Holes in Spoiler or Not

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pictsidhe

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The RAE ones are hit and miss. Quite a few are on the Cranfield site, but I have failed to find many referenced ones anywhere online. I'd love to know of another source. The British Library in London could be worth trying a shopping list at. NACA stuff is far more available. I have acquired a few books with extracts from some. They often cover topics that NACA hasn't looked at.
 

proppastie

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Choices Choices...I read it highest drag and lift reduction no holes or slots......So now will the vibe effects tare off the fabric in the aft half of the wing? Guess I could see how much it affects it and add holes later? NE speed is 65 mph and stall is reported 19 mph.....pretty low Reynolds numbers,.....not 400 mph dive bomber.
 

wsimpso1

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I was thinking that if you get big oscillations with a solid brake, and you give up a 10 percent drag to get a much smoother ride when brake is extended, it would be a decent trade. If you build with no holes and get big vibration amplitudes at an annoying frequency, well, you might remove the brake panels and put a bunch of good sized holes in it to settle it down... And if you get buffeting that lines up with the resonant frequencies of anything else in the airframe, you had better be prepared to do something to drive the vibe frequency someplace else.

The reason holes and slots work so well, is there is still a lot of drag getting air around the edges of the vents. And it suppresses vibration so well because each hole, slot, or edge is shedding vortices at its own rate and strength. Since every holes sees a somewhat different velocity field, every hole will have a different shedding rate and strength. Odds are pretty good that instead of a couple big vortices on a regular cycle, you will have a whole bunch of different vortices shedding at different rates.

I think I would be prepared to change things once I get flying, and be prepared for losing 10-12% of solid brake drag to make it pleasant flying. Then if it turns out your speeds are so low that vibe is no big deal, BONUS!

Billski
 

Hot Wings

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Bumping an old thread:

The first generation or two of composite sailplanes tended to locate the overcenter at the wing root or even in the fuselage. Later generations tended to locate the overcenter inside the airbrake box near the inboard arm. This refinement reduces the loads on the long push-pull tubes to the airbrakes, making their adjustments more robust and reliable.
--Bob K.
How much force is involved here? The reason for asking is that my current CAD project of an existing design seems to have revealed some TLAR engineering ... and the mechanism for the Schempp Hirth spoilers/airbrakes looks like it fits into this category. If I can save a pound or three and some construction time with a redesign, the time may be worth it.

The air loads in the open position are are easy enough to figure out but either I'm missing something or the some of loads are much higher than I'd intuitively think they would be.

The limiting part in the system looks to be an AN276 ball joint but the size of the operating arms at the spoiler would indicate that either they are way overbuilt or there is significant drag difference between the upper and lower plate/fence?

I looked for some pictures of similar mechanisms but my Google skills didn't provide much reward.:(
 

proppastie

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I would still have an overcenter mechanism to keep the airbrakes from sucking out.
whatever you do please absolutely make 200% certain you have a good over-center mechanism or some other POSITIVE locking force on that type of (vertical) Schempp-Hirth spoiler.
Installing my spoilers so I have revisited this thread....

any idea how much force up the lift generates....?

.is it simply the lift divided by the surface area of the top cap? I will have the find the equation of lift in relation to speed......typically the wing loading of this glider is 2 lb/sq ft.....but that has to change in a dive?....aerodynamics is not one of my skill sets.

does bleed air from bottom to top or the inside the wing volume of air vs negative pressure come into play?.....the wing will not be air tight from control openings and attachment fittings etc. If the the whole negative pressure of the wing is pushing up on the inside of the spoiler cap .......that would be a much bigger number than just the lift on the area of the cap.
 

Jay Kempf

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Holes give more edges for more turbulence and so more drag keeps the air from stagnating upstream and building a ramp. Also introduces vortices behind the panel instead of stagnant air. Win/win. Maybe creates less load on the attachment structure for the same deployment distance vertical.
 

