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Slotted flaps, am I on the right track?

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flyingforx

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Not sure I my markings are dark enough, but I posted some photos of my recent flap design. Question: is the gap up top large enough to provide sufficient flow over the top of the flap? I notice that other designs have an inch or greater at full deflection, I assume lowering the hinge point would bring the flap further back, creating a Fowler effect, but would create a larger gap. First photo is flaps up, second is at 50 degrees the half circle is 3/8 inch around my planned trailing edge of the wing. The line at the nose of the extended flap is 1.5% above the chord of the wing. Any input will be greatly appreciated. Maybe I'm off track, but at least I'm still on a hinge.
 

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pylon500

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I dare say there has been a reasonable amount of research done on flaps, but I'll throw in my two cents worth...
Working on the assumption that flaps are usually used at low speed, and as such down at velocities that airflow can start to separate, I like to have the leading edge of the flap slightly protrude into the upper airline as they are deployed.
I have seen remote hinge flaps that drop down as they rotate and feel the airflow coming from bottom to top around the flap leading edge, tends to get dispersed into the turbulent air coming across the top of the wing, basically just generating drag.
This movement is created by moving the hinge point aft rather than down.
The only thing to watch is whether the flap is intended to 'seal' when fully up or still allow a little airflow, I'm often dealing with flaperons, so there is always a gap.
Now going to read the above report...
 

Old Koreelah

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Been there, done those design drawings, FF.
After studying lots of different flap installations (including at Taree airport) I decided to base my flaps on Jabirus’s. Their offset hinges create a gap of about 10mm when fully open, so I presumed that would feed sufficient air over the leading edge of the flaps to energise airflow.

What I didn’t allow for was the boundary layer thickness; Jabiru’s slippery fiberglass skin would have a much thinner boundary layer than my rough fabric skin.
I should have offset my hinges much more to ensure a much bigger gap.
 

Jay Kempf

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The whole idea of the gap is to blow over the upper leading edge of the flap to attach the flow at higher AOA. You have to get it right or it will be no better or worse than a plain flap. The leading edge of the flap must be positioned when deployed such that the flow from the upper surface of the wing and the energized air from the slot merge and don't become turbulent. You also have to make sure the slot is small enough to provide just the right exit velocity. The NACA report above gives all those relevant data. If the goal is to have a large portion of the wing achieve a large CL like more than 2.0 then you have to get it just right.
 

flyingforx

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I'll definitely read up a little more and see if everything agrees, the LE of the flap rolls nicely on the 1.5 above Chord line. The hinge on the wing side is planned to be 3/8 .049 tubing with Bearhawk control lugs at the end, the flap side is going to be similar to an RV-10. My biggest concern will be the spar itself. Below is the wing layout I figured would be best for the larger surfaces. Interesting enough the hinges are shorter, but I imagine the fact they are lower may mean they may have more of a twisting force. Would I be wise to talk to a structural engineer to compute these loads and see if they will work? If so, anybody have any recommendations? In my maximizing of performance I want to do it right and not just slap it on. As I'm not an engineer, I'm taking a simplified logical/romantic approach which isn't always right... I have built a few planes and not completely out there..well hopefully. The red lines are compression members, green-drag wires, pink are the hangar locations.
 

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flyingforx

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Here are the diffences between the two hinge lengths, the flap themselves are larger... 17 inches chord vs 13.25 stock. Seen it done on the backcountry gen 2 cubs with success. Except I'm building my flaps from the ground up vs them modifying the original flaps.
 

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flyingforx

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After reading the article, it seems like the slotted flap in the diagram and optimal nose dimensions match up at 50 degrees, but that's it. I understand that you have to get it perfect for it to work, does one usually hinge for one optimal deflection or I guess there is a way to hit all the ticks (which seem very Fowler like when I plotted them). Also the flap nose... It that a fixed point on the flap or does it roll downward as the flap changes pitch? Another thought, I was thinking about building the trailing edge of the wing to where it can be easily removed and trimmed to adjust the slot, this may be less than ideal method of fine tuning?
 
