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  #11  
Old 05-11-2021, 03:43 PM
Cumulo Cumulo is offline
 
Join Date: Apr 2015
Location: KHMT
Posts: 83
Default Asymmetric flap experiment of yore

I once had a very nice BL75 Funk, a pretty good STOL a/c as it was. But since making things faster or in this case, slower, is in my nature, I set out to put some of my NACA reading to practice.

Now, in this thread, the term Split Flap is used to describe an asymmetrical flap condition. Fair enough.

But in the design world a split flap is a type of flap such as on a DC-3 where where the upper wing surface remains intact and an aft bottom portion swings down. This type of flap is more effective than a plain flap but less so than a well designed slotted flap.

My split flap experiment consisted of a door skin plywood strip 1 foot wide and 8 feet long on each side supported by a vertical 6 inch strip supporting it at the trailing edge. This gave the geometry of a 30 degree 20 percent NACA split flap.

Results were out of the book, considerably slower stall but with a somewhat sharper break.

A group of friends there thought the experiment was a great success but one busybody stranger ranted that I might be in trouble if one side collapsed. So, thanking him for his concern and out of curiosity, I removed the support from one side. I then flew it carefully lifting off slightly and, somewhat surprised, was able to easily counteract the asymmetrical condition.

Had I simulated a split flap at 60 degrees, near full effect, I can't say whether that would be the case though.

\OT
For posterity, I should mention that this experiment was undoubtedly the nexus for the Gurney flap.

My close friend took a Polaroid of the flap setup. Then later that day, he went to a get together with Race drivers Phil Hill and Dan Gurney at Phil Hill's house. (My friend's brother was mechanic/houseboy/assistant for Phil Hill.)

My friend raved about how good this flap was, but the Polaroid from behind the plane only showed the vertical support at the trailing edge. So, since wings were already the thing with race cars, Dan, seeing only the flap vertical support at the trailing edge, tried out adding a vertical flap on a existing race car wing and it worked! Ta-da. The Gurney flap.

He tried to patent it but it was prior knowledge. NACA had tried every configuration of a vertical plate previously. Sadly, I discovered today that Dan, a local boy earlier, passed away in 2018. OT\

Ron

Last edited by Cumulo : 05-11-2021 at 03:46 PM.
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  #12  
Old 05-12-2021, 05:51 PM
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avrojockey avrojockey is offline
 
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Quote:
Originally Posted by kaa View Post
Is there a reason to believe that even a decent degree of flap asymmetry would exhaust the roll authority? These are not exactly high lift flaps, unlike the Twin Otter someone mentioned above (doesn't it also deflect ailerons as well when flaps are deployed?). I wonder if after some degree of deflection the split flap action reverses due to adverse yaw. But yeah, testing it seems way too high on the risk to reward ratio scale.
I would guess there are some major differenced in the Van's models considering the various moments and surface areas of the ailerons. More consistent would be the rudder authority, so, in theory, the best way to recover from this situation is increase alpha and use the rudder for roll. Increased AOA and decreased speed will minimize the effect of the flap asymmetry, but make ailerons less effective to counteract the roll...but this is where big rudder comes in. It is very effective at rolling the airplane at high alpha and low airspeed.
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Last edited by avrojockey : 05-12-2021 at 05:54 PM.
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  #13  
Old 05-13-2021, 06:24 AM
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chepburn chepburn is offline
 
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Default A quick analysis

Hi All,

Some of you may have seen an earlier post on a tool I have been working on to allow someone to do a quick what-if analysis on an aircraft.

So, I generated a model shown in image one. I ran the model and the results are shown in image 2.

Unfortunately, the wake impacting the fuselage in this aero model muddies the results too much to get a believable rolling moment coefficient, so, I changed the approach to model the wing alone in four cases.
Case one: Symmetric full flaps
Case two: Asymmetric full flaps
Case three: Asymmetric full flaps plus full opposite aileron
Case four: Full aileron NO flaps.
I did the full aileron no flap case to give everyone a feel for what the rolling moment magnitude means. Everyone knows what an RV8 rolls like. The actual value is a relative one... not an exact one.


Wing alone Rolling Moment:
Case 1 : -0.001
Case 2 : -0.147
Case 3 : -.0084
Case 4 : 0.1429


So when one flap is floating with no aileron, you are going to roll at about the same rate as when you are performing an aileron roll.
If you feed in opposite aileron, you are going to be able to -almost- stop the roll rate... adding rudder might get you stabilized.

