What are boost tabs???
10-30-2011 | 03:38 AM
#26
Senior Member
RE: What are boost tabs???
ORIGINAL: sopwith
Does anyone know the forces that are generated by the boost tab? It would be great to know how large a servo is required. I used a servo force program to try to get the numbers but it doesn't look like it is correct.
Does anyone know the forces that are generated by the boost tab? It would be great to know how large a servo is required. I used a servo force program to try to get the numbers but it doesn't look like it is correct.
10-30-2011 | 06:13 AM
#27
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From: Canon City,
CO
RE: What are boost tabs???
I have some large scale planes with Ryobi engines and thought I would try adding boost tabs to see how they work. I want to just add a tab on the back side of the control surface and see how that works. I think I should keep the size a little smaller since the radius of the tab will be greater than the entire control surface.
Thanks for the info
Thanks for the info
10-30-2011 | 07:26 AM
#28
RE: What are boost tabs???
Sopwith, you realize that you need to run pushrods and linkages to that tab for it to work? It's not just a case of sticking a tab on the rear. It needs to be powered by the servo or a slave linkage setup.
10-30-2011 | 04:58 PM
#29
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From: Canon City,
CO
RE: What are boost tabs???
Yes, I have already installed it with slave links and have it adjusted. If the weather holds up I will try to fly it tomorrow. I only installed the tab on the elevator for now. If it all goes OK I will do the same on the rudder and alierons. The tab on the elevator has a 1 inch chord and 5 inch length. The elevator has a 4 inch chord and is 25 inches long. The tab is attached centered on the elevator.
10-31-2011 | 11:22 AM
#30
RE: What are boost tabs???
The only real way to tell if this is working is to install a recording current sensor on the power lead running to the elevator servo. Or if you've actually had experiences where the model was slow to recover from a dive and you're pretty sure it was due to overloading the servo then it should recover better with the tab working.
The size you made it may be a bit much. Even with that size of elevator I think I'd have gone with about 1/2 inch width at that length or even a bit less. Hopefully the tab you put on doesn't act to remove too much of the elevator authourity.
The size you made it may be a bit much. Even with that size of elevator I think I'd have gone with about 1/2 inch width at that length or even a bit less. Hopefully the tab you put on doesn't act to remove too much of the elevator authourity.
10-31-2011 | 01:03 PM
#31
Senior Member
RE: What are boost tabs???
It looks like your elevator has 100 square inches of area so you could have up to 10% or 10 square inches of boost tab from my experience. You have only 5 or 5% so that should be fine.
10-31-2011 | 05:00 PM
#32
Senior Member
RE: What are boost tabs???
Wobblewobble mentioned having crashed because of a boost tab. I had my rudder servo linkage disconnect once, and the rudder went hard over - more than 45 degrees, with a very large 3-D type rudder. I had to fly the airplane in knife edge to a gentle upwind crash landing that slightly damaged a wingtip. I had been experimenting with the travel of the passive boost tab, where the servo controlled the rudder directly, with the tab assisting.
It turned out that the tab was too effective, causing the rudder to slam hard over when the resistance of the servo was removed. I simply reduced the travel of the boost tab, and everything was OK. The rudder had been a bit hard to trim, because the tab was forcing the rudder to uncommanded movement within the slight slop in the control linkage.
It turned out that the tab was too effective, causing the rudder to slam hard over when the resistance of the servo was removed. I simply reduced the travel of the boost tab, and everything was OK. The rudder had been a bit hard to trim, because the tab was forcing the rudder to uncommanded movement within the slight slop in the control linkage.
10-31-2011 | 09:29 PM
#33
RE: What are boost tabs???
Another setup would be to use balance tabs to reduce the force needed to operate the control surface. The balance tab is passive as it is not directly controlled. Here's a photo of a balance tab on the elevator on an F4U Corsair. The control linkage is attached to a fixed horn on the horizontal stabilizer. The tab automatically moves proportionally and opposite the movement of the elevator. This and the portion of the elevator tips that projects forward of the hinge line combine to reduse the force needed to operate the elevator.
Scott
Scott
11-01-2011 | 03:24 AM
#34
Senior Member
RE: What are boost tabs???