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flyingforx

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Attached are some pictures of my flap of where it lies vs what is considered ideal. It seems to get pretty close as it approaches 50 and 50 seems dead on. Is this a good optimal location or is it better to improve it in the 10-30 degree range for takeoffs?
 

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Jay Kempf

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Something is off in the pivot location or the upper flap LE geom. You want the gap to be very consistent until the flap closes and tucks in going almost forward at the top of the arc. That upper leading edge has to be a circular arc so that you can control all that and then it should blend tangent at the upper rear surface loft line of the flap. If the gap opens up with varying gaps it will give non linear CL change with flap angle. That could feel really weird like you are sinking until you get to the last notch of flap.

The flap pivot should be hear the leading edge of the flap retracted and below however you intend to deploy.
 

wsimpso1

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After reading the article, it seems like the slotted flap in the diagram and optimal nose dimensions match up at 50 degrees, but that's it. I understand that you have to get it perfect for it to work, does one usually hinge for one optimal deflection or I guess there is a way to hit all the ticks (which seem very Fowler like when I plotted them). Also the flap nose... It that a fixed point on the flap or does it roll downward as the flap changes pitch? Another thought, I was thinking about building the trailing edge of the wing to where it can be easily removed and trimmed to adjust the slot, this may be less than ideal method of fine tuning?
Which article are you referring to? I pointed to TOWS chapter on high lift devices. If you search on "flap" and "Orion" or "wsimpso1" I think you will find discussions of the topic. For slotted flaps we like the airfoil looking flap and off airfoil pivot because you can pretty well seal off the flap when stowed for great cruise performance and then at max flap get a lot of Cl, and it tends to behave well at intermediate settings.

When I did mine, I worked a bunch of stuff:
  • Flap chord and area;
  • How much hump to put on the flap top surface and how far forward for max thickness;
  • Set max flaps down speed;
  • Surface hinge moments with pivot at flap leading edge;
  • How far aft I needed to get the hinge line (aero balancing some of the hinge moment) to be able to deploy flaps at max flaps speed;
  • Set max flaps angle and hinge line height;
  • Set lip position;
  • Checked slot depth and overlap;
  • Checked fit as flaps came up;
  • Then I iterated the design.
To make it all work, the forward third of the upper surface is a little lower than I might like, but the slot and flow shape through the slot and onto the top surface looks good at all settings. Most of the info out there really goes no further, so you end up having to test and then think about VG's etc with a good estimate of boundary layer thickness when you do.

Billski
 

wsimpso1

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Attached are some pictures of my flap of where it lies vs what is considered ideal. It seems to get pretty close as it approaches 50 and 50 seems dead on. Is this a good optimal location or is it better to improve it in the 10-30 degree range for takeoffs?
Looking at your photos, I have a couple comments:

Your flap thickness really looks big in these photos. Are you sure you want them that thick? Anybody else flying flaps on your type of bird that thick? IIRC using 0.15-0.3%c at the trailing edge and maybe that much above the airfoil contour added over the upper flap surface. Low surface just follows the airfoil line.

I can not see the whole flap shape, but TOWS data encourages a flap that looks like a nicely shaped airfoil, as does Orion's extensive history on the topic.

The big trick for me was getting the flap stowed nicely, getting the flap and lip relationship right at max flaps, and then finding the hinge line that did both while being deployable at max flap speed with one hand on the handle.

Good luck,

Billski
 

wsimpso1

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I'll definitely read up a little more and see if everything agrees, the LE of the flap rolls nicely on the 1.5 above Chord line. The hinge on the wing side is planned to be 3/8 .049 tubing with Bearhawk control lugs at the end, the flap side is going to be similar to an RV-10. My biggest concern will be the spar itself. Below is the wing layout I figured would be best for the larger surfaces. Interesting enough the hinges are shorter, but I imagine the fact they are lower may mean they may have more of a twisting force. Would I be wise to talk to a structural engineer to compute these loads and see if they will work? If so, anybody have any recommendations? In my maximizing of performance I want to do it right and not just slap it on. As I'm not an engineer, I'm taking a simplified logical/romantic approach which isn't always right... I have built a few planes and not completely out there..well hopefully. The red lines are compression members, green-drag wires, pink are the hangar locations.
Hmm. I have not done much looking at strut braced wings, so take all of this with at least a grain of salt.

