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

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

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

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

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.

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.

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.

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

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.

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).

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

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

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.

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

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

I put them on my old beater 1/4 scale Corby Starlett's ails. 10 % of ail area. Best performance was about 10% pos. incidence to airflow. I kept coming in on the tab horn and all of a sudden the roll rate just about doubled, or servo had twice the torque. I'll see if I can find a pic.

Da Rock.

I just activated my PM. It was off.

Where is the power for the BOOST coming from ? If added to the main surface power need, the 2 should equal a non- boosted surface.

This sounds like a " Free Lunch " idea.

It is a free lunch. I don't know why the guys with the 1/3 scale Cap 232s and such that have to use two or three servos on the surfaces could have the same thrust with one, using booster tabs. Seems I read somewhere, where the bigger transports, 747s and such, only drive the booster tab with no mechanical,hydraulic connection to the elevators.

How do boost tabs work? It is a simple matter of physics, a small force and a long lever arm pushing at the end of the moveable surface. I've been told that Burt Rutan once flew his full scale VaraViggen with the ailerons powered by a single Futaba 148 servo driving the boost tabs.

You have a valid point, one I have been convinced on for more than ten years. When planes first started getting really big, a few people tried tabs but for some reason they never caught on. I think it's mostly a paradigm. The big names all use multiple servos, so everybody else wants to. Big models could be finished cheaper (maybe, though the labor cost for the airframe would be greater) and maybe a little lighter with better aerodynamic counterbalancing and one servo per surface. But, many people validly worry about the loss of redundancy with only one servo. Also, (and this is a biggie and probably the main reason tabs didn't catch on) with the big 3D models doing blenders and walls and such, boost tabs will not help hold the surface in position against the massive drag forces present during those maneuvers. You would need nearly equal forward counterbalancing such as 25%-33% offset hinging on ailerons (which almost no-one does) and large leading counterbalances on elevator to make up for the reduced holding power of only one servo. Also, with the aileron supported at fewer points, it would have to be built stiffer and a little heavier to prevent flutter, though servo tabs can help with that particular issue if istalled well away from the servo arm. It would really be accomplishing the same thing with a different technique. Offset ailerons hinges are more complex and heavier but you can lose servos if you use them. So if you're willing to do the work, you can save money on servos, wires, match-boxes, servo programmers, and batteries. It's just plain faster and cheaper for the ARF builders to make butt aileron joints than it is to go like full-scale and make offset aileron hinges.

Flypaper,

Thanks for your comments. After your comments I tested in the close hole on the tab horn and the roll rate was diminished. Obviously my linkage is different and the sweet spot was not found. Your front horn is not clear in the picture. Something about the geometry is still a mystery.

Bill

I bet the tab was stalling at the higher deflection, negating it's effectiveness.

Bill :
The horn on the wing is as close to the surface as I could get it and close to the trailing edge of the wing as I could get it. With the deflection of the aileron shown, you can see the deflection of the trim tab is a little lower than the airflow of the wing and maintains that deflection all the way to full aileron travel. The horn pushrod on the trim tab is right over the hinge line as you would do on the elevator or ail horn. When the ail. is at the neutral point. the trim tab is too, although that goes without saying.

Flypaper,

After your comments I tested for several flights at the close hole on the front horn and the far hole on the rear horn. The airplane would twitch in the air but I kept testing anyway. This morning I moved to number 3 hole on the rear horn and the twitch stopped.

I am getting closer. Thanks for your help and comments.

Bill