I started a thread on Boost Tabs in Giant Scale and thought it might be better here.

http://www.rcuniverse.com/showthread...515&forumid=38]

Roger,
since i'm building a "relatively" small plane, at 60" span, I set up my tabs on the elevator with 2/56 ball and socket joints, and threaded rod, my problem now, is that i have more movement in one direction than the other, because of the offset from the hinge-line, is this ok? or should i figure out how to get the balls even with the hinge line for even boost each way? i assume i would want more boost in the up elevator direction than down.
thanks,
Dennis

If it were an aerobatic airplane, you would want the same movement in the up and down position, but being a Warbird if I remember correctly, it would be OK.
You should as much as possible get the ball in line with the hinge line.
But because you are using a very small boost tab, you should consider the nor 1/2a control horn and the small nylon clevis. This would help keep the weight down which would also help in the balance of the control surface.

Hi Roger,

Since, for the purpose of this thread, you are our resident expert, at what size would you think it necessary to begin to use boost tabs, i.e. wing spans greater than 80" or a certain amount of wing area or is it related to how fast the plane files?

Andy

Well Andy,
That would be difficult to say because it would depend on a lot of parameters.
I had them on my 80" Giles some time ago and was using regular type servos (42oz) even on rudder. And everywhere on the airplane for that matter.
I would say anything above 80". But if you are going to install powerfull servos anyway, then it might be useless work.

We started using powersteering in our cars because it made more sense then spend a lot of time at the gym and build our muscles

Roger - I tried the link to your web site, but "Cannot find..." Are you over your traffic quota?

Anyway, I know an article or two have been written about this before - I think one was in MAN some years ago. Anyway, I'd like to see the work that's been done if anybody has a source. Specifically, how to calculate how many oz-in of torque will a certain setup account for. "Flight Surface creates X oz-in torque minus Y oz-in from the trim tab = oz-in needed from servo". Solve for Y and then size your servo.

I have just bought a Lassek A-10 which will be begging for tabs on the ailerons - both scale and functional.

I'm not sure on the calculation of torque, I'll have to re-read the article.
And yes with Geocities, me being cheap and don't pay for the web site, it goes out of comission every once in a while because of traffic quota as you said,

Now if I start selling more engine standoffs and Fiber Optic extensions, then I will be able to afford a higher quota for my web site.

http://www.geocities.com/roger_forgues/Boost-tabs.html

I'd also like to know the calculations that Johng asked about. I like the idea of boost (trim) tabs reducing the servo torque required, specifically applied to giant scale 3D aerobatic planes. However, I'm not real keen on trying it on any of my giant scale aerobatics until I get some ball park formulas. Anyone have any ideas?

First they are Boost Tabs and not Trim tabs, big difference.

You would want to stay in the 6 to 10 % of the control surface.
Also, you should not go more then 30% of Boost Tab throw versus that particular control surface.

If you stay within these parameters, you will be all set.

Boost vs Trim tabs: I just assumed that there were similar. Enlighten me, what's the diff?

I get the 6% to 10% and no more than 30' throw, but what surface unloading should I expect? I don't need an exact number, but what are we talking? 10%? 50%? What about extreme throws, such as 45 degrees. How well do the tabs work in that situation?

Thanks.

I'm affraid I don't have specific numbers. Not even a guess except that with a 25 oz servo controlling the boost tab only with the proper setup you could end up with as much as 5000oz of force.

Normally for extreme throws such as 45 degrees, they will not be as effective, but then at these throws, your airspeed is much lower, or it should be anyway.

Boost tabs operate in unison with the control surface while the trim tab operates independent of the contol input..

I've had good luck in making my boost tabs mechanical linkage such that the boost tab stayed parallel with the fixed surface no matter what degree of throw was on the movable surface. I did find out that greater than 10% area did cause a problem (would hunt for neutral and tend to gallop when the elevator boost tab area was greater than 10% of total elevator area). I never made any actual measurements but effectiveness seemed to be exponentially proportional to air speed; i.e. more effective at higher speeds.

Rodney,
Keeping them parallel is exactly what I do and it works out quite nicely.
Would you have some pictures of your installation with the control horn. I have been successfull with my installation but there is allways room for improvement.