Drag spars on structural skinned wings (single strut) tend to be little more than just the bottom skin brought up to the top skin and braced with the ribs.

Go fabric covered and two struts, and now both spars need to carry some of the bending accumulated from the tip to the strut mount AND compression imposed by the strut loads, which can make the drag spar kind of substantial, and maybe allow placing the hinge points where ever you want.

Until I knew better, I would sure consider the drag spar to be pretty flexible. If the drag spar is a flexible flyer and the skin is fabric, then I sure would want to place the hinges where a rib also connects to the drag spar.

So, let's do some benchmarking. I call this Monkey-see-monkey-do engineering, and it has a lot to recommend it. I just went to the basement and checked the Buttercup plans. Steve Wittman used a pretty substantial wooden drag spar and then spread his flap hinges evenly along the flap, not aligned with ribs at all. What is the practice in the Super Cub and Bearhawk world? Do they run slotted flaps with off-wing hinges? Do they have stiff sturdy looking drag spars? Do they hang the hinges at ribs? Lots of good practice exists in these airplanes.

Billski
 

BJC

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Attached are some pictures of my flap of where it lies vs what is considered ideal. It seems to get pretty close as it approaches 50 and 50 seems dead on. Is this a good optimal location or is it better to improve it in the 10-30 degree range for takeoffs?
Attached is a photo of my Glasair slotted flaps jig. The Glasair slotted flaps reduce the standard plain flap stall speed by 5 MPH, and also lower the nose on approach, providing significantly better forward visibility.airplane2.jpg
Note that the upper wing surface / trailing edge will be modified to meet the flap upper surface at the correct position.

Here is a photo showing the modified TE:

Wing 2.jpg

BJC
 

Jeffd

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Overland Park, Kansas USA
Attached is a photo of my Glasair slotted flaps jig. The Glasair slotted flaps reduce the standard plain flap stall speed by 5 MPH, and also lower the nose on approach, providing significantly better forward visibility.View attachment 106592
Note that the upper wing surface / trailing edge will be modified to meet the flap upper surface at the correct position.

Here is a photo showing the modified TE:

View attachment 106593

BJC
just this week I saw a GI TD with slotted flaps. Beautiful workmanship. My thoughts immediately went to h-tail volume and elevator effectiveness. Could be fine but I am curious. At this point, for me, I am happy with the pattern behavior of my I TD with the standard h-tail.
 

flyingforx

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So where do I start?
I just reread the thread that Orion posted on and everything seems to line up. Airfoil is thicker,think I'm going to try to match it up with Naca 4416.7 if there's such a thing. Right now, I just used a full rib of an old wing along with a printout of USA 35B, chopped off the rear 17 inches, and started with a rough shape of a flap similar to Piper's. The article I was referring to is the first reply that BJC made. Very useful so thank you for that. As a result, I have refined the airfoil shape to something more contemporary. I was thinking the thicker the flap, the greater the lift and everything is somewhat proportional to the stock piper flap, I wanted to slot it mainly to get the nose down on approach as pacers are notorious for nose high approaches that ride on power for shortest landings. I'm shooting for peak geometry at 50 degrees. I feel like I may be overbuilding the setup slightly as each flap will have four hinges and two bellcranks, each hinge will have a compression member adjacent to it for direct transfer of loads to front spar (except for outboard which will be 10 inches off. As for horizontal, I'm starting with enlarged tail feathers from a cub that is now 20 inches further back and placing Naca 0008 ribs on the horizontal. The engine is not moving from original position so I'm slightly concerned it will be heavy on the tail, which isn't great for ski operations. With that said, I'm pretty sure nobody has ever placed flaps this big on a pacer. I'm using the old outboard aileron hinge as my outboard flap hinge. Ailerons are going to be 20 inch chord by 4 feet which actually match very closely with stock surface area (assuming I can fit everything in there). The bellcrank to the ailerons is going to have another arm in case I feel that roll spoilers are needed after test flight. Some will find my wing to be similar a shortened version of double ender. Wish I could chat with those guys as they probably have a bunch of good info, but you know. I'm kinda enjoying all this R&D stuff.
 
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