that was controlled. When deflected, this tab pushed the aileron in the opposite direction aerodynamically, thereby causing the airplane to roll. So it works like this:

1) stick goes left
2) left boost tab goes down (YES, down)
3) this pushes left aileron UP aerodynamically (NOW I get it!)
4) same thing in reverse on other side
5) plane rolls left

It's all about using leverage - that's why these planes had very large control surfaces chordwise - to get the torque that could be exerted by the boost tab up. All in all, it's a really elegant solution.

See if you can find a good picture of a china clipper parked - you might be able to see that both ailerons are drooping.

See, Dr. King - I was listening that day in Flight 2!

What are boost tabs???
 

I have been using boost tabs for abut 15 years and they work.

Here is a better explanation
boost tab

Extra with boost tab

What are boost tabs???
 

Some medium to large lowspeed aircraft have what is known as a 'spring/blow off servo tab'. This has a tab, as mentioned earlier, that operates in the opposite direction to the parent control surface but only comes into operation when the aerodynamic load on the main control surface is above a certain value. This means the pilot has direct contol over the surface at low speed, but at medium speed he is assisted by the servo tab. However another spring device comes into play when the aircraft is at high speed that allows the tab to blow back. This to prevent over- control of the surface at high speed and gives the pilot adequate 'feel' . The two 'springs' are usually in the form of torsion bars.

RE: What are boost tabs???
 

boost tabs crashed my plane. beware!!!!

RE: What are boost tabs???
 

Boost tabs are very safe, if your airplane crashed because of the boost tabs, they weren't installed properly. Been using them for more then 20 years, never a problem ...

Roger

RE: What are boost tabs???
 

Boost tabs can be very beneficial, do you have any pictures of your failed installation? I can not even visualize how they could be a cause of crash if properly installed.

RE: What are boost tabs???
 

UNFORTUNATELY, 'anything' has caused a plane to crash !
At least for some folks.

I'm afraid fact, reason, logic, proof, etc. etc. has little to do with these claims.

They chose the airplane; they chose the equipment; they did the installation(s); they were at the controls; yet "they" are always able to define something else as being the 'cause' AND they can do so almost at the instant of the crash.
Sound familiar?

Oh well, ....................so what else is new in RC ?

Interesting sidelight, RF type or equipment brand may vary BUT the claims stay the same. :([X(]:eek:
26.995MHz to 2.4GHz same folks same results. [:o]

I guess we could award an "A" for consistency .

RE: What are boost tabs???
 

My small hitec servo failed on the boost tab and that is all I had powering my big elevators!

RE: What are boost tabs???
 

That will do it,

the reason for not going too small on servos, is not for the flight loads but rather because of vibrations, also if you do go small on the servo, make sure you mass balance the control surface properly.

On my 34% Turbo prop laser, I am using only one 75 oz. nylon gear servo per control surface. Two years now.

Roger

RE: What are boost tabs???
 

Just so we are clear, then, there are two ways of operating the tab. You can directly control the tab, and use it to move the surface. This requires that there be enough airspeed over the surfaces before the tab will move anything. The other way is to power the surface and mechanically connect the tab to assist the movement, sometimes called a 'balance tab'. Of the variations, there's plenty, but they all work in a similar fashion. Which way you go is then up to you, though as at least one has found, if the servo stops working it don't matter which bit you are controlling, it still won't move.
Evan, WB #12.

RE: What are boost tabs???
 

i was using a 75oz servo...

RE: What are boost tabs???
 

You had a "small" 75oz servo on a boost tab driving your big elevators???

What airplane? A picture would go a long way to clearing up how you got one boost tab to drive elevators. Or did you have a boost tab on each elevator half of those big elevators but being driven by one servo? If so a picture of the linkage over to the other tab would be worth seeing.

RE: What are boost tabs???
 

was this servo operating the elevator or the boost tab, ??? If the boost tab, it is quite big for the job, as an insight, on my 35% Yak 54, I installed a small hitec 81 servo directly controlling the boost tab on the rudder. The rudder itself was free floating. On the ground for rudder/tail wheel operation, I would need a blip on the throttle to get some air but it worked beautifully.
In the air it worked very powerfully, did some very nice Knife Edge.

There was one problem though, since the Rudder also had an aerodynamic balance on the top of the rudder, in straight flight it would tend to oscillate. What I needed to do was install dampers on the rudder itself..

