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All Forums >> RC Airplanes >> Aerodynamics >> flap turbulance
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flap turbulance - 3/18/2008 9:14:56 PM   
fredsedno


 

Posts: 64
Joined: 10/19/2006
From: Winchendon, MA, USA
Status: offline 		Gentlemen,
I am building a 6' Beaver from plans. These plans call for a "slotted flap type hinge.In building a sample set up with Robart hinge points,the flap rotates down and away from the wing appx.1/8-3/16"from the trailing edge of the wing.The pivot point of the hinge is 1/4"below the lower surface of the wing. My question is;Will the gap between the trailing edge of the wing and the leading edge of the flap create sufficient turbulance to adversly affect the flight of the aircraft and/or cause premature failure of the wing components?
What say you,Darock, Opjose, B.Mathews, et al?

Yup,still not the sharpest tack in the box,
Fredsedno
       Post #: 1

RE: flap turbulance - 3/18/2008 10:14:51 PM   
da Rock



Posts: 6695
Joined: 10/11/2005
From: western, NC, USA
Status: online 		Slotted flaps have worked on many, many models. They certainly work on full scale airplanes, thousands and thousands of them.

If you've copied the proportions of the Beaver exactly, you'll have an good chance of having no problems.

If it's an existing plan drawn by an experienced modeler, it'd be close to 100% probability of no problems.

The only thing that would be a worry would be how tightly the hinging held the flaps retracted and extended.

(in reply to fredsedno)
       Post #: 2

RE: flap turbulance - 4/4/2008 1:02:32 PM   
bogbeagle


 

Posts: 37
Joined: 4/21/2003
From: York, UNITED KINGDOM
Status: offline 		I've used similar flap arrangements on an O/D stol model of about 6 feet span. In my case, the hing line was about 3/4" below the wing's lower surface, so there was a very substantial gap when the flaps were extended. I contoured the underside of the wing in way of the flap gap; so as to direct and smooth the airflow. Used about 50 degrees of flap at max extension. Fantastic arrangement...drag city....great fun to fly with.

My only advice to you is to keep everything as stiff as possible; perhaps fabricate ali hinges, it doesn't take long. It will be fine.

(in reply to da Rock)
       Post #: 3

RE: flap turbulance - 4/4/2008 6:02:50 PM   
BMatthews



Posts: 8951
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		From a scale persepective I'd include it as shown if you're after points from the judges.

For the real world aspect of flying I'm on the fence about them in our models. Flaps certainly work but at OUR sizes I feel that some extra research in wind tunnels or using computational flow study that takes "our" reynolds number range into account is needed. To get the most out of slotted or fowler flaps the air off the bottom needs to be focused by the slot over the top of the flap such that the airflow is encouraged to stick to the flaps upper/rear surface and thus encourage a greater downwash force. If that flow isn't there then I think the flap isn't doing much more than a regular plain flap. And with the leakage through the gap it may not even be as good as the plain flap. Without some way to test what is happening I'd suggest that to save a lot of hassles with making the fancy hinging that you just stick with plain hinged flaps. Plain flaps having been shown to work well for us in a number of model styles.


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(in reply to bogbeagle)
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RE: flap turbulance - 4/4/2008 6:14:15 PM   
IL2windhawk



Posts: 387
Joined: 4/7/2006
From: Ogden, UT, USA
Status: offline 		I'm in the idea-generating stages of designing a small electric pusher aircraft.
I contemplated adding flaps, but they would likely end up just a couple of inches
in front of the prop, aligned right with the motor's rotational axis. Anybody have
predictions about what this would do to the airflow over the wings? Assuming that
I could build them strong enough to not get ripped out of the wing and sucked into
the prop, I wonder about the aerodynamic implications of having a flap right in front
of a prop.

I tell myself that I would probably only use them on landing approach, so the prop
would be at low enough speed to have minimal effect. But of course the first time I
had to abort a landing, I might have to raise flaps before I could punch throttle!
Not a good idea at flair speed!

Regardless of the practicality, I'm interested in the aerodynamic considerations strictly
to satisfy my curiosity. Any takers?

