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servo/boost tabs

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Old 11-30-2010, 07:13 PM
  #1  
Otter Guy
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Default servo/boost tabs

has anyone set up controls like very large pre-hydraulic controled aircraft with a servo tab, similar to a trim tab, hooked to the servo to use the airflow to actuate the control surface instead of monster servos? ive got dreams of things like a large martin mars, or something, and might do some kind of experimenting with this through the winter and next summer. if im not having to much fun with my beaver and cub lol
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Old 12-01-2010, 05:20 AM
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Default RE: servo/boost tabs

A search has returned these previous threads on boost tabs:

http://search.rcuniverse.com/search....hwhere=subject

Regards!
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Old 12-01-2010, 10:46 AM
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Default RE: servo/boost tabs

Servo tabs can function per full scale but the results are NOT the same .
Simply put - the model is too small for the forces produced to be of much use
Also servo tabs only work around the neutral position- Again simply put - you can't get something for nothing.
Think about it - If you are going to produce a down force (or side force) you have to to put at least that much force into the task
Aerodynamically counterbalanced control surfaces will produce high forces with small energy inputs.
You can use linked tabs (tab linked to the stationary surface such that the tab moves a LOT in opposite direction of the control surface). This works but if all you want to do is decrease cost of servo - ferget it
You can buy really good 300inch ounce servos for 10-20 bucks -really -that are extremely good. The only other necessity is a really good power supply to the servo -
I would not use anything on a large model but A123 batteries and good 22ga wiring.
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Old 12-01-2010, 02:08 PM
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Default RE: servo/boost tabs


ORIGINAL: rmh

Servo tabs can function per full scale but the results are NOT the same .
Simply put - the model is too small for the forces produced to be of much use
Also servo tabs only work around the neutral position- Again simply put - you can't get something for nothing.
Think about it - If you are going to produce a down force (or side force) you have to to put at least that much force into the task
Aerodynamically counterbalanced control surfaces will produce high forces with small energy inputs.
You can use linked tabs (tab linked to the stationary surface such that the tab moves a LOT in opposite direction of the control surface). This works but if all you want to do is decrease cost of servo - ferget it
You can buy really good 300inch ounce servos for 10-20 bucks -really -that are extremely good. The only other necessity is a really good power supply to the servo -
I would not use anything on a large model but A123 batteries and good 22ga wiring.
Your comments above are contrary to my experience with using boost tabs on models. I have found them very useful and effective. In fact, it you overdo their size (more than about 10% of the movable surface) you will get to much force and find a hard time holding neutral position; i.e. the surfaces tend to gallop. I have seen many effective uses of this on all surfaces, especially rudder and ailerons.
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Old 12-01-2010, 05:45 PM
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Default RE: servo/boost tabs

There is an interesting link on Welcome to Alan's Hobby, Model & RC FAQ Web Links which I found from Airfieldmodles web site ( http://www.airfieldmodels.com ).
http://homepages.ihug.co.nz/~atong/
Look under Aircraft Design. Airfoils, Pull-Pull Systems, Flight Trim & Static Thrust Calculator.
Boost tabs.
it is an archived page but worth the wait for it to load.

http://web.archive.org/web/200502121...oost-tabs.html

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Old 12-02-2010, 08:07 AM
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Default RE: servo/boost tabs

OK boys- just how much actual control surface deflection do you think these tabs will accomplish?
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Old 12-02-2010, 08:14 AM
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Default RE: servo/boost tabs


ORIGINAL: rmh

OK boys- just how much actual control surface deflection do you think these tabs will accomplish?

The couple I saw had all the deflection they needed. Never saw anything suggesting any kind of limitation. Nor have I read that any of the WWII aircraft that used them had any limitations. MOF they retrofitted them on one airplane to boost the response. Might be where the nickname came from?

If I remember correctly, the paper I read about them some years back implied their performance was a function of their location within and proportion to the driven surface, that it was possible to have them too small. But I do remember the paper leaving out how to tell they were big enough.
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Old 12-02-2010, 08:26 AM
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Default RE: servo/boost tabs

Actually, the article that iron eagle links to is very thorough. It even mentions the Douglas commercial jets that use them today and how boost tabs perform on those.

http://web.archive.org/web/200502121...oost-tabs.html
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Old 12-02-2010, 08:41 AM
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Default RE: servo/boost tabs

Aerobatic setups where throws typically reach 45 degrees - are not candidates for trim tabs except as trimming devices
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Old 12-02-2010, 11:19 AM
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Default RE: servo/boost tabs

Actually boost tabs are a lot of work And I think just getting the proper torque servo and good linkages are the way to go.

