Tandem-wing LSA/microlight concept and poll

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

Which tandem-wing configuration would interest you the most (pick one in each of four categories).

  • A1 - High wing forward, low wing aft (Flying Flea) OR

    Votes: 17 36.2%
  • A2 - Low wing forward, high wing aft (Quickie);

    Votes: 26 55.3%
  • B1 - Two-axis controls (no rudder pedals like an Ercoupe) OR

    Votes: 9 19.1%
  • B2 - Three-axis controls (with rudder pedals like a Cessna);

    Votes: 33 70.2%
  • C1 - Conventional (taildragger) gear OR

    Votes: 22 46.8%
  • C2 - Tricycle (nosewheel) gear;

    Votes: 20 42.6%
  • D1 - Tractor engine (engine and propeller at front) OR

    Votes: 31 66.0%
  • D2 - Pusher engine (engine and propeller at rear);

    Votes: 13 27.7%

  • Total voters
    47

rtfm

Well-Known Member
Joined
Jan 3, 2008
Messages
3,522
Location
Brisbane, Australia
Seems intuitive to me. The gap induces airflow from the bottom of the wing, and pulls airflow down from the top. If anything, it reduces any tendency to stall.
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
8,251
Location
Fort Walton Beach, Florida, USA
My understanding was that a lowered control surfaces increases lift by increasing the effective angle of attack of the control surface, which seems to suggest that the part of a wing with a lowered control surface would stall before the rest of that same wing.
 

Sockmonkey

Well-Known Member
Joined
Apr 24, 2014
Messages
2,192
Location
Flint, Mi, USA
That's an interesting concept, Sockmonkey, and not unlike what I was describing with elevons on the front wing (though I was still going to have a conventional rudder). The question is what happens if you stall the front wing in a turn so only one half stalls? That seems like a possible stall-spin scenario.
I'm not certain if it would flip it into a spin, but it might. Washout on the tips perhaps?
Or since this kind of wing doesn't need to fold it could use front-wing elevons.
Spratt managed to get away with differential pitch wings in his designs, but I'm not sure what he did about stall prevention.
My understanding was that a lowered control surfaces increases lift by increasing the effective angle of attack of the control surface, which seems to suggest that the part of a wing with a lowered control surface would stall before the rest of that same wing.
Junkers flaps are essentially a separate surface from the wing so they can't actually stall the wing they're attached to unless you use them to pitch the whole plane up to a high enough AOA that the wing would stall anyhow.
In a flea, the fore wing stalls first and drops the nose so I don't think you could use them to make the rear wing stall at all.
Junkers flaps are mounted below and behind the wing, so they can't interfere with the airflow over it, If the top airflow is fine, you don't stall.
 

rotax618

Well-Known Member
Joined
Oct 31, 2005
Messages
1,119
Location
Evans Head Australia
The Savannah and CH701 have Junkers flaperons, the Junkers flaperons are split in the middle of each wing and the angle of incidence reduced on the outward portion to delay the stall at the tip. Junkers ailerons will induce the wing to stall, they can effectively increase the angle of attack of the wing.
 

Sockmonkey

Well-Known Member
Joined
Apr 24, 2014
Messages
2,192
Location
Flint, Mi, USA
The Savannah and CH701 have Junkers flaperons, the Junkers flaperons are split in the middle of each wing and the angle of incidence reduced on the outward portion to delay the stall at the tip. Junkers ailerons will induce the wing to stall, they can effectively increase the angle of attack of the wing.
Really? Is there a diagram somewhere I can look at to see why that is?
 

rotax618

Well-Known Member
Joined
Oct 31, 2005
Messages
1,119
Location
Evans Head Australia
You can think of a Junkers aileron as a conventional aileron with a slot, when moved down it induces increased airflow over the top surface of the wing effectively increasing the angle of attack.
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
8,251
Location
Fort Walton Beach, Florida, USA
i think there is some confusion here in terms of how you are measuring AOA. My understanding is that by lowering or raising a trailing-edge control you are effectively changing the angle of attack of the airfoil while also changing its camber. The change in AOA is measured by comparing straight lines from the leading edge of the wing to the old and new positions of the trailing edge of the control surface. So, without changing the position of the airfoil relative to the oncoming airstream, you have increased the AOA by lowering the flap.