ORIGINAL: Rotaryphile
Wobblewobble mentioned having crashed because of a boost tab. I had my rudder servo linkage disconnect once, and the rudder went hard over - more than 45 degrees, with a very large 3-D type rudder. I had to fly the airplane in knife edge to a gentle upwind crash landing that slightly damaged a wingtip. I had been experimenting with the travel of the passive boost tab, where the servo controlled the rudder directly, with the tab assisting.
It turned out that the tab was too effective, causing the rudder to slam hard over when the resistance of the servo was removed. I simply reduced the travel of the boost tab, and everything was OK. The rudder had been a bit hard to trim, because the tab was forcing the rudder to uncommanded movement within the slight slop in the control linkage.
Wobblewobble mentioned having crashed because of a boost tab. I had my rudder servo linkage disconnect once, and the rudder went hard over - more than 45 degrees, with a very large 3-D type rudder. I had to fly the airplane in knife edge to a gentle upwind crash landing that slightly damaged a wingtip. I had been experimenting with the travel of the passive boost tab, where the servo controlled the rudder directly, with the tab assisting.
It turned out that the tab was too effective, causing the rudder to slam hard over when the resistance of the servo was removed. I simply reduced the travel of the boost tab, and everything was OK. The rudder had been a bit hard to trim, because the tab was forcing the rudder to uncommanded movement within the slight slop in the control linkage.
Interesting experience and good that you saved the model.
But one question. How do you know your fix actually was better, or the change actually did anything at all? Did the rudder servo linkage fail after you reduced the travel? If you think about it, removing our model servos from the system removes the absolutely rigid, constant lock the servo has on rudder position. As long as a servo has a solid grip on model surfaces, we really wouldn't know how much boost the tab gives. OK, unless we put a recorder into the servo wires and checked the amps drawn before and after.
Did your "3-D type rudder" have significant area forward the hinge line?
11-01-2011 | 03:39 AM
#35
RE: What are boost tabs???
I''m a bit confused. Some time ago I had known about servo tabs, and when reading RC forums over a decade ago heard of boost tabs. Never heard of full scale pilots or mechanics calling them boost tabs. Concluded that "boost" tab was an RC slang to avoid confusion with the servo. But now seeing full scale pilots refering them as boost tabs.
Is this the same thing, or is there a differance between boost and servo tabs?
Is this the same thing, or is there a differance between boost and servo tabs?
11-01-2011 | 03:21 PM
#38
RE: What are boost tabs???
It depends on what you want them to do...there are tabs to assist the pilot, tabs to oppose the pilot, tabs to trim, tabs for the pilot to alter the trim...moving tabs and fixed tabs. There are tabs the pilot moves, tabs that move because the surface it's attached to moves, tabs that don't, and tabs that only move after a specific load is applied to the surface it's attached to...but they boil down to three distinct types, those that assist surface movement, those that oppose surface movement, and those that bias the whole surface circuit.
Evan, WB #12.
Evan, WB #12.
11-01-2011 | 06:59 PM
#39
Senior Member
RE: What are boost tabs???
Hi Da Rock: My tab-equipped rudder that slammed hard over when the servo linkage disconnected (because I had neglected to adequately tighten the output arm screw) measured about 18" high, with average chord of about 7", on an 84 inch span full aerobatic bipe. It was activated by a servo with only about 60 ounch-inch torque, which was utterly inadequate at its full speed of about 85 MPH, providing solid rudder authority only up to about 40 MPH airspeed. The tab had a chord of about 1/2", spanning the entire length of the trailing edge, and needed only about half as much movement as the rudder to do a good job, without trying to take the bit into its mouth and overpower the servo. With the tab connected, the relatively weak servo was able to slam the rudder over past 45 degrees nearly instantly, at 85 MPH airspeed
The rudder had no area ahead of the hinge line - I gave up on that idea several years previously. after discovering that boost (or balance) tabs did a much better job. I also found that that balance area ahead of the hinge line seemed to introduce non-linearities into the rudder's hinge line moment that sometimes resulted in a low frequency oscillation of the rudder, while not supplying all that much assist at extreme rudder movement.
That same model's ailerons used only a single servo to obtain twice the roll rate that previously was supplied by two of the same servos.
After experimenting with passive boost (or balance?) tabs for years, I have found that the tabs can reduce the needed servo torque by a factor of up to a maximum of about 85% (80% may be safer), before the control surface may tend to hesitate a bit before going to full travel, particularly with rudders. In other words, relatively small and light servo can do a job that would require a super-torque servo, and such servos were not available when I designed and built that airplane.
