boost tab question

I made aileron boost tabs for a gas pattern airplane that seems to have a sluggish roll rate. The tabs turned out to be 15% of the area of the barn door ailerons. Cannot wait to test the concept but it is too cold to fly.

I am guessing that no convenient method exists to measure servo loads and verify effectiveness.

When you are uncertain about the geometry where in the slipstream should the tab be for initial testing? I would guess that a few degrees of positive into the slipstream when the aileron is deflected would avoid oscillations or flutter due to linkage slop.

Bill

What do you meen by boost tabs? I've never heard of this. Are you talking about counterbalnces on the ailerons?

I've done this on several planes, yes it does relieve a lot of the servo load. However, I feel that anything over 10% of the area is to much, especially on the elevator. You may have trouble at neautral positions with 15%, it will try to hunt.

You may want to check this out...

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

Hope this helps...

AerodynamicServo/Boost Tabs
Frequently called boost tabs in the full-scale world, thse are the best aerodynamic balancing devices. They're relatively unobtrusive, unlike the odd-looking paddle balancers, and produce liitle drag. Introduced in the 1920s these small, auxiliary control surfaces are mounted in the trailing edges of the main control surfaces, just like oversize trim tabs. Unlike trim tabs, however, theyre actuated by a linkage that moves them in opposition to the control surface.
For example, when an elevator moves to the "up" position, the boost tab moves "down" and tends to lift the trailing edge of the elevator -- thus assisting the servo. The farther the elevator moves, the farther the boost tab moves in the opposite direction to counter the increasing aerodynamic force that opposes the elevator's motion (see Fig.3). The result is a nearly linear assist (almost like hydraulic boost) that's far more efective than putting balancing area ahead of the hinge line, particularly at large deflections. Boost tabs can be retro-fitted without great difficulty to perk up sluggishly responding models. Thanks to boost tabs, one of my large models needed only one servo on the ailerons to get twice the roll rate that two servos had previously provided.
The degree of assist is adjusted by varying the relative travel of the boost tab to that of the control surface. This can be accomplished by experimenting with various linkage holes in the control horns. It can also be overdone, resulting in a control surface that wants to move off center of its own accord; this forces the servo to hold it on center. The most noticeable result of this is a control that just won't trim, as the control surface flips from side to side within the free play that's present in even the tightest linkake.

Boost tabs are usually small, not any larger than a normal trim tab..
On the T-6... and a BT-14

this is a great idea,I know about the trim tabs on light aircraft and the hydraulic boost on large ones but never thought about something so simple to help to move the control surface.

Used on full scale sailboat, they assist self steering devises. Normally maybe 6-8% of the chord of the rudder or self steer rudder/vane.

Everything was made by guess prior to calculating areas. I will test at 15% and cut the ends of the aileron stock if the tab area appears too large.

What is really needed is instrumentation to measure the force at the servo. Maybe measure the current to the servo or a strain gauge in the linkage. Maybe as technology improves the rocket scientists among us will eventually come up with a way for the rest of us to measure servo loads and such. Seems like everyone is always guessing and speculating about the dynamics of the airplane (servo load, battery size, servo torque required, battery voltage under load, rpm, etc.). Oh well … my flying skills are not likely to observe oscillation unless the thing self-destructs. Wish you were close enough to test fly for me.

The concept is fundamentally sound and the boost tabs and linkage were simple and easy to make. So I am forging ahead like I know what I am doing. I don’t.

I have also noticed that the linkage would be extremely easy to connect directly to the boost tab. Pulling the tab into the slipstream would move the aileron. Hum … piano hinges maybe. Oh well … cannot measure the results and shouldn’t destroy the airplane.

Bill


Cabin fever

Too cold to fly, not enough to do, and cabin fever brings on a touch of insanity. Maybe I could hang the wing out the car window at 70 mph measure the servo current and observe the deflection. Another day or two of cold weather and the idea will sound better and better. Oh oh, after only a few minutes of thought the idea sounds better.

The wing will fit inside the van. It sticks out about 30” to the right and can be held in place by the windows. The servo leads are inside the van and an extra RX is available.

Anyone have any thoughts about what and how to measure before I get arrested?

Bill

You're going to be surprised at how quickly the wing out the window vanishes when you deflect the surface.
You're working with many pounds of force, not ounces.

I would say that measuring the current woud give you an idea of the servo load. But as Paul pointed out lots of force involved in the test be sure your wing is secure. Unlike NASA and the aircraft Manufactures we do not have the money for instruments to make measurements so there is a lot of guess work involved...
perhaps a pull scale like used to weigh fish could be connected to the control surface could be used to measure the force to deflect the control surface.

Well Paul I was not planning to turn the wing sideways hanging out the window. I only want to know the force at the servo connected to a 2 1/2 by 20” aileron deflected 30 degrees.

I just finished driving down the road at 55 to 60 mph and the wing is very stable and secure. A slightly better arrangement to make sure the wing is parallel to the slipstream would be desirable but probably not necessary.

I am leaning towards assuming that servo current is proportional to the mechanical force against the servo. If data cannot be found the assumption can be verified on the bench at a later time.

Bill

Huh? I've heard of too windy, too busy, too nervous, late, dark, broken, heavy, and even too drunk.
But too cold? Thats a good one BillS!

go faster...................and take video.

I would say that that is a fairly good point to start. You can take your data and by using something to measure the amount of force you apply during the bench test you can say that x force = x current.
Would you be so kind as to post the data you come up with?
thanks
Paul

It is 41 degrees, feels like 33 according to the weatherman and the flag is standing straight out which means 15 mph plus which is too cold for me.