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(in reply to BMatthews)
       Post #: 5

RE: flap turbulance - 4/5/2008 8:03:38 PM   
fredsedno


 

Posts: 64
Joined: 10/19/2006
From: Winchendon, MA, USA
Status: offline 		Thanks gentlemen,
I felt reasonably certain you would not only provide a correct answer,but would also provide me with background info. I was not disapointed,my brother who is a licensed pilot had no idea as to what effect a gap would have.
Thanks again,Fredsedno

(in reply to IL2windhawk)
       Post #: 6

RE: flap turbulance - 4/6/2008 6:33:51 AM   
khodges


 

Posts: 5091
Joined: 7/3/2003
From: newton, NC, USA
Status: offline
quote:

ORIGINAL: fredsedno

My question is;Will the gap between the trailing edge of the wing and the leading edge of the flap create sufficient turbulance to adversly affect the flight of the aircraft and/or cause premature failure of the wing components?



On the contrary; the reason for the space is to allow high pressure air from under the wing to pass through the slot and over the flap surface, maintaining a smooth boundary layer airflow and delaying the stall. Like any flap, there is a maximum airspeed recommended beyond which the benefits of the flap are either negated, or there is too much stress placed on the flap mechanism by the speed of the airflow. Used within the proper envelope, there should be no problem with increased or premature failure.

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RE: flap turbulance - 4/8/2008 1:06:01 AM   
TRCX


 

Posts: 177
Joined: 7/15/2004
From: capetownwestern cape, SOUTH AFRICA
Status: offline 		khodges is correct.
the gaps are meant to be there as they re-energise the boundry layer causing a delay in flow seperation which would leat to a stall.. they are known as slotted flaps. as long as you dont extend the flaps at too high a speed they should be fine... they are after all designed to be there
pce
kyle

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(in reply to khodges)
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RE: flap turbulance - 4/8/2008 4:19:39 AM   
beenie



Posts: 379
Joined: 10/16/2002
From: Baton Rouge , LA, USA
Status: offline 		I modified a 1/6 scale Sig Cub into a super cub a few years ago. Although the super cub uses a flap hinge that is more akin to a plain flap, I wanted something else. So I made mine like a Husky's with the offset Robart hinge. I used aluminum tubes to get a little more length out of them. I think that the hinge point is offset 5/8" down and 1/2" aft of the flap LE. The Beaver's flap hinge looks similar. The LE of the flap is rounded into an airfoil shape and the TE of the wing where the flap sits is contoured to allow a nice slot.
Bruce is probably quite right regarding the aerodynamic advantages of a slotted or Fowler flap in a model, but it certainly looks neat and is much more unique and attention getting than a plain flap. As for any degradation in flight control or fatigue in regards to the flap, I havn't noticed any. The cub is not well known as a speedy airplane and I have not been observing any max flap extention speeds. I just throw them out in the downwind leg. I haven't tried any powered dive tests to see how long they will hang on, but I can dive the airplane straight down at idle with no problem. The descent rate is frightening.
With flaps in general, you get the most lift with the least drag penalty around 14-20 degrees (or something like that. It has been awhile since I have seen the graph). After that, drag goes up a lot with little increase in lift. I have mine set for takeoff around 23 degrees, that seems to be where the takeoff is shortest. I'm sure that 20 or 18 or 23.45 degrees would work just as well, 23 is just where it was easiest to program the radio. Full flaps works out to 55 degrees due to the linkage. The only thing that I have noticed is that sometimes the rudder loses some effectiveness with full flaps. Slipping most high wing Cessnas with full flaps is discouraged due to some "funny" airflow getting to the elevator, but I havn't noticed any of that.
Ben

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(in reply to TRCX)
       Post #: 9

RE: flap turbulance - 4/8/2008 4:03:41 PM   
TRCX


 

Posts: 177
Joined: 7/15/2004
From: capetownwestern cape, SOUTH AFRICA
Status: offline 		those flaps on the cub really look the part!!!! ive always loved the look of hinged flaps or fowler flaps... it looks so cool just seeing pieces of the wing sliding backwards and down!!!

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(in reply to beenie)
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RE: flap turbulance - 4/9/2008 12:52:36 AM   
fredsedno


 

Posts: 64
Joined: 10/19/2006
From: Winchendon, MA, USA
Status: offline 		Bonjour Baton Rouge,
Merci,I have copied your post.
Ayuh,not the the sharpest tack in the box, Fredsedno

(in reply to TRCX)
       Post #: 11

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