Slotted flaps, am I on the right track?

[TFF]

The Backcountry Cub I have seen has fixed slats on the flaps, just like a smaller version of the wing leading edge ones. The slats tuck into the flap cove when up. Flaps up supposedly the plane makes 2000 lb of lift at 20 kts. Flaps down, who knows. I do know on floats he can land and takeoff in a 300 ft lake.

 

[Jonny o]

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.

Flyingforks,

My background is building Backcountry Revs, Mackey SQs and Experimental cubs.

20 inch fuselage extension: Great mod. Converts a Pacer to a Producer, which can be Certified. Twice as effective as larger tail feathers and only 3 pounds more weight. Larger tail feathers add 2 lbs.
Might have to move engine forward ( Which do you have? Electric start ? Prop type ? ) if you are planning on hauling loads over 700 lbs. As the need for nose weight starts to get really important.

20 x 48 inch flaps are much shorter than stock Supercub flaps that use 1 bell crank. Double bell cranks are not necessary on 98 inch flaps that are stock chord. 24 x 144 inch flaps require double bell cranks if the flaps aren't very stiff in torsion and the bell crank is not centered.

Keeping the same hinge point on longer chord flaps will make them harder to pull. With your 48 inch length it should be fine. The key is to move the pivot point back so less effort is required; not by adding more cables, springs and bell cranks, to deal with extra load of an unbalanced system.

Adding thickness on the top of the flap, that protrudes above the wing will help "pull" the flaps down. This is because of the increased suction (lift) on the forward part of the flap. Don't worry about it adding drag. The opposite will happen. By the time the air makes it around your fabric airfoil to the flaps, the boundary layer is over .5 thick, in cruise. Much more at higher angle of attack. Having the flap or aileron stick up 3/8 to 1/2 will help reattach the flow and your speed should increase. Husky's, Carbon Cubs and more modern acrobatic planes use this technique.

Jonny O

 

[Jonny o]

The Backcountry Cub I have seen has fixed slats on the flaps, just like a smaller version of the wing leading edge ones. The slats tuck into the flap cove when up. Flaps up supposedly the plane makes 2000 lb of lift at 20 kts. Flaps down, who knows. I do know on floats he can land and takeoff in a 300 ft lake.

Completely empty (1350), with a 150 pound pilot, a Backcountry Rev 3 will land with no wind and full flaps at 20 mph. With no flaps it lands at 36.

Never seen a Backcountry Rev 3 take off from a measured 300 foot lake, on floats. I tie my float tails to shore with rope that snaps at 850 lbs, so I am at full thrust (950 lbs) without moving. Even doing that it would be sketchy.

The airplane is amazing.

 

[TFF]

A friend pointed at a lake he lands at just because he can and no one else in the neighborhood can. There are a couple that might be in the ballpark soon. He has a Whirlwind three blade on his. He showed me some of the switch back streams he lands on. They were hardly anything. We were not flying in it at the time.
If someone didn’t tell me it was almost two feet longer, I would have never known although the fuselage is a little more boxy.

 

[flyingforx]

Hi there Jonny o,
I almost didn't lengthen but decided to since I wanted to upgrade the flaps and thought it would be a good way to do so. Running an O-360 with a whirlwind c/s prop. Otherwise light on everything else up there. The flaps I plan to add are 17 inches by 120 inches, I originally thought I would build 48 inch by 20 inch chord ailerons but have read recently that greater than 30% chord for the ailerons might not be the best. I might have to shorten the flaps 8 inches or so if I decide to reduce the chord or the ailerons to maintain effectiveness. It might be better that way anyhow. Haven't decided yet. I'm redesigning the hinge to match the flap I think it will be smart to add a second bellcrank as I am using actuating points at hinge 1 and 3 from inboard side. Four hinges on the flap to maintain equal distribution for both the rear spar and flap. It also lands nicely with the compression members as well. I think there is a layout I added earlier on this thread.