Bill

Paul,

Oops, a conceptual problem. The wing will not be able to deflect with aileron movement, which will present a worse case scenario that will never be seen in actual operation. Compensation will be necessary to get reasonable data. I suspect the first and simplest approach will be to back the speed down.

I am making amp meter leads now and will post findings even if they appear silly. Will also post a picture of the test rig.

Found it necessary to brief the test driver and wife. She giggled furiously and seemed perplexed.


Bill

I like to set the travel of boost tabs a bit on the low side of what I think will be needed, for initial testing, then gradually increase travel until I get the response that I want. If you use excessive travel, the control surface will try to move off center on its own, forcing the servo to center it, rather than deflect it. This is evidenced by a control that just refuses to trim, since the surface tends to flip and flop back and forth within the linkage slop, unless the boost tab is initially deflected a bit to act as a trim tab, something that boost tabs do very well, incidentally.

I came to grief with an over-boosted rudder that deflected to full travel on its own, after I neglected to properly secure the linkage after adjusting. Had to land the bird in knife edge.

Have done a lot of experimenting with boost tabs and servo tabs, and wrote that article that Iron Eagle posted - it may still be available in Air Age's "How To's, Vol. 2".

That was one of the best articles I had ever read on the subject geared towards modelers...
Please pass on any others you may have done, great work!

Rotaryphile,

I read and enjoyed your work while researching. The information was very informative.


Bill

The meter is a Fluke 73 and the error is probably operator. The readings are incorrect by a minimum of 0.01 which is the reading without servo connected. The meter is not auto ranging on the amp scale and will not resolve in the third decimal place. The no load value could be between 0.005 and 0.015.

Aileron barn door – 2 1/2” by 19 3/4” = 49.4 sq in
Boost tab – 1” by 7 1/2” = 7.5 sq in

Servo - S-148 Futaba analog
Stall current – 0.60 amp
No load current – 0.02 amp

Both pictures show the test setup. The boost tab hinges have not been epoxied since reference values are needed before permanently adding tabs.

The driver returned after dark so the data will be tomorrow.


Bill

Some years back Al Rabe did some airfoil research and used his pickup truck somewhat like you're using that van. He lived in Texas and used flat roads to make the runs. He was looking for lots "purer" data than you are. But a couple of details should be mentioned.

He discovered that he needed to make back and forth runs and average the data. The idea was to consider the effect of the wind encountered at the time of the test. Now, you're not looking for the same kind of test results, but it is worthwhile to suggest that comparing your runs to each other should make some consideration of the winds. If the wind is at different direction and strength on the run with tab versus the run without, unless you're keeping track of the wind, there's a good chance the comparison will be skewed.

He also found that the test item needed to be in clean air, that the airflow near the vehicle was extremely turbulent. It's not only screwed up by the vehicle's body, but the body shields or exposes the test item depending on the winds. His results suggested that placing the test item close to the side of the car and back from the front of the car placed the item into extremely random and variable turbulence.

It's obvious you're looking for quite a different level of test results than Al was looking for. But it is worth noting what he discovered about testing with a car. His advice was to have the item as forward as possible, as far away from the body as possible, and to make runs both ways on as straight a road as possible. The difference in test results might give an indication of the variability of the test setup's effects or of the test conditions (wind speed and direction that day).

BTW, Al was quite an innovator. Seems he isn't the only one.

I wonder if a digital servo might not give readings that would suggest the quality of the surface's neutral position.

It seems that digital servos work to maintain neutral settings and their current draw at neutral should give some idea if the boost tab rigging is causing problems. There is some question about boost tabs being setup with appropriate travel. Seems the digitals would tell you with their current draw at neutral whether or not they're having to keep the surface at neutral because the tab is rigged with too much travel or there's too much slop in the rigging.

Just a thought.

BTW, your tests are kewl. Modelers are amazing people.

Thanks for your compliment re my boost tab article, BillS.

Here is a neat little Java servo torque calculator:

http://www.csd.net/~cgadd/eflight/calcs_servo.htm

I haven't as yet checked it, but it looks to be in the ballpark. Multiplex used to have a similar one on their website, but I haven't been able to find it lately, and anyway, this one also figures in the servo travel, which the Multiplex calculator neglected to do, calculating only the actual torque at the control surface, called the hinge moment.

No accidents. No arrests. Lots of comments from the peanut gallery driver. Lost one of the driver’s towels. Data varied 10% during the NO tab run due to 8 to 10 mph wind and direction of travel. Decided removable tabs would be necessary to verify results on a calm day and to facilitate changes.

NO tabs - to 8 to 10 mph wind
20 mph Right (down) ail 0.10 amp
20 mph Left (up) ail 0.13 amp
-------------------
30
R ail 0.11 amp
L ail 0.14 amp
-------------------
40
R 0.14 amp
L 0.15 amp
-------------------
50
R 0.14 amp
L 0.16 amp
-------------------
60
R 0.15 amp
L 0.20 amp

With boost tab - 15 to 18 mph wind
20
R ail 0.12 amp
L ail 0.06/0.07 amp
-------------------
30
R ail 0.13 amp
L ail 0.09 amp
-------------------
40
R 0.12 amp
L 0.11 amp
-------------------
50
R 0.12 amp
L 0.11 amp
-------------------
60
R 0.09 to 0.13 amp - erratic wind and bad data
L 0.10 amp - bad data and abort


Generally a gain with boost tab appears above 30 mph. The gain gets larger as the speed increases. The 40/50 mph gain is in the 15% neighborhood. The 60 mph current reduction appeared to be in the 30% range but the test was aborted due to wind speed variations. Based on the higher current on right or down aileron the tab was probably improperly centered.

Bill