Norman

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Spoilers have holes to eliminate some nonliterary in their operation. Spoilers without holes have a delay in both the buildup of drag and loss of lift due to a starting vortex (there could even be some extra lift during the lag period). After a few seconds the vortex breaks up into random turbulence and the full effect of the spoiler kicks in. The airflow through the holes prevents the lag vortex from forming.
 

proppastie

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I had the spoiler caps suck out and flutter up and down on those springs one beautiful day, in a near-vertical dive through the start gate
Did that happen on the recovery from the dive when pulling Gs ?
 

lr27

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If you have flaps and ailerons, the flaps can be used as air brakes. With flaperons, the roll response would be much less when fully deployed. One drawback with flaps is that they allow you to fly slower, so that you can't put them back up to stretch the glide without stalling. Keep the airspeed up and it's no problem. This is based on RC soaring experience, but I see no reason it wouldn't apply to full scale as well.
 

proppastie

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The original Carbon Dragon had flaperons and the flap portion was not effective for landing, which is why they added an air brake/spoiler over the pod/boom juncture. I have eliminated the flap function and added spoilers. I will not feel bad if everything is not perfect, because even Irv Culver and Jim Maupin had problems.....
 

mcrae0104

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One drawback with flaps is that they allow you to fly slower, so that you can't put them back up to stretch the glide without stalling. Keep the airspeed up and it's no problem...
You can always push the stick forward as you retract the flaps, no?
 

lr27

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So you'rd saying they had separate flaps? If not, you wouldn't want to deploy them enough to create much drag, because you'd lose aileron control.
 

lr27

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You can always push the stick forward as you retract the flaps, no?
If you're trying to stretch the glide because your approach is too low, dropping a few feet to get the speed up might be a problem.

BTW, I think one of the holes in a spoiler should have a whistle to scare people off the runway. (Especially if they don't know their back 40 is really a glider port.)
 

proppastie

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If not, you wouldn't want to deploy them enough to create much drag, because you'd lose aileron control.
perhaps that was the problem...they had full span flaperons, I have full span ailerons and spoilers.
 

Pops

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The only thing I can add with regard to holes or not:

The ones without holes stung more.

View attachment 78739

School policy at one time was to wear a belt. I was a bit of a nonconformist. :gig:
I always thought the ones with holes hurt the worse. But, maybe its all in who is swinging .
 

Victor Bravo

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Did that happen on the recovery from the dive when pulling Gs ?
Not a recovery from the dive in the traditional sense, but yes I was pulling a fairly small amount of G to level off at high speed... to stay just underneath the top of the start gate.

So yes, the wings did indeed flex upwards a little during that maneuver and yes this likely did contribute to the event. But the primary contributor to this event was stupidity, and especially the manner in which stupidity was translated into physics.

(This happened at the world's greatest soaring site, Minden, NV, where the airport is at 4750 MSL. The top of the start gate was 1000 meters above that, so just above 8000 feet. Then add the fact that it was earlty afternoon in summertime, with cumulus clouds and the associated heat and humidity. The glider I was flying was a nearly brand new Schleicher AS-W20BL, the heavy and fast racing version of the series. If I recall, the Vne on the airspeed indicator was somewhere around 150-160 MPH.

But diving through the gate at the Vne on the airspeed indicator at that density altitude maaay possibly have created a verrry slightly higher actual velocity. Starting at about 10,000 feet and diving toward the gate, I saw a white flash cross in front of me, and it was a fantastic guy named Rob Sjostedt in his homebuilt racing sailplane, contect number S6, who also was diving toward the start gate. So although the vast majority of the laws of physics simply didn't apply to someone as handsome and gifted as me, the laws of mid-air collisions did apply, and I aborted the start run somewhere around 8500 feet, electing to pull up and out of Rob's path.

At the moment I started to ease out of the dive, maybe 1.5G pull, is when it sounded like I was inside a metal dumpster with an AK-47 going off inside. I thought that I had fluttered the ailerons or elevator, and the probability I would have to leave (we all wore backpack parachutes). A few seconds later, when I realized I hadn't lost any of the controls, and it was still in one piece, I figured that it had been the spoiler caps.

So I found another thermal, climbed back up to 10K, made another start at a slower speed and a lower dive angle, got a good start, and went out on course. Turns out I had the fastest time on that day, and got to give the "winner's speech" the next morning at the pilot briefing. )

Like I said, youth, stupidity, and testosterone is usually fatal, but once in a while the conservative people who design and certify gliders in Germany are smart enough to overcome it :)

Sorry for the mostly OT rant... as the great Jethro Tull song says "let's go livin' in the past". This is the glider in question, at the time and place of that incident:

n20vb.jpg
 
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