 

[flyingforx]

TFF, believe it or not. I have made it out of the water in 300 feet a few times with my kitfox 4. Light weight is definitely in my favor in that aspect. By no means you will find me in a 300 ft lake with my plane though...

 

[Jonny o]

Changing the chord of ailerons or the hinge point can be an expensive, both in money and time. Doing both can deliver some large performance gains, on STOL aircraft.

1. 30% ailerons with the same hinge point as 20% ailerons are going to be heavier to actuate. ( plain old leverage )
2. Keeping the same hinge point is going to make the 30% ailerons tail heavy ( more prone to flutter )
3. Having the aileron spar in the same place for the 30% aileron as the 20% is going to make them less stiff ( more. prone to flutter )
4. 30% ailerons allow for larger flaps.
5. 30% ailerons allow the aileron area to be further outboard ( more leverage )
6. 30% ailerons will have a higher local Reynolds Number ( higher efficiency both for stall avoidance & force )
7. 30 % ailerons are going to be more efficient per area.

Take an 8 foot 2x4 and stand up to your waist in a swimming pool. Hold the board in the middle and curl it downward 20 degrees like an aileron. Now hold the board out away from you. Try and force the board (With the same mass and area ) down at a 20 degree angle. The board sticking out away from you is going to take much more force to move, but it will also have much more resultant force. With an aileron or flap we can move the hinge point and get the much larger resultant force with small a actuation force !!!

The hinge point that Piper used was close to the quarter chord point. It made good structural and aerodynamic sense.

Jonny O

 

[Jonny o]

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?

Flyingforx,

The nose of the flap in your first post looks really good. The nose of the wood mock up looks pretty sharp on the lower edge. With full flaps the separation point will likely be below the sharp edge. If so, there will be a high probability that the boundary layer on the upper surface, of the flap, will separate ( stall ).

A rounded upper surface will help the flow follow it around the corner. ( Coanda Effect ) Stick the side of a glass under the faucet and watch the flow bend around and follow the outside of the glass.

Jonny O

 

[Pilot-34]

How can the slipstream best be applied to a flap?
Assuming you have a propeller you can move would it be best centered slightly below the airfoil directly in line with the airfoil slightly above the airfoil or significantly above the airfoil?
It looks to me as if the Custer wing works so much better because it channels was slipstream above the airfoil

But I am unclear how is it working relationship to a flap

 

[BJC]

Answers here Slotted flaps, am I on the right track? | HomeBuiltAirplanes.com


BJC

 

[Pilot-34]

I guess I’m just not seeing it.

 

[Jonny o]

How can the slipstream best be applied to a flap?
Assuming you have a propeller you can move would it be best centered slightly below the airfoil directly in line with the airfoil slightly above the airfoil or significantly above the airfoil?
It looks to me as if the Custer wing works so much better because it channels was slipstream above the airfoil

But I am unclear how is it working relationship to a flap

Pilot-34,

NACA/NASA wind tunnel reports show the following at general aviation speeds and airflows. Remember that air acts very differently at 30 mph than it does at 100 mph. It acts even more differently as the airfoil gets smaller (chord).

With no flaps and a propeller in front of the wing the center of the slipstream should be 5 percent below the wing for highest lift. Best L/D is also below the wing. ( Propwash is not a steady flow. If it was a perfect steady flow the slipstream would make the most lift slightly above the center of the wing, which is called UPPER SURFACE BLOWN )

With flaps and a propeller in front of the wing the center of the slipstream should be at the center of the deployed flaps.

The increased lift of a Channel wing cannot be directly correlated to a flat wing. Each pulse of the prop on top of a flat wing causes huge flow disturbances. Air rushes inward (sideways) toward the prop and then away as each blade passes. This does not happen when the prop is in a Channel Wing duct.

Jonny O

 

[Pilot-34]

Doesn’t the increased flow of the prop stream under the wing cause lower pressure there ? Sucking the wing downward ?