I have not, as yet, tried true servo tabs, where the servo moves only the tab, and the mass-balanced control surface is free to weathervane, actuated by the tab only, as in the DC-9, which used a combination of servo tabs and passive boost (or balance) tabs on its elevators and ailerons. Such a setup might easily multiply the effective servo torque by 100 or more, enabling a normal model-type servo to control truly gargantuan models, and probably even human-carrying aircraft, quite handily. Some homebuilt full-sized aircraft use normal model servos to provide electric trim, with good results.
The rudder had no area ahead of the hinge line - I gave up on that idea several years previously. after discovering that boost (or balance) tabs did a much better job. I also found that that balance area ahead of the hinge line seemed to introduce non-linearities into the rudder's hinge line moment that sometimes resulted in a low frequency oscillation of the rudder, while not supplying all that much assist at extreme rudder movement.
That same model's ailerons used only a single servo to obtain twice the roll rate that previously was supplied by two of the same servos.
After experimenting with passive boost (or balance?) tabs for years, I have found that the tabs can reduce the needed servo torque by a factor of up to a maximum of about 85% (80% may be safer), before the control surface may tend to hesitate a bit before going to full travel, particularly with rudders. In other words, relatively small and light servo can do a job that would require a super-torque servo, and such servos were not available when I designed and built that airplane.
I have not, as yet, tried true servo tabs, where the servo moves only the tab, and the mass-balanced control surface is free to weathervane, actuated by the tab only, as in the DC-9, which used a combination of servo tabs and passive boost (or balance) tabs on its elevators and ailerons. Such a setup might easily multiply the effective servo torque by 100 or more, enabling a normal model-type servo to control truly gargantuan models, and probably even human-carrying aircraft, quite handily. Some homebuilt full-sized aircraft use normal model servos to provide electric trim, with good results.
11-01-2011 | 07:10 PM
#40
Senior Member
RE: What are boost tabs???
Here is a link to a MAN article with some pics of my boost tab-assisted 84" span bipe that uses only one standard servo for each flight control to get full-aerobatic control response at its top speed of about 85 MPH.
http://media.radiocontrolzone.com/ma...aneSecrets.pdf
http://media.radiocontrolzone.com/ma...aneSecrets.pdf
11-01-2011 | 07:20 PM
#41
RE: What are boost tabs???
This may help.
http://web.archive.org/web/200502121...oost-tabs.html
check out this thread as well...
http://www.rcuniverse.com/forum/m_10168294/tm.htm
http://web.archive.org/web/200502121...oost-tabs.html
check out this thread as well...
http://www.rcuniverse.com/forum/m_10168294/tm.htm
11-01-2011 | 07:45 PM
#42
RE: What are boost tabs???
I have been working in the airline industry for nearly, well, a long time and I have never heard of 'boost' tabs except in model airplane speak, and even then only in the last couple of years or so. It seems that the 'boost' tabs are the same as full size 'balance' tabs, pilot assistance tabs. As noted, they can be directly driven, driven by surface movement, or driven by surface aerodynamic forces, 'spring' type tabs. They move in the opposite direction of the main surface, and therefore assist the surface movement.
Evan, WB #12.
Evan, WB #12.
11-01-2011 | 08:24 PM
#43
RE: What are boost tabs???
Boost tabs (not to be confused with trim tabs) were used on a quite a few early aircraft prior to hydraulics, the most famous aircraft to use them was the Corsair (F4U).
11-02-2011 | 06:32 AM
#44
Senior Member
RE: What are boost tabs???
ORIGINAL: Rotaryphile
Hi Da Rock: My tab-equipped rudder that slammed hard over when the servo linkage disconnected (because I had neglected to adequately tighten the output arm screw) measured about 18'' high, with average chord of about 7'', on an 84 inch span full aerobatic bipe. It was activated by a servo with only about 60 ounch-inch torque, which was utterly inadequate at its full speed of about 85 MPH, providing solid rudder authority only up to about 40 MPH airspeed. The tab had a chord of about 1/2'', spanning the entire length of the trailing edge, and needed only about half as much movement as the rudder to do a good job, without trying to take the bit into its mouth and overpower the servo. With the tab connected, the relatively weak servo was able to slam the rudder over past 45 degrees nearly instantly, at 85 MPH airspeed
The rudder had no area ahead of the hinge line - I gave up on that idea several years previously. after discovering that boost (or balance) tabs did a much better job. I also found that that balance area ahead of the hinge line seemed to introduce non-linearities into the rudder's hinge line moment that sometimes resulted in a low frequency oscillation of the rudder, while not supplying all that much assist at extreme rudder movement.
