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Sep 12, 2014, 08:18 AM
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miniphase's Avatar
Thanks Herk, it's ok I'm up to speed on the pitch neutral flaps. My previous wings exhibit the 'front end' stall and they too had pitch/moment neutral flaps.

The nosing down only seems to occur only when the model is near to the ground, almost as if there's a sudden breakaway of the airflow over the centre section as the model comes down through the wind gradient.

I'm hoping by extending the flaps as close to the root as possible (whilst still keeping them pitch neutral) I will reduce or remove the problem.
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Sep 12, 2014, 08:57 AM
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nmasters's Avatar
How close to the ground does this pitch down occur? Is it close to 1 chord length or more like 1/2 the span. There shouldn't be a flow breakdown but it could be some weird ground effect interaction with the flaps and elevon settings

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Last edited by nmasters; Sep 12, 2014 at 09:02 PM.
Sep 12, 2014, 09:54 AM
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Knoll53's Avatar
Quote:
Originally Posted by miniphase
With all my 'flapped' wings I've noticed a tendency for them to dive at the ground on final approach (though this markedly less pronounced with the thicker wing sections).
Really ! Well that's a useful tidbit to know.
Apparently the problem has happened enough to convince you that it is tied to this configuration. We expect the wing to stall upon final flare but it is the abrupt part that has caught your attention..............I assume.
If you are willing to accept a wildly pitch positive flap, then you can extend them all the way to the root and thus have a more even airflow. My SW-1 had pitch positive flaps and were useful with opposite elevon mixed in. That plane has a big gap between the elevon and flap so the flap did not disturb airflow at the elevon, which may be worth considering.
Also at tapered flap (skinny towards the root) should lessen the positive pitch up.
If they are for landing only, maybe split flaps would solve some problems.
Sep 12, 2014, 10:22 AM
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miniphase's Avatar
Norm- It seems to be less about ground effect. I would say (the majority of the time) that the nosing down occurs at a height in excess of 1 wing span altitude. It also seems to be all but absent in very smooth air. An extra bit of down trim seems to help keep it at bay otherwise.

Kent- I like the idea of tapering the flap. If there's any fact in my supposition I ought to take the flaps all the way to the root to get a firm conclusion.

Edit- I am reminded of the Flair30 http://www.delta-club-82.com/bible/7...er-flair30.htm which killed its test pilot with a severe nose dive following flap deployment (if my memory serves)
Sep 12, 2014, 02:27 PM
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Knoll53's Avatar
Quote:
Originally Posted by miniphase
I like the idea of tapering the flap. If there's any fact in my supposition I ought to take the flaps all the way to the root to get a firm conclusion.
I would think that the transition that occurs from a 90 degree deployed flap and the adjacent un-flapped airfoil would create all sorts of boundary layer disturbances and vortex too. Eliminating that transition, by having a continuous flap thru the root would be a good thing to clean up the flow. Based on Nickels formula, this would push a pitch neutral flap waaaaay out on the wing, leaving a potentially undersized elevon.

Based on only TLAR, your wing appears to have somewhat short elevons already, so this configuration might lend itself well to test the optimum arrangement for flaps. That is one that does NOT have any surprise bad behavior when landing and only a little elevon (elevator) compensation needed to pitch neutralize the flaps.

My SW-7 has shown me that trying to neutralize a pitch negative flap with elevon is nearly impossible. It just falls out of the sky (nose down) when I try to do so. I'm guessing that the elevons are stalling. Here's why I think it is the elevon stalling and not the root. The flaps create a very high AoA, so the adjacent root area that is not flapped is at a much lesser AoA. So the root shouldn't stall due to AoA. Maybe it is stalling due to other disturbance reasons, but that seems unlikely.

I like the idea of flaps continuous thru the root. The lift distribution would be much smoother and more efficient and thus justify any needed elevator compensation needed to "make it fly".

BTW, I haven't noticed any surprise or abrupt stalling with my swept wings upon landing. The SW-7 certainly has a tiny flight envelope with it's 4" chord and high wing loading, but that is to be expected. Who knows, maybe I'm just not paying attention enough to notice. To be honest, my landings are more along the lines of crisis management as opposed to a graceful meeting of bird and earth.
Last edited by Knoll53; Sep 12, 2014 at 02:34 PM.
Sep 13, 2014, 06:16 AM
Can I crash it?
cheapncheerful's Avatar
Crisis management!! ROFLLLLLLL!
Yup! I can live with that description!
I do live with it.
Sep 13, 2014, 02:58 PM
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miniphase's Avatar
Kent, I'm thinking that the flapped area has much more camber (with flaps deployed) and therefore a lower stall speed than the inboard (unflapped) area. I'm guessing that when I deploy flaps, the AOA doesn't change (we're aiming for pitch neutral after all),and that the outboard part of the wing is at a lower AOA due to washout, So you therefore have the centre most likely to stall....what do you reckon?