The Multiplex web site has a servo torque calculator program at:
http://www.multiplexrc.com/calcservo.htm
You can use this program to calculate the torque that is necessary to give the maximum desired control surface deflection at the maximum airspeed. You can use this maximum torque to find the force that the boot tab must supply by dividing the torque by the distance from the control surface hinge line to the expected center of area of the boost tab. Pick a boost tab size and deflection and put it into the multiplex torque calculator program. This will give the torque required to operate the boost tab. Divide the torque by the distance from the center of area of the boost tab to the boost tab hinge line to get the force perpendicular to the boost tab. Find the vector component of that force that is parallel to the control surface and compare it to the required control surface actuating force. Adjust the design of the boost tab and its deflection and iterate the force calculations until the component of the boost tab force perpendicular to the control surface meets the control surface actuating force requirement.

The above design algorythm seems reasonable to me but I have never tried it. It needs experimental verification.

Well I made some assumptions, and cranked some math out. I did something similar to what Ollie suggested using my rudder as the test parameters. I tried several variations of throws, speeds, ect. Rudder is 24" span, average chord of 8.5", boost tab at 8%.

As best I can tell the actual "boost", or off loading of the primary control surface servo in my case fell in the 15%-30% range when using a tab that follows Aerografixs' guidelines. I'm not sure if that is enough to really matter much in my application, especially if it is at the 15% range. Additionally, I lose ~8% of my rudder authority as it went to the boost tab.

I hate to wing stuff like this. Does anyone really know how much off loading will occur?

I'm not sure that those calculations are correct, as the Boost Tabs done correctly are much more powerfull then that. Did you read the article on my web site? I didn't write this but the one that did knows quite a bit about this.

I have looked at the calculator from the Multiplex site and it didn't worked very well I thought.

Here is a bit to try and understand better

Anton Flettner invented them and have been called Flettner tabs untill recently. Me 109 used them on rudder and corsair used them on ailerons and elevator. Nice to have something, finally, that doesnt require batterys!! In fact you could probably a smaller batt and 4.8 volt conventional servos.

Thanks for the infor on there originalety.

Guys, I put a boost tab on our Ziroli P-51 rudder and I can tell you it is a royal PITA. To actually make it operate is the next level followed by truley functional. Keep in mind this is a 100" wing span plane at about 40 pounds with a 5.8 motor. I I removed the the functional part and made it look real instead. The 5.8 destroyed it the first run up anyway.
Having flown the wings off our Z corsair (94" wing) over the last 9 years and replaced a number of damaged servos, my personal opinion is that you are really asking for some disapointment. I have spent considerable effort to prevent flutter on all control surfaces and have been very successful using more or less proven methods..ie tight sealed surfaces, husky linkages, balanced control surfaces, all gaps are tightly sealed, powerful servos with good leverage. this plane has been on countless near vertical dives (bomb runs) aerobatics, nasty winds, encounters with flying and non flying objects. plus more than a few very rough landings. through all of this it still flies very well. I have trashed at least 8 75oz or better servos in flight. several flights were obviouly flown with one servo stripped or jambed with no loss of control. One rudder servo 135 oz (F- giant scale)was destroyed when the gear collapsed and it ground looped snagging the tail wheel on a rock. One 135 oz elevator servo failed after a hard landing. the whiplash of the tail hitting the ground caused the elevator to cycle opposite the command. I would not have any objection the the scale detail of boost tabs, they certainly will look nice, but I would not depend on their functional effectiveness to be able to cut back on the servo size for a few dollars. The second factor to consider is weight. My rudder and elevator servos are very near the CG to keep from having to add anymore weight than necessary to the nose of this plane. Still have 2-3 pounds. Now if you are building a smaller plane weight becomes more important. Keep in mind that weight at the tail take about 4 to 1 balance at the nose. One oz in the tail takes 4 oz in the nose so you have added 5 oz to the plane for 1 oz of detail. I think I would make these look functional and leave the servo do the driving.
This is just my opinion and not any criticism of creative thinking.
goos luck
bentwings

Bentwings makes a good point. The knowledge and data necessary to design and build a reliable boost tab system are far from trivial. There are other ways of reducing servo load. An all moving tail surface that is aerodynamically balanced has virtually no load on the servo and linkages. An aerodynamically balanced, hinged control surface can have as much servo load reduction as wanted. With these solutions available to all but the scale buff, why not keep it simple?