 

[Jonny o]

Doesn’t the increased flow of the prop stream under the wing cause lower pressure there ? Sucking the wing downward ?

Yes. Anytime there is an increase in speed there is a drop in pressure. There are dozens of places on an airplane cowling, wing, gear and etc. that cause negative lift. The trade off is that the over all lift is positive.

But, that is only focusing on a very small part of the big picture. To quote John Roncz, "Flaps throw ( a large mass/weight ) of air at the ground".

When a wing stalls you only loose the suction part of the lift on top of the wing. You still have the larger proportion of the lift on the bottom of the wing pushing up AND air, which has mass, getting thrown at the ground.

Air that makes it from the bottom of the wing through a well directed flap slot helps pull air on the top side of the wing down. That is why above 30 degrees a slotted flap makes more lift than a split flap.

 

[Pilot-34]

Then why not use the additional flow from the propeller and lace the entire propeller below the wing blowing against the flaps?

 

[flyingforx]

Johnny O,

So if I have 114x17 in slotted flaps on a hinge dropped three inches, would you run two bellcranks or one? I guess my concern would be loads on the piper rear spar, all hinges are within a few inches of compression struts. I remember you mentioning flap stiffness, as I am building the flaps from the ground up with 16 ribs each I don't think it'll be an issue.

 

[Jonny o]

If the flap is not stiff in torsion then you need 2 bellcranks.

A one piece leading edge without the cutouts for the Piper flap hangers will give you a lighter and very torsionally stiff flap. Several people have built flap hangers that connect as a truss to the rear spar and do not depend, as much, on the false spar (wing trailing edge) for vertical stiffness. This also helps limit spar twisting. On planes that have a 2400 lb gross weight it helps to run the flap hanger to the front spar. Piper spars fail because of twisting, not vertical movement. The most important thing is to just not engage the first notch of flaps below 60 (I start at 50) and full flaps until 40. A tangent is that if you wait; there is very little or no trim change needed.

Moving the pivot point, of the flaps, a half an inch aft, would make a huge difference in the amount of effort to apply the flaps and the twisting load on the flap hangers and spar. When this was done on the 24 inch chord flaps double return springs and bent flap handles went away. I have never built 17 inch chord long flaps. Only 24. If I did I would put nut plates on the outside bottoms of the flaps, so I could move around the flap attach point bracket to find the sweet spot.

 

[Jonny o]

Then why not use the additional flow from the propeller and lace the entire propeller below the wing blowing against the flaps?

I don't understand what you mean by lace.

Propeller flow is a circle not a rectangle. With the diameter of the propeller arc centered on the center of the flaps, you would have the greatest amount of flow hitting the flaps.

The width of the high speed air, 3 feet behind the propeller, is about 7/8 the diameter of the propeller in cruise. Around 3/4 at 30 mph and 5/8 at zero forward speed.

Jonny

 

[flyingforx]

Attached are pictures of the hinges I have built. They are basically Bearhawk style wing side (spherical bearings go in the holes) and RV-10 flap side. I rounded the front of the flap itself as you mentioned. So you wouldn't be concerned about the torsional loads on the rear spar from the bellcrank itself. (Which I will have to double the arm length on the flap side while keeping 4 inches on the cable side) I like the idea of simplicity. I feel that I could probably set the hinges to transfer the loads across the hinges as they will only be 1/2 inch between them.

 

[Pilot-34]

I don't understand what you mean by lace.

Propeller flow is a circle not a rectangle. With the diameter of the propeller arc centered on the center of the flaps, you would have the greatest amount of flow hitting the flaps.

The width of the high speed air, 3 feet behind the propeller, is about 7/8 the diameter of the propeller in cruise. Around 3/4 at 30 mph and 5/8 at zero forward speed.

JonnyView attachment 107737

You need a P ...place.

At what point does the high speed flow from a propeller start to increase in size?