A properly installed boost tab on a control surface, using only a 25 oz servo controlling the boost tab, gives the control surface as much as 5000 oz of torque.

So, it would be nice to see how every thing was installed....

Roger

RE: What are boost tabs???
 

I just crashed my new 40%r today....because of Boost Tabs..... I was using HS-56HBs on my Boost Tabs on all surfaces including throttle......apparently Boost Tabs aren't what they're cracked up to be......:(

RE: What are boost tabs???
 

Or something else in your case.......http://www.rcuniverse.com/forum/js/f...ey/msn/lol.gif

Funny that we fly planes with normal setups andhavevery fewproblems........

RE: What are boost tabs???
 

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RE: What are boost tabs???
 

Boost tabs have been employed on full-scale airplanes of much later vintage than the China Clippers. The DC-9, which later became the MD-80, uses a combination of boost tabs and servo tabs to control its ailerons and elevators. The elevators and ailerons are statically counterbalanced. The pilot has direct cable manual control solely over the servo tab, which is assisted by a separate boost tab. The result is relatively light control forces, weight reduction, and excellent feel. Should hydraulic oil be lost, the airplane remains fully controllable - a boost to safety as well.

RE: What are boost tabs???
 

I did everything that was called for to make the boost tab work right and it failed.

RE: What are boost tabs???
 

If it might be of any help, here is how I operated the boost tabs on one of my planes that worked very well.

RE: What are boost tabs???
 

Here is one I used successfully,

RE: What are boost tabs???
 

which part of the setup failed , if installed properly, they are bullet proof..can you post a picture of your setup...

Roger

RE: What are boost tabs???
 

Does anyone know the forces that are generated by the boost tab? It would be great to know how large a servo is required. I used a servo force program to try to get the numbers but it doesn't look like it is correct.

RE: What are boost tabs???
 

I do not know the answer to your question but did find out through both experimentation and others experience to limit the size of the boost tab to 10% or less of the control surface it is mounted on. At that size, it takes most of the load off the servo allowing fairly weak servos to drive some pretty large surfaces.

RE: What are boost tabs???
 

I have some large scale planes with Ryobi engines and thought I would try adding boost tabs to see how they work. I want to just add a tab on the back side of the control surface and see how that works. I think I should keep the size a little smaller since the radius of the tab will be greater than the entire control surface.

Thanks for the info

RE: What are boost tabs???
 

Sopwith, you realize that you need to run pushrods and linkages to that tab for it to work? It's not just a case of sticking a tab on the rear. It needs to be powered by the servo or a slave linkage setup.

RE: What are boost tabs???
 

Yes, I have already installed it with slave links and have it adjusted. If the weather holds up I will try to fly it tomorrow. I only installed the tab on the elevator for now. If it all goes OK I will do the same on the rudder and alierons. The tab on the elevator has a 1 inch chord and 5 inch length. The elevator has a 4 inch chord and is 25 inches long. The tab is attached centered on the elevator.

RE: What are boost tabs???
 

The only real way to tell if this is working is to install a recording current sensor on the power lead running to the elevator servo. Or if you've actually had experiences where the model was slow to recover from a dive and you're pretty sure it was due to overloading the servo then it should recover better with the tab working.

The size you made it may be a bit much. Even with that size of elevator I think I'd have gone with about 1/2 inch width at that length or even a bit less. Hopefully the tab you put on doesn't act to remove too much of the elevator authourity.

RE: What are boost tabs???
 

It looks like your elevator has 100 square inches of area so you could have up to 10% or 10 square inches of boost tab from my experience. You have only 5 or 5% so that should be fine.

RE: What are boost tabs???
 

Wobblewobble mentioned having crashed because of a boost tab. I had my rudder servo linkage disconnect once, and the rudder went hard over - more than 45 degrees, with a very large 3-D type rudder. I had to fly the airplane in knife edge to a gentle upwind crash landing that slightly damaged a wingtip. I had been experimenting with the travel of the passive boost tab, where the servo controlled the rudder directly, with the tab assisting.

It turned out that the tab was too effective, causing the rudder to slam hard over when the resistance of the servo was removed. I simply reduced the travel of the boost tab, and everything was OK. The rudder had been a bit hard to trim, because the tab was forcing the rudder to uncommanded movement within the slight slop in the control linkage.

RE: What are boost tabs???
 

