boost tab question
 

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

RE: boost tab question
 

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

RE: boost tab question
 

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.

RE: boost tab question
 

You may want to check this out...

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

Hope this helps...

RE: boost tab question
 

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.

RE: boost tab question
 

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

RE: boost tab question
 

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.

RE: boost tab question
 

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.

RE: boost tab question
 

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

RE: boost tab question
 

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.

RE: boost tab question
 

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.

RE: boost tab question
 

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

RE: boost tab question
 

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!;)

RE: boost tab question
 

go faster...................and take video.:D

RE: boost tab question
 

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

RE: boost tab question
 

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

RE: boost tab question
 

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

RE: boost tab question
 

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".

RE: boost tab question
 

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!

RE: boost tab question
 

Rotaryphile,

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


Bill

RE: boost tab question
 

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

RE: boost tab question
 

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.

RE: boost tab question
 

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.

RE: boost tab question
 

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.

RE: boost tab question
 

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

RE: boost tab question
 

The airflow under the wing is at a higher pressure than over the top, may be part of the difference in amp readings.

RE: boost tab question
 

Incidentally I was surprised at the low current draw in all circumstances. At 60 mph and full surface deflection (w/o tabs) the current was 1/3 of stall current.

Not having a way to make simple in air measurements is a pain.

Dirtybird’s web site with many current vs. torque plots indicates torque is linear to current for most purposes between 10 and 85% of stall current.
http://www.servotestsite.com/


Bill

RE: boost tab question
 

Rotaryphile,

The weather will present a flight window for a few days later this week. How would you suggest testing? The boost tab hangs off the back end of the aileron and they are removable. Flying skill is marginal.

Primary interest is to determine the reduction of servo torque. Intuitively I believe the airplane will fly better.

Bill

RE: boost tab question
 

The airplane has been flown with and without boost tabs and I cannot confirm that the servo load in flight is bigger or smaller. Would welcome a simple method to determine flight load reduction.

Bill

RE: boost tab question
 

You will not notice a difference in flying qualities if the servo had plenty of torque before addition of the tab. The least expensive experiment might be to try to do several "identical" flights with and without the tabs, each starting with a fully charged battery, and see if the battery is "fuller" on average at the end of the boosted flights. This might not be conclusive if the difference is small, the temperature changes, etc. Otherwise you might need a recording of in-flight current draw. If the difference is so small that you can't tell a difference this way, one might argue that they don't provide enough benefit to be worth the extra trouble in your particular application, though they might provide some increased life to the servo and/or battery.

RE: boost tab question
 

There is an inflight measurement device which records current draw.
The name escapes me, but those that have used it have found that large electric pattern planes use much less current than similar sized i.c. powered planes for the same manuvers and flight durations.

RE: boost tab question
 

Like mesae says, if you are not pushing the capability of the servos, you will see little difference in performance. However, one exception to this is, if you have weak (springy or bending pushrods under minor loads) you will see a big improvement as the backforces on the pushrods is reduced.

RE: boost tab question
 

Tab linkage

Originally I thought the boost linkage should be set so the tabs would always be parallel to the air stream. The horns are at center of the hinge line and the linkage is in equal holes, which makes the tab always parallel to the airflow over the wing.

Tab parallel to airflow
The tabs seem marginally effective.

The tabs seem to be more effective when the rear of tab pinches towards the wing with aileron in the up position. – unequal holes – front in #4 hole and rear in #3 hole
The ailerons seem to be more effective but some twitchiness has developed. However it was windy today.

Has anyone verified where the tab linkage is most effective?

Bill

RE: boost tab question
 

You get to decide that based on your particular installation. If your ailerons have indeed become twitchy then they are over-boosted and you should reduce the throw of the tabs. Otherwise you may increase the risk of flutter.

RE: boost tab question
 

I've always had best luck with the boost tabs staying in alignment with the fixed surface (parallel with the main airflow as mentioned above).

RE: boost tab question
 

Thanks Rodney and mesae.

I am reducing the size to approximately 12%. Since the tabs are will be off I will fly another reference test and then test at reduced size. I still have not been able to prove that the servo load has been reduced.

Bill

RE: boost tab question
 

Flew with and without boost tabs. There is about a 15% to 20% difference in effectiveness with tabs. Still cannot prove that the servo load is less. Tabs that are about 12% of aileron seemed to stop the twitchiness. I know that you told me to use 10%.

Bill

RE: boost tab question
 

Bill S

I have a old article by Mike Leasure who ran his car down a runway to get actual Control-Surface loads for different surface areas at different angles. The thrust to deflect and hold position is in OZ. IN. Would you like me to scan them to you ? 10-30- and 60 SQ. IN. were done at up to 45 degrees.

RE: boost tab question
 

How about to everyone here? Like attach them to the next post.

RE: boost tab question
 

cyclops,

Thanks but I don’t know how the data would be used.

I believe the loads are less with a boost tab and the concept is sound from an engineering perspective. But I cannot come up with a method to probe how much less.

Bill