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Old Jun 19, 2012, 10:37 PM
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Spoiler placement

So I'm making the leap back into sailplanes after a long time away. I built a GL in the 90s while stationed in Germany just to give you an idea how long its been. Anywho...On commercial airliners, the B-52 on the AF Academy and other aircraft, the spoilers are located near the trailing edge of the wings. Why on 2M plus sailplanes are the spoilers consistently built on the back edge of the spar? Is it a function of servo placement, rebuild ease, or general consensus?
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Old Jun 19, 2012, 10:50 PM
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Spoilers mounted just aft of the high point in the wing kill lift in the most effective way possible. They not only disturb the air directly behind the spoiler, the also kill it outboard of that for a way along the wing. These are lift killing devices, not speed brakes.

Things mounted on the trailing edge that pop up or drop down are really speed brakes, helping to slow you down, rather than kill lift.
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Old Jun 20, 2012, 08:56 PM
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[QUOTE=Curare;21944347]Spoilers mounted just aft of the high point in the wing kill lift in the most effective way possible. They not only disturb the air directly behind the spoiler, the also kill it outboard of that for a way along the wing. These are lift killing devices, not speed brakes.

Things mounted on the trailing edge that pop up or drop down are really speed brakes, helping to slow you down, rather than kill lift.[/QUOTE

Hummmmm. Gives me something else to experiment with when building a modified second wing. Just can't leave somethings undone!!!
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Old Jun 21, 2012, 01:43 AM
Art Schmitz
United States, TN, Crossville
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My 'old school' open class experience with spoilers has them located at 1/3 of the airfoil and just outside of the h.stab span.
Std class Aquila, Legionair 140, and a Grand Esprit with conventional tail.
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Last edited by lindart; Jun 22, 2012 at 01:57 AM. Reason: added info
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Old Jun 21, 2012, 02:18 AM
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Outboard placement is a matter of taste it seems.

I have sailplanes similar to what you describe, my old sagitta being the obvious example.

Things have been changed with Mark Drela's Bubble dancer and allegro models featuring a central spoiler. I guess if you have a flat centre section, with tip dihedral, a single central spoiler is a lot less hassle than two.

As for people worrying about the tail being blanketed by the central spoiler, having flowing models with them, elevator authority isn't comprimised. I put it down to the increased angle of decent actually has the disturbed air going well above the fresh flow over the tailplane.
If however you put it on a T-tail it may be a different story.
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Old Jun 21, 2012, 09:29 PM
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Also.. you can have them deploy like doors hinged in the front. These have a tendency to lift due to airflow over the wing if not secure well in the down postion. A second way is a sort of scissor popup kind allowing for a much thinner topside footprint and also much less likely to lift up. These rise up more like a wall form inside the wing
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Old Jun 21, 2012, 09:49 PM
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yup, Like ziz:



orr like ziz!

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Old Jun 21, 2012, 11:40 PM
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Quote:
Originally Posted by Curare View Post
Outboard placement is a matter of taste it seems.

I have sailplanes similar to what you describe, my old sagitta being the obvious example.

Things have been changed with Mark Drela's Bubble dancer and allegro models featuring a central spoiler. I guess if you have a flat centre section, with tip dihedral, a single central spoiler is a lot less hassle than two.

As for people worrying about the tail being blanketed by the central spoiler, having flowing models with them, elevator authority isn't comprimised. I put it down to the increased angle of decent actually has the disturbed air going well above the fresh flow over the tailplane.
If however you put it on a T-tail it may be a different story.
Outboard placement of spoilers is definitely not a matter of taste. I first installed spoilers on my sailplane over 40 years ago primarily to get the weak winged sailplane out of thermals without destroying them. The spolers could not be placed outboard of the stab and the extreme pitch down on spoiler deployment made them unusable for landing. Spoilers outboard of the stab tips were almost essential before transmitters with mixers made it possible to mix elevator with spoiler deployment. I owned an Ava with the center spoilers and found it extremely difficult set the elevator mixer to correctly compensate for spoiler deployment at all speeds. I know of only one Ava flier who prefers the original center spoiler to the new Ava with two outboard spoilers.
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Old Jun 22, 2012, 11:54 AM
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Quote:
Originally Posted by CO Hiker View Post
So I'm making the leap back into sailplanes after a long time away. I built a GL in the 90s while stationed in Germany just to give you an idea how long its been. Anywho...On commercial airliners, the B-52 on the AF Academy and other aircraft, the spoilers are located near the trailing edge of the wings. Why on 2M plus sailplanes are the spoilers consistently built on the back edge of the spar? Is it a function of servo placement, rebuild ease, or general consensus?
You are actually trying to compare control surfaces used for different purposes on very different purpose aircraft.

The 737 uses what might be construed as spoilers actually for roll control and to dump lift. Pay close attention to the control surfaces the next time you fly commercial.
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Old Jun 22, 2012, 06:58 PM
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I could see where spoilers would cause issues at the stabs as they're usually mounted on the fuse. Is the effect that noticeable on T tails or if you move the stab a little lower/ higher in reference to the trailing edge of the wing (with relationship to angle of attach as the glider is coming in). I know that I'll have to build a removable horizontal stab for transportation purposes so I've been studying the workmanship here on the board (mighty fine). Understanding flow separation and how AOA affect flight controls and lift/stall, I was looking at how little separation there is between the stab and wing TE.
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Old Jun 23, 2012, 10:12 AM
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Sign Riser ordered yesterday. Wife goes to visit family for two weeks this weekend. Huuummm. What to do?
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Old Jun 24, 2012, 08:41 PM
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Quote:
Originally Posted by ChuckA View Post
Outboard placement of spoilers is definitely not a matter of taste. I first installed spoilers on my sailplane over 40 years ago primarily to get the weak winged sailplane out of thermals without destroying them. The spolers could not be placed outboard of the stab and the extreme pitch down on spoiler deployment made them unusable for landing. Spoilers outboard of the stab tips were almost essential before transmitters with mixers made it possible to mix elevator with spoiler deployment. I owned an Ava with the center spoilers and found it extremely difficult set the elevator mixer to correctly compensate for spoiler deployment at all speeds. I know of only one Ava flier who prefers the original center spoiler to the new Ava with two outboard spoilers.
So are you saying Drela got it wrong?
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