But, at 400 feet short final, I think you may still have a very bad day, or at least you will need new pants.
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Last edited by chepburn : 05-13-2021 at 06:32 AM.
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  #14  
Old 05-13-2021, 09:25 PM
kaa kaa is offline
 
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Cool! This is not a floating flap situation, but a fully asymmetric deployment, right?

Results seem a bit counterintuitive though, I thought RV flaps don't really increase lift past the first position, which is approximately the same deflection as aileron (on RV-7 at least). If that's true, how can a single flap create the same rolling moment as two ailerons that are further apart?

Edit: I guess flaps are bigger.
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  #15  
Old 05-13-2021, 11:39 PM
RhinoDrvr RhinoDrvr is online now
 
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For all those cautioning the OP about this test....heís very humble, but is a renowned US Navy Test Pilot, as well as combat vet A-4 pilot. Iím certain heís well aware of the workup concept to flight test.
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  #16  
Old 05-14-2021, 04:55 AM
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chepburn chepburn is offline
 
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Quote:
Originally Posted by kaa View Post
Cool! This is not a floating flap situation, but a fully asymmetric deployment, right?

Results seem a bit counterintuitive though, I thought RV flaps don't really increase lift past the first position, which is approximately the same deflection as aileron (on RV-7 at least). If that's true, how can a single flap create the same rolling moment as two ailerons that are further apart?

Edit: I guess flaps are bigger.
You're not incorrect. That analysis does not take separation into account so the actual lift generated by the flap at 40 degrees will be reduced. I also took some liberties in modelling the edges of the control surfaces, so this wing model generates more lift than in reality.
This analysis is basically a back of the envelope one... CFD is not the 'truth' its just a tool to help visualize the impact of change. My earlier reaction that there isn't a safe way to test an asymmetric condition was a gut response based on past experience, and in the interest of my fellow RVators safety....
That's actually why I gave it a second look using a CFD model to see if I could learn anything.
That being said, I think that if a single flap rod did fail on a full flap down low final, it would be a true test of our own flight test ability... and I'm pretty sure I would fail that one.

Regards,
Chris
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  #17  
Old 05-14-2021, 05:48 AM
chopperchops chopperchops is offline
 
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Quote:
Originally Posted by n82rb View Post
I had it happen in my twin Comanche on a go around one stuck about half way up. Luckily Comanche flaps donít come up really fast and I noticed the roll coming in as I brought them up and identified it immediately. I just stopped bringing them up and matched the m up and came around and landed.
My father had one stick full down on his twin Comanche on takeoff. Sadly it didnít end well.
PLEASE donít try this!
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  #18  
Old 05-14-2021, 11:19 AM
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Ironflight Ironflight is offline
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Ever since I saw Paul’s (the OP) post, I have been thinking about how to do this testing, just to see what kind of rolling moment you’d get, and how much aileron it would take to counteract it. I suppose you could disconnect one flap and remove the pushrod, then use speed tape to tape the flap up (its going to want to trail in flight anyway). Then you get to altitude with flaps up, establish level conditions, and deploy just a tiny amount of flap and see the result. Add a little more, see the result. Record stick forces in roll with a fish scale maybe.

With a good test plan and safety analysis, you could get some great data for the community! Tip of the hat to Chris for the modeling, but in this case, I think you’d need test data to back up the analysis....which would be interesting to compare in itself.

WARNING - this is a thought experiment, and if someone would want to try it, they need to be a VERY experienced test pilot with a deep background in flight test safety!

Paul
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  #19  
Old 05-14-2021, 11:33 AM
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Mel Mel is offline
 
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Quote:
Originally Posted by Ironflight View Post
WARNING - this is a thought experiment, and if someone would want to try it, they need to be a VERY experienced test pilot with a deep background in flight test safety!
Paul
AND A PARACHUTE! Of course, that would go without saying. This would already be on the checklist for an experienced test-pilot.
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  #20  
Old 05-14-2021, 11:44 AM
kaa kaa is offline
 
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I'd probably want a more positive means of securing the disconnected flap than just speed tape. Could also reprogram the stops on the remaining flap so that it can't deploy past a certain angle (adding a physical stop would be even better).
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