Red B.

Hi all aerodynamic gurus!
After many years of flying .40 - .90 sized aircraft my latest aircraft will be a little bit bigger and hence need more powerful servos, especially for the rudder. Since good quality hi-torque, hi-speed servos are quite expensive I am thinking about the possibility of using a boost tab on the rudder in order to lessen the strain on servo and linkage.

My question is about the size and effectiveness of boost tabs. What size should they be and how much should they deflect relative to the rudder. Is there a way to calculate (at least approximately) the influence of the boost tab on how much torque is needed to deflect the aileron by a certain angle at a certain speed?

Any help is very much appreciated.

/Red B.

barto

Here you go, [link=http://www.gsal.org]GSAL[/link]

It's under the projects menu, click on the '33% RadioCraft Extra 330-LX'


Hope it helps,


Bart

Tall Paul

I don't see a "boost tab" on the rudder there. The overhang at the front isn't a tab, it's a balance horn.
A true boost tab is hinged to the surface, but driven by a pushrod connected to the structure ahead of the surface. The moment arm of the tab provides an aerodynamic assist to the surface deflection.
They are generally fairly small because they are effective.
Most frequently seen on late pre-hydraulic controlled planes, such as the T-6.
I've seen 4 servos paralleled on rudders in the larger scale models.
A recent magazine cover showed one such with 6 on ailerons, and 4 on the elevators. With probably another 4 on the rudder, and one for the motor, @ $150 each... I'll never have THAT airplane!

barto

Scroll down on the page and click on rudder boost tab.

I think it isn't a 'true' boost tab in accordance to your description because it isn't 'driven by a pushrod connected to the structure ahead of the surface'. The owner of the airplane calls it that way though.

Sorry I didn't mention you had to scroll down.


Bart

Flypaper 2

Have them on the ailerons on a Corby Starlett. Rule of thumb is 10% of the area. Had three holes in the boost tab horn. I kept moving in on the holes till it felt like you put the next size bigger, faster servos in it. When it does that, don't go any further as it will overdrive the surface and will not stop oscillating. Ended up, with full deflexion (sp) the tab was about parallel with the slipstream. Hope this helps.

feihu-RCU

Red B -

I have seen servo tabs on a couple of TOC airplanes (Austrailian competetors who were practicing at our flying site); and the practice flights were outstanding.

For details on aerodynamic boost tabs, go to http://www.geocities.com/roger_forgues/Boost-tabs.html

feihu

Rotaryphile

Tall Paul is right - a boost tab is a completely different animal from a balance tab. Balance tabs are simply a portion of the control surface that is ahead of the hinge line. I experimented a lot with rudder balance tabs before largely abandoning them in favor of boost tabs. About 10% of the area is more than adequate for most applications, but be sure to make the linkage adjustable, and start with about 50% of the angular movement of the control surface, then gradually increase the tab travel until you get the control response you want.

Balance tabs can reduce servo torque requirement by about 85%, after which non-linearities rear their ugly heads, resulting in "funny" feel and response, particularly on rudders. I equipped a rather large bipe with aileron balance tabs, and one servo produced at least twice the roll rate that two servos had previously produced - a vast improvement, and it saved weight and battery current too.

Red B.

Interesting information!
I will definitly try this on my next aircraft.
Thanks!

/Red B.

tIANci

Wooow ... a reduction up to 85%. Interesting ...

barto

Ok, thanks for the info, learned something new again.


Bart

Johng

My Feedback: (4)

The GSAL link does show a rudder boost tab, it's just driven by it's own micro servo instead of a hard link to the fuse. Iteresting programability there. It could be mixed only to deflect when the rudder is more than xx degrees deflected. Practically though, the hard linked pushrod will do a fine job.

Tall Paul

I can't find any words at the GSAL link, just this picture...

Johng

My Feedback: (4)

Paul! Wake up Paul! Lookee here



Check the link here:

http://www.gsal.org/projects/BranExt..._boost_tab.htm

barto

That's the one I meant.