But, if you can't get a servo with enough torque it is a method to try, I think the range of deflection they will work at is governed by the control linkage geometry.

Wouldn't it be a lot easier to set up dedicate trim surfaces if that is what you wanted?
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Old 12-10-2010, 05:40 AM
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Default RE: servo/boost tabs

These are on my old 1/4 scale Corby Starlett. When I was test flying with them on. I moved the pushrod on the tab horn one hole at a time. When I hit the magic spot, it was as if I put the next size bigger servo in it and increased the control throw, much better roll rate. Servos are JR 821.
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Old 12-10-2010, 07:31 AM
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Default RE: servo/boost tabs

Extra work? OK, sure, but is there a downside?

I did them so long ago, I can't remember what the airplane was. The goal was to drive a much larger surface than my servos had the power to drive. The first time I saw a monster IMAC with 3 servos in each wing the light went on. Heck, that airplane had more cost in the servos than my entire 60 size Extra cost for everything. Funny thing, talking to the owner gave me the impression he was proud of the expense and wouldn't think of doing any differently. And of course, he also didn't build.

But not having to tune grouped servos, and the increased exposure to failures....... yeah, silly me, that wouldn't matter either.
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Old 12-10-2010, 08:09 AM
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Default RE: servo/boost tabs

Let's have a little reality check.
the "opposing action" boost tap , DECREASES the effective size of the control surface.
Y/N?
At some point in the travel of the control surface, the boost tab is no longer working to move the control surface in the desired direction.
Y/N?
Does the tab help around lower deflections?
Yes
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Old 12-10-2010, 08:36 AM
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Default RE: servo/boost tabs

Yes, by 10%, and it does drive the surface to the limit of the servo throw. When the ail. is at its limit, the tab is still at a positive angle of attack by about 10 degrees. The tab works through the whole range.
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Old 12-10-2010, 10:41 AM
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Default RE: servo/boost tabs

ORIGINAL: Flypaper 2

Yes, by 10%, and it does drive the surface to the limit of the servo throw. When the ail. is at its limit, the tab is still at a positive angle of attack by about 10 degrees. The tab works through the whole range.
The deflected tab becomes counterproductive as angle of the control surface increases-at a certain angle
Imagine the tab was far larger - to see how this works
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Old 12-10-2010, 11:15 AM
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Default RE: servo/boost tabs

The tab counters the control force by ten to fourteen percent, or equal to the percentage of surface area the boost tab is of the control surface. So there is that loss...
This loss is actually reduced somewhat as the boost tab is below the trailing edge of the wing, granted not much.

They are a real good option if want to save some money and do not mind the time and work it takes to set them up.
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Old 12-10-2010, 12:36 PM
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Default RE: servo/boost tabs

I am just making the servo balance panels on an 1/8th scale 16.25ft span Handley Page HP42. They are separate surfaces altogether. These are hinged at the rear edge, so the airflow tries to push them further over when operated, helping to equalise the 'self centering' of the rudders themselves. Heres a pic of the full size. The whole lot works on a single closed loop system to the centre rudder.

Ian.
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Old 12-10-2010, 02:15 PM
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Default RE: servo/boost tabs


ORIGINAL: rmh

Let's have a little reality check.
the ''opposing action'' boost tap , DECREASES the effective size of the control surface.
Y/N?
At some point in the travel of the control surface, the boost tab is no longer working to move the control surface in the desired direction.
Y/N?
Does the tab help around lower deflections?
Yes

The reality of my use of them was.............

It worked so well a couple of other guys did the same and none of us noticed those limitations. We were facing a limitation of the power and price of available equipment at the time. It saved what would have been a ton of money at the time.

The fact that boost tabs probably won't work on TOC or 3D airplanes to do those maneuvers is good to know if that's where you wish to use them.

They might not work for every application, but they are an option for lots of applications. And won't even be considered by most modelers today because ARFs don't usually come with them, do they.
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Old 12-10-2010, 04:55 PM
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Default RE: servo/boost tabs

For scale type or nice sport flying stuf - sure- go for em -
It's fun to experiment

I pondered that some may think they replace the force needed to drive surfaces against heavy aerodynamic loads
Having said that
On ailerons which are joined (left to right )
The stick (servos) forces can be reduced to -0- or less (not a good idea )
stick snatch can occur
Aerodynamic aids such as these type tabs and/or spades can be a great aid - or a real problem
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Old 12-10-2010, 05:20 PM
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Default RE: servo/boost tabs

