STOL AIRCRAFT

Some of the most challenging and fun things I have done with RC aircraft is to explore the low speed envelope.
This page is for the Aircraft designs, lift devices and all sorts of doodads and widgets that fit in with STOL.


The Planes

Piper Cub -

  

Love or hate them, the Cub has its place embedded in Aviation history. In the hands of an experienced pilot, they can T/O & land on a dime & and give you change. No fancy lift devices here just lots of wing area to weight.

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Fieseler Fi 156 Storch-

A WW2 design that incorporates full span Leading edge fixed slots and full length flapperons

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Warszawa-Okecie PZL-104 Wilga -

Polish design than uses full span slots and slotted flaps find more about this aircraft at

Kiwi Aircraft Images: PZL-104 Wilga

 


Understanding lift

First off we need to study the pressure curve of an airfoil and see what causes a wing to fly and what causes it to stall.   If you don't know already, air is incompressible.  That means it has to increase speed to move around an object, like a wing.   When it speeds up, the pressure gets lower.  This is how a wing creates lift.  It displaces more on top than the bottom,  since the air has to go farther to get around the top it has to flow faster.  Since it flows faster the pressure is lower and the difference between pressure on the top and bottom creates a force called lift.   What we are interested in is the pressure curve.   The pressure curve is represented in a graph of the coefficient of pressure vs. the chord of the wing.   The graph is set up so the pressure is shown on the Y axis and reads backwards so the low pressure is on top. 

The diagram below is the pressure curve for a Clarke Y airfoil at 0 degrees Angle of attack (AoA) and the stalling AoA.

As you can see from the graph, the pressure is basically a function of how much air is displaced.   The lowest pressure point on the 0 degree AoA occurs where the airfoils is the thickest.  When the airfoil is getting thicker rapidly between the leading edge and the thickest point, the pressure decreases rapidly also.   The point we are concerned with, is the lowest pressure point.   As shown in the graph, it is a smooth transition from decreasing pressure to increasing pressure at  0 degree AoA.  But at the stalling AoA there is a rapid decrease followed by a rapid increase in pressure.  At the stall AoA, the air flows fastest around the tight radius near the leading edge.  That is why the stall AoA graph has the strong peak near the leading edge of the wing.     The air goes to a very low pressure point at the top of the peak, then it has to flow rapidly into a high pressure area.  At a certain point it does not have enough energy to flow into the high pressure and separates from the wing, (stall).


Lift devices

 

Leading Edge Slots

The leading edge slot is not so much a lift device as it delays the stall to a higher AoA. 
It delays the stall 2 ways at the same time.  First, the pressure peak occurs on the slat, not the main wing.  The air separates from the slat, but leaves air unaffected on the main wing.  Second it delays stall on the main wing by allowing high energy air from below and in front of the wing to flow through the slot where it is accelerated.  The higher speed of the air,  the lower the pressure,  creating more of a pressure differential from the bottom (lift).   The high energy air also energizes the boundary layer, helping it stay attached to the wing at higher AoA.  The diagram below shows what is happening when a slot is effective.

 

The shape of the slot is critical.  it must be wider at the intake (front) that the exit.  the opening should be 2 to 3 times as big as the exit.  3/16" on  40 -90 sized models and slightly larger on giant scale.   Its also critical the the trailing edge is parallel to the wing so the air flows even and straight to avoid adverse flight characteristics

 

The graph below shows the correlation between lift and AoA.
  If a normal wing creates a lift coefficient at about 1 at a AoA of about 12 degrees.  The line represents lift generated that climbs as the AoA rises.  At the top of the line the curve is the stall.  The tighter the curve the sharper the stall.     

Airfoil and size of wing vary, but generally on models,  the overall effect is far less than on a full scale.  That's not to say its doesn't work.  I have flown planes with slots at thirty degrees or more AoA with full control.  These were not 3D capable models, but trainers that were modified.  the biggest drawback is loss of elevator control or a blanking effect as the wing covers the airflow over the tail. - EK

 

To gain the benefits of slots, you have to fly differently than conventional plane.  For slow flight, idle back, slowly ease up the nose to decrease speed, as the plane slows down and pitches up, slowly add power.  It takes a little practice, but you should be able to fly level,  with the nose above 20 degrees AoA at 1/4 to 1/2 elevator and how ever much throttle it takes to maintain speed, usually about 1/2 or less.  Don't forget to use rudder and coordination when turning

Take offs can be shortened providing you have ground clearance.  Modifications may be needed to address these issues depending on design.


Flaps

Slotted Flaps

Slotted flaps work the same way leading edge slots do.  Air is accelerated through the slot, allowing greater angles of the flap before the air separates into a stall.  The more angle increases lift by effectively increasing angle of AoA and camber in the wing.  

Slotted flaps are harder to make than regular flaps, but no too difficult.

 

1st the trailing edge needs to be made at an 45 degree angle instead of 90, this can be accomplished with appropriate tri stock.  A lip extending past the top trailing edge to cover the slot gap when flaps are up can be made with sheeting. 

 

Flap can built conventionally allowing for curve on front to make the slot shape.

 

Once the flap is built and trailing edge completed,  Draw full scale plan looking at it end wise as above.  Trace the end of the flap onto a small piece of paper and erase the flap on the drawn plan.  trim the paper from flap drawing so you can see the wing, but leave enough paper below to hinge it.   Lay the flap on the back of the trailing edge where it should be.  Use the pencil or a pin and hold a point on the flap paper about 1" below and 1/2" back from the gap as a pivot point.  Rotate the paper flap to about 30 degrees, and see what shape the slot makes.   Move the pivot point until the slot when opened to 30degrees is the proper shape, ( leading edge gap 2 to 3 times as large as the trailing edge 3/16" gap). 

The hinge block is attached to the bottom of the surfaces since it needs to be below  and behind the gap between wing and flap.  The block can be made from balsa and cut as shown above, and then hinged with Robart hinges. 

 

These 2 drawings show  an approximation of what it should look like when completed

Care should be take to ensure the gap is parallel and there is no binding.


Many thanks go to Ty Frisby who put most of this info together and sent it to me.

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revised 11/15/04