In practice you can see this in something like a C-150 by holding it near the stall with one notch of flaps and then dropping full flaps without changing the nose-up attitude of the aircraft...you will induce a stall because the flaps increased the effective AOA without moving the rest of the wing. At least that’s how I understand it.

Sure, but the airfoil is no longer limited to 14 degrees (or whatever), but the max AoA increases.
 

Sockmonkey

Well-Known Member
Joined
Apr 24, 2014
Messages
2,192
Location
Flint, Mi, USA
Ok, I get it now. I was thinking in terms of those Junkers flaps I've seen where the gap between the flap and wing is huge. More than the chord of the flap itself.
 

rotax618

Well-Known Member
Joined
Oct 31, 2005
Messages
1,119
Location
Evans Head Australia
Matthew is correct, what I was trying to say is that anything that increases the downward deflection of the airflow behind the wing increases the camber of the wing increasing the effective angle of attack.
Its a bit dark in my hangar, but this is the arrangement of the Junkers flaperons on the Savannah. BD44499E-62EC-4941-AE72-AA266B2D0178.jpeg
 

Victor Bravo

Well-Known Member
HBA Supporter
Joined
Jul 30, 2014
Messages
8,918
Location
KWHP, Los Angeles CA, USA
If Mark Drela, or Dave Lednicer, or Al Bowers, or the ghost of Ludwig Prandtl wishes to correct me, they can feel free.

A Junkers flaperon may increase the camber of the wing, but I don't see how it increases the actual Angle of Attack.

Due to circulation theory and all of that... I can see how the effective angle is influenced, and I can see how the deflection could put more "load" on the airflow to induce a separation.

But if 80-85% of the wing is still moving through the atmosphere at the same angle it was doing before the flaperon deflection, to me the AoA hasn't changed.
 

mcrae0104

Well-Known Member
HBA Supporter
Log Member
Joined
Oct 27, 2009
Messages
3,819
Location
KBJC
Just speculation: with the Junkers flap down, more list is being created. Therefore the whole “wing system” is creating more upwash ahead of the wing, effectively creating a higher angle of attack.

There are several NACA reports out there on Junkers flaps, although I haven’t sifted through them to see what light they shed on the question.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,712
Location
Rocky Mountains
Just speculation: with the Junkers flap down, more list is being created
Depends on what side of the boat you are on. ;)

Seriously: If you look in TOWS the X axis of the lift and moment graphs are labeled "Section angle of attack". I've always understood this to mean that the the AOA used is the line drawn through the leading and trailing edges of the unmodified airfoil. This seems, to me, to be the logical reference point to use because once we attach the wing to an aircraft we then use the fuselage angle of attack for the remainder of the calculations....................same as considering the junkers flap as a "wing system"
 

cluttonfred

Well-Known Member
HBA Supporter
Joined
Feb 13, 2010
Messages
8,251
Location
Fort Walton Beach, Florida, USA
OK, VB, I think I am going to have to throw down a flag on this play. Angle of attack is defined as the angle between the relative wind and the chord line of the airfoil, and the chord line is defined as a straight line between the leading and trailing edges. If you lower the trailing edge (via a flap or any other trailing edge control surface, which are just two-way flaps) then you have to have increased the angle of attack. That chord line must tip up as you lower the trailing edge, so the angle of attack must increase, just like reflexed ailerons will actually create washout by decreasing the angle of attack. I agree that it's a little less intuitive with something like a Junkers control surface or even a split flap, but I would argue that in practice the former is really just a sort of extreme slotted flap and the latter is just a very draggy plain flap.

wing_profile_nomenclature.jpg
 

Sockmonkey

Well-Known Member
Joined
Apr 24, 2014
Messages
2,192
Location
Flint, Mi, USA
I think the core question is how far from the wing can a Junkers flap be mounted before the air treats it as a separate airfoil rather than part of the wing.
 
Top