It's why I didn't go any further when I got the control response I wanted. One hole further may drive the surface to the end and not have enough servo to pull it back. Would more than likely oscilate back and forth around the neutral point too. One experiment I didn't want to try.
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Old 12-11-2010, 12:24 AM
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Default RE: servo/boost tabs


ORIGINAL: rmh

Aerobatic setups where throws typically reach 45 degrees - are not candidates for trim tabs except as trimming devices
I always worry when someone suggests that you can use tabs on a control surface (trim,boost, servo, or otherwise) to trim a model. Full size yes, but a model no. (Apologies to Dick if this isn't what you mean). I remember reading of a large model Lancaster bomber crashing in the UK due to this. This was back in the early 80's when large models were relatively new and servos were relatively weak. The model was fitted with an adjustable tab on the elevator. The model needed nose up trim so they adjusted the tab trailing edge up. This pushed down on the trailing edge of the elevator, which was held immobile by the elevator servo, and therefore lifted the nose of the model. Problem solved right?

All was good until one day they flew the model a bit faster than previousely. The tab overpowered the servo and pushed the elevator down.... Splat! Up elevator was no use, the servo wasn't strong enough to overpower the tab. This occured at a model display IIRC, luckily the crash missed the spectators.

In a full size plane (with non powered controls) the pilot holds the controls where required (boost tabs will assist if fitted) and then the trim tab (could be the same tab) is adjusted to reduce the control force to zero.

In a model the servo holds the control surface where commanded, any trim tab is only going to load the servo adding stress to the linkage and using up battery power. Fly it fast enough and either the linkage or the tab or the control surface will break, or the servo will be overpowered.

I read an article where a french modeller mounted a fuselage to the roof of his car and measured the elevator deflection at up to 100 mph or so. Even though the servo was strong enough there was still a significant reduction in throw due to flexing of the linkage and control surface it self. A servo tab gave full throw.

Other aerodynamic balances work well on models eg horn balances (like on a cub rudder), inset hinging, spades etc. Even on aerobatic set ups at 45 degrees. Boost tabs also work in these situations although the linkage might get a bit difficult.

So the way I see it:

pros:

Reduces required power (weight and cost?) of batteries and servos
Improves speed and centreing of servos
In a scale model, if the original had them...

cons:

Fiddly and possibly not reliable
Don't achieve anything that a stronger servo wouldn't come close to achieving.

To th OP, it's your model, could be a cool feature.
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Old 12-11-2010, 03:43 AM
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Default RE: servo/boost tabs

They are called boost for a reason. The boost tab reduces the load on the pilot or servo. Their use can actually be compared to the use of a servo with no tab present. They're all either up to the job or not. And the effect of the boost tab can actually be compared to the effect of any surface, because they aren't much different than the surface they "boost". They are either up to the job or not.

Understand surfaces.... understand trim tabs.... understand boost tabs (something quite different than trim tabs).... understand linkages.... understand stalls.... there is certainly a lot to understand.

When you don't have a stronger servo....... lots to understand.
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Old 12-11-2010, 03:51 AM
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Default RE: servo/boost tabs

There the two types of tabs, the boost tabs and the trim tabs, and a few combined. Most of these problems are just ignorance of the mechanics of them. The guys with the scale model that was using it for a trim tab just didn't have enough servo power, period. Trim tab or not. One of the assets of the Corsair was its agility when taking on the Zero because of the servo tabs. One elev. had the servo tab, the other a trim tab. One of these days I'd like to take one of my 40 odd inch foamys and put micro servos on it with servo tabs just as an experiment.
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Old 12-11-2010, 06:18 AM
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Default RE: servo/boost tabs

As I stated elsewhere above, I have found that boost tabs can be very beneficial if properly installed. What I did not mention and should have is that static balance is also very helpful and more desire able when using boost tabs than otherwise due to the usually added weight aft of the aileron hinge line caused by the boost tabs. Static balance is especially important on the larger models as G forces on the less than perfect landings can seriously strain the aileron and elevator servos. You can get by perfectly safely with quite weak servos if both static balance and boost tabs are used.
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Old 12-12-2010, 03:40 PM
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Default RE: servo/boost tabs

Hi chaps, any ideas on the following?

On the set up in this pic and in the doodle, would the balance panels be set parallel, or set with slight toe in or toe out??? What say ye? The rudders are all closed loop controlled, slop free with a short pushrod to the balance panels, again slop free. I'm just wondering whether they'd pre load them against airflow at neutral to possibly stop any flutter around centre?

Many thanks, Ian.
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