That same model's ailerons used only a single servo to obtain twice the roll rate that previously was supplied by two of the same servos.
After experimenting with passive boost (or balance?) tabs for years, I have found that the tabs can reduce the needed servo torque by a factor of up to a maximum of about 85% (80% may be safer), before the control surface may tend to hesitate a bit before going to full travel, particularly with rudders. In other words, relatively small and light servo can do a job that would require a super-torque servo, and such servos were not available when I designed and built that airplane.
I have not, as yet, tried true servo tabs, where the servo moves only the tab, and the mass-balanced control surface is free to weathervane, actuated by the tab only, as in the DC-9, which used a combination of servo tabs and passive boost (or balance) tabs on its elevators and ailerons. Such a setup might easily multiply the effective servo torque by 100 or more, enabling a normal model-type servo to control truly gargantuan models, and probably even human-carrying aircraft, quite handily. Some homebuilt full-sized aircraft use normal model servos to provide electric trim, with good results.
Hi Da Rock: My tab-equipped rudder that slammed hard over when the servo linkage disconnected (because I had neglected to adequately tighten the output arm screw) measured about 18'' high, with average chord of about 7'', on an 84 inch span full aerobatic bipe. It was activated by a servo with only about 60 ounch-inch torque, which was utterly inadequate at its full speed of about 85 MPH, providing solid rudder authority only up to about 40 MPH airspeed. The tab had a chord of about 1/2'', spanning the entire length of the trailing edge, and needed only about half as much movement as the rudder to do a good job, without trying to take the bit into its mouth and overpower the servo. With the tab connected, the relatively weak servo was able to slam the rudder over past 45 degrees nearly instantly, at 85 MPH airspeed
The rudder had no area ahead of the hinge line - I gave up on that idea several years previously. after discovering that boost (or balance) tabs did a much better job. I also found that that balance area ahead of the hinge line seemed to introduce non-linearities into the rudder's hinge line moment that sometimes resulted in a low frequency oscillation of the rudder, while not supplying all that much assist at extreme rudder movement.
That same model's ailerons used only a single servo to obtain twice the roll rate that previously was supplied by two of the same servos.
After experimenting with passive boost (or balance?) tabs for years, I have found that the tabs can reduce the needed servo torque by a factor of up to a maximum of about 85% (80% may be safer), before the control surface may tend to hesitate a bit before going to full travel, particularly with rudders. In other words, relatively small and light servo can do a job that would require a super-torque servo, and such servos were not available when I designed and built that airplane.
I have not, as yet, tried true servo tabs, where the servo moves only the tab, and the mass-balanced control surface is free to weathervane, actuated by the tab only, as in the DC-9, which used a combination of servo tabs and passive boost (or balance) tabs on its elevators and ailerons. Such a setup might easily multiply the effective servo torque by 100 or more, enabling a normal model-type servo to control truly gargantuan models, and probably even human-carrying aircraft, quite handily. Some homebuilt full-sized aircraft use normal model servos to provide electric trim, with good results.
Thanks for the informative reply. I've tried "assist tabs" once and was baffled trying to figure a way to predict how to measure their effectiveness. It was before electrics had come up with recording devices and back when I believed more in the magic of science.
I'm still baffled at how you know that reducing the throw on your tab would not produce the same max movement as before unless you disconnect the servo in flight now that you've made the adjustment.
It's really puzzling to try and design assist tabs for a model when the real world offers so very little information about something they employed so seldom. Pictures of the Corsair's surfaces has been about everything I found way back when. Maybe I should look again, although the price of powerful servos nowadays ain't nothing like it was back then.
11-02-2011 | 04:50 PM
#46
Senior Member
RE: What are boost tabs???