Another setup would be to use balance tabs to reduce the force needed to operate the control surface. The balance tab is passive as it is not directly controlled. Here's a photo of a balance tab on the elevator on an F4U Corsair. The control linkage is attached to a fixed horn on the horizontal stabilizer. The tab automatically moves proportionally and opposite the movement of the elevator. This and the portion of the elevator tips that projects forward of the hinge line combine to reduse the force needed to operate the elevator.

Scott

RE: What are boost tabs???
 

Interesting experience and good that you saved the model.

But one question. How do you know your fix actually was better, or the change actually did anything at all? Did the rudder servo linkage fail after you reduced the travel? If you think about it, removing our model servos from the system removes the absolutely rigid, constant lock the servo has on rudder position. As long as a servo has a solid grip on model surfaces, we really wouldn't know how much boost the tab gives. OK, unless we put a recorder into the servo wires and checked the amps drawn before and after.

Did your "3-D type rudder" have significant area forward the hinge line?

RE: What are boost tabs???
 

I''m a bit confused.  Some time ago I had known about servo tabs, and when reading RC forums over a decade ago heard of boost tabs.  Never heard of full scale pilots or mechanics calling them boost tabs.  Concluded that "boost" tab was an RC slang to avoid confusion with the servo.  But now seeing full scale pilots refering them as boost tabs.

Is this the same thing, or is there a differance between boost and servo tabs?

RE: What are boost tabs???
 

No.
Evan, WB #12.

RE: What are boost tabs???
 

So if they are not the same thing what is the difference?

RE: What are boost tabs???
 

It depends on what you want them to do...there are tabs to assist the pilot, tabs to oppose the pilot, tabs to trim, tabs for the pilot to alter the trim...moving tabs and fixed tabs. There are tabs the pilot moves, tabs that move because the surface it's attached to moves, tabs that don't, and tabs that only move after a specific load is applied to the surface it's attached to...but they boil down to three distinct types, those that assist surface movement, those that oppose surface movement, and those that bias the whole surface circuit.
Evan, WB #12.

RE: What are boost tabs???
 

Hi Da Rock: My tab-equipped rudder that slammed hard over when the servo linkage disconnected (because I had neglected to adequately tighten the output arm screw) measured about 18" high, with average chord of about 7", on an 84 inch span full aerobatic bipe. It was activated by a servo with only about 60 ounch-inch torque, which was utterly inadequate at its full speed of about 85 MPH, providing solid rudder authority only up to about 40 MPH airspeed. The tab had a chord of about 1/2", spanning the entire length of the trailing edge, and needed only about half as much movement as the rudder to do a good job, without trying to take the bit into its mouth and overpower the servo. With the tab connected, the relatively weak servo was able to slam the rudder over past 45 degrees nearly instantly, at 85 MPH airspeed

The rudder had no area ahead of the hinge line - I gave up on that idea several years previously. after discovering that boost (or balance) tabs did a much better job. I also found that that balance area ahead of the hinge line seemed to introduce non-linearities into the rudder's hinge line moment that sometimes resulted in a low frequency oscillation of the rudder, while not supplying all that much assist at extreme rudder movement.

That same model's ailerons used only a single servo to obtain twice the roll rate that previously was supplied by two of the same servos.
After experimenting with passive boost (or balance?) tabs for years, I have found that the tabs can reduce the needed servo torque by a factor of up to a maximum of about 85% (80% may be safer), before the control surface may tend to hesitate a bit before going to full travel, particularly with rudders. In other words, relatively small and light servo can do a job that would require a super-torque servo, and such servos were not available when I designed and built that airplane.

I have not, as yet, tried true servo tabs, where the servo moves only the tab, and the mass-balanced control surface is free to weathervane, actuated by the tab only, as in the DC-9, which used a combination of servo tabs and passive boost (or balance) tabs on its elevators and ailerons. Such a setup might easily multiply the effective servo torque by 100 or more, enabling a normal model-type servo to control truly gargantuan models, and probably even human-carrying aircraft, quite handily. Some homebuilt full-sized aircraft use normal model servos to provide electric trim, with good results.

RE: What are boost tabs???
 

Here is a link to a MAN article with some pics of my boost tab-assisted 84" span bipe that uses only one standard servo for each flight control to get full-aerobatic control response at its top speed of about 85 MPH.

http://media.radiocontrolzone.com/ma...aneSecrets.pdf