Bart

Tall Paul

.
???
Going to the GSAL link, I get images of a plane WITHOUT any boost tabs..."33% RadioCraft Extra 330-LX"
I see nothing anywhere on the pages for that plane that mention boost-tab... in fact there's no text, just images.
And it's even a different plane than the "Bran Extra"..
Now, the new image from a different location... (guess how much time I will spend looking for a needle in a haystack when the haystack isn't the correct one).. is a boost tab, as Johng mentions.

Johng

My Feedback: (4)

There are two 'haystacks" called "33% RadioCraft Extra 330-LX". Paul looked at the first link by that name. For some reason, I looked at the 2nd one. The 2nd one is where the boost tab is shown.

barto

Ouch, sorry Tall Paul.[]

I never even noticed there were two RadioCraft build links.


Anyway, I was only trying to help. Sorry for all the confusions.


Bart

Rotaryphile

The servo-operated tab in the photo is not a boost tab, but a servo tab. Servo tabs are vastly more effective than boost tabs, but they are more complicated, and require good static balance of the control surface. Boost tabs are passive devices that deflect in response to control surface movement. Servo tabs move the free-hinged, weathervaning control surface directly. Some full-scale airplanes use both - the DC-9, for example, requires no hydraulic boost on its ailerons and elevators. The pilot moves the servo tabs on the trailing edges of the ailerons and elevators by cables connected directly to the control column, and the boost tabs greatly reduce the force required from the servo tabs.

The result is an amplification of control force that may easily exceed 100 times. Normal R/C servos could probably exert sufficient torque to control many light aircraft, in case anyone has a yen for really gigantic models, by a combination of servo tabs and boost tabs. Incidentally, boost tabs are sometimes called balance tabs.

beavertail

Do the tabs move in the opposite direction than the control surface itself?? I may be totally lost here. And If so what does the FS linkage look like?

Warbirdz01

My Feedback: (1)

Rotaryphile is right on the money......DC-9/MD-80...known as the Maintenace Delay-80....or 100 ft long "Sewer Pipe"....has servo tabs( and "geared" tabs)...... for ailerons/elevators...with hydro boost for elevator down only for stall recovery......rudder is hydro powered.........after working on these for a number of years....still amazed on how well the servo tabs worked.......yes...servo tabs move the opposite direction of the primary surface that it is attached to......Bill......

Rodney

I have used boost tabs on many 1/4 scale craft. They are very helpful, lets a low torque servo do the job of a giant servo. do limit the area of the tab to 10% max of the moveable surface however. Every time I tried to use more, the surface would tend to oscillate near neutral position while still working correctly at any other deflection. Maybe oscillate is not the right term, but the plane would seem to hunt or wag when the surface was at neutral. They worked well on ailerons as well as rudder. I had best results when the tab always stayed lined up with the fixed surface no matter which way deflection went. On rudders, I have found that balance tabs (a part of the moveable surface forward of the hinge line) worked as well as the boost tab and was somewhat simpler to set up than the boost tabs. Again, 10% seemed to be the maximum area you wanted ahead of the hinge line when using balance tabs. One big advantage of the balance tabs, it gives both aerodynamic and static balance enhancement.

Rotaryphile

Here's a website that has an article that I wrote on boost tabs and servo tabs for M.A.N a few years ago. http://www.geocities.com/roger_forgues/Boost-tabs.html

When initially setting up the travel on boost tabs, it is best to start with the boost tab moving about half as far, angularly, as the control surface, then gradually increase it. If you overdo the boost, you end up with a control surface that constantly tries to deflect, rather than to weathervane until the servo attempts to force it off center. The result is a control that may refuse to trim - the control surface tends to flip one way or the other within the linkage slop that is always present. On one of my early rudder boost tab experiments, the rudder disconnected from its control linkage when I stupidly forgot to replace the servo output arm screw. The rudder went hard over in flight, making things get interesting fast. That particular model would only fly in knife edge with its rudder at full deflection, so I had to "land" the model in knife edge, but very little damage resulted since that fully aerobatic big bipe could fly much slower in knife edge than its normal stall speed.