Some, like the mountain climber, may take the challenge just because it is there. My hat's off to those that succeed.

Sorry to hear about your back luck with them, but I have never had any problems with them yet. I started using them way back in the mid 70's
If they are made right, they will work, that is a sure thing. Why do you think all large aircrafts have them.
Sealing gaps won't stop flutter it will make the controls tighter. No slop, and balanced surfaces will stop flutter to a degree.
Were you flying this airplane at a much greater speed then recommended?
To tear off the Boost tab on the ground, maybe the engine was vibrating too much.

aero....,
To answer your questions....yes the plane does vibrate..both of them do ..quite a lot infact. We expected this and allowed for it in construction. The tab was an afterthought for some scale detail. Whether it provided boost or not didn't matter. It simply was a scale operating feature. It proved to be too delicate for everyday flying in a hotrod plane. that being the case..implied functionality took over from realism when we reinstalled it.
I wouldn't say the plane is flown out of scale speeds if you use dynamic scalling. In fact 110 is still on the slow side for a 22% warbird. Faster than average..yes but not excessive.
I agree with you when you say 'reducing flutter'. The purpose of 'anti' flutter measures are to put the flutter speed higher than the maximum speed your plane ever will attain not necessarily to eliminate it. Since we as modelers don't have the resources to do endless research, we have to take the experts' advice. For example the full sized Corsair went through at least 6 aileron designs before the flutter was fixed. My counter balances and other measures do the job and I made them so they "look scale" A purist will quickly shoot me down. Personally i don't care as long as my plane survives normal..and abnormal flight.
I also agree that full size planes have them. for very good reasons. Flight loads are far higher per unit of strength and simply adding a bigger and heavier control mechanism just makes the problem worse. (modeler method)
The large 40-50% aerobatic guys are finding this out. 4 servos per control surface is getting complex and overloading the rest of the electrical system. A well done boost tab works here. And reduces load to a manageable level. I'm sure youv'e seen them. They are large enough area wise to provide meaninfull loads. I think you will see more of these. They may not fit the classic formulas but work they will.
You obviouly found this out and are making good use of them. Very wise.
The whole idea of the boost tab effectivness rolls around dynamic scalling which is a complex subject in itself.

whew... this is getting deep....how about picking on someone else for a while. ... peace??
bentwings

Bentwing, (I wish I knew your name)
It might be getting deep, but brain storming is what this is all about.
I have been asked about installing boost tabs in smaller airplanes, and I personnally don't think it would help much. So with the larger airplanes to me there is no otherway.

I am presently installing a rudder boost tab on a 80" Giles. The servo that I installed on this is a Futaba S28. Not very powerfull and not digital, but plenty powerfull with a boos tab.
I'll have pictures here soon of this installation.

Hey aerografixs,
That's a pretty interesting tab setup. A while back I did an 80" Cap for a guy with 2 Jr 4131 (90 oz ea) on the rudder. He claimed it was marginal at best. I would say the claim vs. skill might be questioned. Actually it was more pain than anything as it was constantly buzzing and very difficult to keep both servos from fighting each other. Not to mention radio programming.
If you are only using a 40 oz servo plus boost tab....that's great. Just saved about $75 in servo cost + 3/4 the cost of the second and ??? $$$$ in labor and pain setting up. From the picture it looks pretty tough too. Nice job. The neat thing is that you always have "power" assist like power steering as long as there is air flow over the surface. more or less proportional....more air flow more power assist. Even with no air flow the 40 oz can move the surface easily. Utopia!!!
As I was saying earlier, on larger planes it is much more effective than smaller planes. Likewise on larger surfaces it is more effective. Also the farther away from the hinge line the better too..as is your system.
Let me know how it performs.
By the way I checked out your web site listed. You got some very interesting stuuf there I like the fiber optic cables. As J.O. noted these have been in industry for years. I use them all the time just like wire in our machine control sensors. I'm about ready to start another large war bird so I'll be interested in using some of these. RC has needed this for a long time.

more later
bentwings