Here are servo tabs I put on a 1/4 scale Corby Starlett quite a few yrs ago as an experiment. I used the 10% rule. I started by moving in on the servo tab horn one hole at a time to avoid overcontrolling it. At the third hole in, it was like I put the next sized bigger/faster servos in the wings. Roll rate was much faster with more authority.
11-02-2011 | 08:57 PM
#47
RE: What are boost tabs???
Da,
Boost tabs were, if I remember correctly,only used on some experimental fighters and some racers (primary on elevator surfaces) just before hydraulic assist became the norm. Hughes did some of the first hydraulic assist systems, and it may even be some of his earlier stuff were boost tabs were used. That is basically the era you'd find any mention of it (end of the "Golden Age"???) just before or near the beginning of WWI.
Boost tabs were, if I remember correctly,only used on some experimental fighters and some racers (primary on elevator surfaces) just before hydraulic assist became the norm. Hughes did some of the first hydraulic assist systems, and it may even be some of his earlier stuff were boost tabs were used. That is basically the era you'd find any mention of it (end of the "Golden Age"???) just before or near the beginning of WWI.
11-02-2011 | 10:06 PM
#48
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From: Christchurch, NEW ZEALAND
RE: What are boost tabs???
Yep this is one of those areas where it seems 'model speak' reigns supreme. When I was taught aerodynamics there were only three types of tabs (and all of Pimmnz's examples fit into these);
Trim Tabs;
Both fixed and adjustable, for trimming the neutral point of a control surface so the pilot didn't have to continually have to hold pressure on the controls, allows for more accurate and less tiring flying.
Never ever try to trim a model plane with these.
Balance (or Antibalance) Tabs
Makes the controls easier for the pilot to move, usefull on larger or faster aircraft (may also be adjustable for trim)
Antibalance tabs make the controls harder to move, this may be reguired where large controls have horn balances. As Rotaryphile says horn balances can be non linear and cause controls to overbalance at large displacements, antibalance tabs can fix this.
Balance tabs can be used on models with great success and allow the normal servos to drive larger and/or faster models.
Servo tabs
With these the pilot 'flys' the tab only. The tab then acts as a servo (not the rc kind) and flys the control surface for the pilot. As Pimmnz points out these normally have a spring connection to the surface so that at low control loads the pilot flys the control surface, as control loads increase the spring deflects allowing the tab to fly the control surface. The pilot only ever feels the spring loading. They may also be used for trim.
I never realised that these were being used in models but clearly they are.
I don't know where boost tabs come from, maybe it's the American name for Balance tabs or Servo tabs, maybe it's a term invented by aeromodellers. Passive boost tabs? I have no idea, did we just invent that term in this thread.
Iron Eagle you would be hard pressed to find a higher performance or heavier aircraft since the 30's or so that doesn't have tabs of some sort, Nearly every WW II aircraft has them. Even the light low perrformance aircraft will have at least a plain trim tab.
Dave H
Trim Tabs;
Both fixed and adjustable, for trimming the neutral point of a control surface so the pilot didn't have to continually have to hold pressure on the controls, allows for more accurate and less tiring flying.
Never ever try to trim a model plane with these.
Balance (or Antibalance) Tabs
Makes the controls easier for the pilot to move, usefull on larger or faster aircraft (may also be adjustable for trim)
Antibalance tabs make the controls harder to move, this may be reguired where large controls have horn balances. As Rotaryphile says horn balances can be non linear and cause controls to overbalance at large displacements, antibalance tabs can fix this.
Balance tabs can be used on models with great success and allow the normal servos to drive larger and/or faster models.
Servo tabs
With these the pilot 'flys' the tab only. The tab then acts as a servo (not the rc kind) and flys the control surface for the pilot. As Pimmnz points out these normally have a spring connection to the surface so that at low control loads the pilot flys the control surface, as control loads increase the spring deflects allowing the tab to fly the control surface. The pilot only ever feels the spring loading. They may also be used for trim.
I never realised that these were being used in models but clearly they are.
I don't know where boost tabs come from, maybe it's the American name for Balance tabs or Servo tabs, maybe it's a term invented by aeromodellers. Passive boost tabs? I have no idea, did we just invent that term in this thread.
Iron Eagle you would be hard pressed to find a higher performance or heavier aircraft since the 30's or so that doesn't have tabs of some sort, Nearly every WW II aircraft has them. Even the light low perrformance aircraft will have at least a plain trim tab.
Dave H