On elevon sizing, my thoughts were the same as yours....too small for aileron function, but if I use the flaps as ailerons too, then roll should be ok. Elevator should be ok I hope!...my wings never need much elevon throw for pitch so I'll just have comparatively more movement. Maybe a smaller area of pitch control surface further from the cg is more efficient than that covering more span? if all else fails, I can chp the flap in two, install 2 more servos and experiment with 6 control surfaces!!
Sep 13, 2014, 05:22 PM
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Knoll53's Avatar
Quote:
Originally Posted by miniphase
Kent, I'm thinking that the flapped area has much more camber (with flaps deployed) and therefore a lower stall speed than the inboard (unflapped) area. I'm guessing that when I deploy flaps, the AOA doesn't change (we're aiming for pitch neutral after all),and that the outboard part of the wing is at a lower AOA due to washout, So you therefore have the centre most likely to stall....what do you reckon?
Certainly there is a lot of wing twist with flaps deployed, so just cruising you won't tip stall, but when you really pull back hard (when landing) and attempt to force that highly cambered section at the flaps to a higher AoA, you might stall the elevons because they are highly loaded and deflected so much that you get a separation at the elevon. In this case the elevons are stalling upside down.

The bigger issue seems to be the disturbance the 90 degree flaps cause to airflow downstream along the swept wing and for that matter towards the root as well. I assume the 90 degree deflected flap has already stalled most of the flow on top of the wing and traded that loss for higher pressure on the bottom of the wing. I have a hard time thinking of this as increased camber (maybe with a 20 degree deflected flap). Just how that disturbance affects adjacent areas is anybodies guess, but it can't be pretty. Some carefully placed tufts of yarn would probably be quite instructive.

The flaps on my SW-7 are proving to be so problematic that they are not useful for landing approaches. It seems to fly the same with full flaps, but any attempt to raise the nose causes an abrupt stall. My vote is certainly for flaps that touch at the root for the sole purpose of a more predictable lift distribution. This may lead to pitch positive flaps which require nose down mixing from the elevons, thus reducing the elevon wing loading ( they are pushing down normally).

Relatively short elevons should be effective as elevator, especially if you can manage an aft CG, in which case they can become too effective.
Last edited by Knoll53; Sep 13, 2014 at 07:24 PM.
Sep 15, 2014, 08:13 AM
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miniphase's Avatar
Quote:
Originally Posted by Knoll53

The flaps on my SW-7 are proving to be so problematic that they are not useful for landing approaches. It seems to fly the same with full flaps, but any attempt to raise the nose causes an abrupt stall .
Wow! I'm amazed that they're proving so troublesome Kent. I've never had those kind of results with my flapped wings. Despite the nosing in trait, all other aspects seem positive....a nice increase in lift when needed, slower landing approaches, and increased response when coupled with the elevons for roll.
Sep 15, 2014, 09:03 AM
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Knoll53's Avatar
I'm pretty sure that the flap limitations of the SW-7 are all about the tiny 4" airfoil and the relatively high wing loading.
Oct 09, 2014, 02:52 PM
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Dec 17, 2014, 05:38 PM
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iron eagle's Avatar
Looking good.
Dec 17, 2014, 06:05 PM
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Knoll53's Avatar
I couldn't help but to notice that you didn't specify which Christmas. My project has been struck by a slow down stick, so I'm glad to see the dust flying on your's.

Nice reflection off the top of the wing.........you're getting handy with those mylars!
Like the sliced up tip rib. Neat trick to get perfect alignment.

I can see the charm of having a big slippery pod for landing. It gives you a fighting chance at skipping over the odd rock or two without wing damage. I would think that touch and goes will certainly be doable, at least at the beach.

Just for fun, I checked your weather forecast on weatherunderground. Holy Moly you guys get some killer storms! Winter flying must be just a bit of a rarity.


Dec 19, 2014, 08:52 AM
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miniphase's Avatar
This Christmas Kent......perhaps. Gliders and deadlines have a wonderful property in common; the lovely whooshing sound they make as they pass by!

The pod helps beef the nose up a bit and gives a bit of room for shoving stuff in....also can help with launches.

As to the weather, we get a real mix from the frontal systems. Can't really predict more than 2 days ahead but we do get as much flying in during the winter as possible. It's only the rain that stops us....though I do remember 'back in the day' flying with a carrier bag over my transmitter!


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