Slotted flap slot design

AERADE Reports Archive, search keyword slotted

http://aerade.cranfield.ac.uk/ara/dl.php?filename=1947/naca-tn-1395.pdf

http://aerade.cranfield.ac.uk/ara/dl.php?filename=1947/naca-tn-1463.pdf

http://aerade.cranfield.ac.uk/ara/dl.php?filename=1950/naca-tn-2149.pdf

http://aerade.cranfield.ac.uk/ara/dl.php?filename=1949/naca-report-942.pdf

Slotted flap slot design

AERADE Reports Archive, search keyword slotted

http://aerade.cranfield.ac.uk/ara/dl.php?filename=1947/naca-tn-1395.pdf

http://aerade.cranfield.ac.uk/ara/dl.php?filename=1947/naca-tn-1463.pdf

http://aerade.cranfield.ac.uk/ara/dl.php?filename=1950/naca-tn-2149.pdf

http://aerade.cranfield.ac.uk/ara/dl.php?filename=1949/naca-report-942.pdf

I have been thinking how the efficiency of a tandem wing aircraft could be improved. In tandem wing aircraft the front wing determines pretty much how high total Clmax the aircraft is going to have which translates then to the required wing area. To maximize the efficiency, because flaps can not be used in the rear wing, the Clmax of the front wing is desirable to be as high as possible. Usually on tandem wing and canard aircraft the Clmax of the front wing is around 2.0 and the elevator is a single slotted flap.

It may not be mechanically very practical, but theoretically it could be possible to increase the Clmax of the front wing by adding more high lift devices into it. There could be a fowler flap implemented so that it increases the wing area substantially while the single slotted elevator flap remains working as usual in the trailing edge. The problem is that how you do that when the whole elevator system moves as consequence when the flaps are lowered or raised (the fowler is either inside the wing or protruded out of the trailing edge). But if this was practical with any other means than using servo motors for the elevator too, it could increase the available lift from the front wing somewhat. A challenging thing in this obviously is that the shear web that connects the spar caps goes through the front wing, and the flap system can not break the integrity of the shear web. More limiting factor also is that the chord length of a tandem wing aircraft front wing is low and there is not that much space for the high lift device.

Anyway, it would be interesting to try this out with a radio controlled model.

I hereby license this invention under the terms and conditions of GNU General Public License, version 3, or any later version. (C) 2008 Karoliina Salminen. All rights reserved. By reading this text, you aknowledge this and agree with the terms and conditions of the GPL license.

I changed the LS(1)-417 so that the trailing edge gap is zero (=sharpest achievable) instead of the large gap present in that airfoil (Janne's Mini-Sytky does not have this gap while Panu's Mini-Sytky has). According to simulation with Javafoil, this decreases the drag quite significantly. The airfoil has good Clmax at the same time with the low drag (approaches almost NLF414F).

I calculated that Clmax of 2.88 is possible with this profile with full span flaps with fowler inboard section. 

Quick calculation with aerocalc shows that the following might be theoretically achievable:

AR 9

Clmax 2.88

Wing area 4.6 m2

span 6.4 m

Wing loading 144 kg/m2 29 lbs/sqft

L/D max 22

Stall speed 55 kts

Max level speed 260 kts 480 km/h with Rotax 914 (90 hp required out of 115, max continuous 100 hp->ok)

best glide speed 150 kts

empty weight 366 kg

mtow 666 kg

Full span flaps with flapped ailerons:

In board wing has 60% span fowler flaps. Outboard wing, the remaining 40% consists plain flap type flaperons with similar mechanism than used in Mini-Sytky.

deltaClmax_fowler = 0.6 * 1.67 + 0.4 * 0.9 = 1.362

For airfoil with Clmax 1.2 the maximum Clmax on landing configuration is thus 1.32 + 1.362 = 2.68

This allows smaller wing area and higher wing loading to be used without sacrificing takeoff and landing performance too much.

Another variation with single slotted flaps:

deltaClmax_singleslotted = 0.6*1.18 + 0.4*0.9 = 1.06

+1.06 in Clmax still is a very good value and better that would be obtained with full span flaperon (+0.9). For airfoil with Clmax of 1.32 this yields Clmax of 2.37.

This idea has not been tested in practice and is not guaranteed to work.

Effects on aircraft:

Aircraft with 60% span plain flap and Wortman FX 38-153 (no full span high lift device):

Clmax = 1.3 + 0.9*0.6 

deltaClmax = 0.54

Clmax => 1.84

86 hp required for 200 kts cruise

wing loading: 92 kg / m2

wing area: 7.2 m2

stall speed: 55 kts

design cruise: 200 kts

Cdtot = 0.011 (with boundary layer suction)

Same aircraft with full span flaperon and Wortman FX 38-153:

Clmax = 1.3+0.9 =  2.20

Same aircraft parameters:

76 hp required for 200 kts cruise

wing loading: 110 kg / m2

wing area: 6 m2 

Aircraft with full span flaps with slotted inboard section:

Clmax = 1.3 + 1.06 = 2.36 

Same aircraft parameters:

74 hp required for 200 kts cruise

wing loading: 118 kg / m2

wing area: 5.6 m2

Aircraft with full span flaps with fowler inboard section:

Clmax = 1.3 + 1.362 = 2.66

70 hp required for 200 kts cruise

wing area: 5 m2

wing loading: 134 kg / m2

For the most extreme case theoretical savings over usual configuration:

Power = 86-70 = 16 hp (18%)

wing loading: 134-92 = 42 kg/m2 (31%)

wing area: 7.2 m2 - 5 m2 = 2.2 m2 (30%)