Combat / Fixed Wing Design Options

Introduction

There are seven possible options for Combat or Fixed Wing performance designs. Conventional designs do not need the same performance requirements and may use a much longer MA(moment arm).

These options affect the following  main characteristics ;

a) The ability to loop quickly and in minimum radius.

b) The ability to turn quickly in minimum radius.

c) The ability to stay in a following "groove" which is stability related.

d) Speed.

1)  Looping / Bunt Radius.

This is dependent on four factors - MA; Elevator size; Elevator position as regards propeller wash; speed. A longer MA gives quicker loop at the expense of fuselage weight.

2) Turn / Bank Radius,

This is dependent on three factors - aileron area; aileron position as regards propeller wash; speed. Larger ailerons give quicker roll rate but less precise control, especially in a wind.

3) Stability.

A quick but precise response to control input is required. Unfortunately, this is a compromise between CG , MA , Aileron and Elevator configurations.

4) Speed.

Speed is directly related to airfoil depth (drag), engine power / weight ratio. At 18mm depth / 3.5lbs(hot BB.40) is very quick, and quicker than most pilot's reactions. A hot .46 under the same conditions is certainly not short of performance.

References

References to SPAD free plan designs ("Dagger"; "DPS"; "Debonair"; "Dogfighter" are from www.spadtothebone.com.

Note that I consider these designs as "experimental" and not developed into a finished product. To do this takes time and effort (and persistance !) in refining a design (KT4 took 4 versions) test model which the original designers did not have.

There is no point in designing a plastic product that does not fly identically to a good balsa product. If it flies better than the balsa product then it must be due to the design !

My Plans have to come up to this standard, otherwise they do not appear.

Experiments

I have been experimenting with these options and my findings are below. All possible options have been tested.

The diagrams show a) the side view relationship between Wing, Elevator and Ailerons,  b) the rear view relationship, and c) the plan view.

The following Keys are used in the Diagrams ;

A= Ailerons

E=Elevator

T = Tailplane (if used)

A+E = Elevons

A & E = Ailerons & Elevator are on the same datum line side view.

Fuse = Fuselage

MA = Moment Arm (although the MA is shown as typically constant on all diagrams, the actual MA used on the plane is also given for information. This is especially important where the wing and tailplane are on or close to the same datum line. Typically, the longer the MA, the more stable the plane. The downside is the Fuselage is longer and weight increases.

Option 1 - Wing and Elevator on same Datum line

This is the configuration used by most popular SPAD designs due to the rectangular Fuselage used. It suffers from wingwash pushing the tailplane down and needs down elevator trim to compensate. This leaves little throw for inverted or bunts. It also slows the plane down due to increased drag.

Vlad-1 (all Correx SPAD "Dagger" copy) used this configuration initially.

Vlad-2 ("Dagger" with mods.) used this configuration initially.

Modified Vlad-2 to a "V" tail but it did not improve the flying characteristics.

Attributes

1) Short MA.

2) Full strip Ailerons

3) Wingwash.

4) Airfoil depth = 34mm (1" spar  (26mm) + 8mm Correx ) symmetrical.

Side View

Rear View

Plan View

Characteristics

a) Pretty stable laterally.

b) Pitch / Trim  issue but stable.

c) Lack of down elevator.

d) MA = Approx 6"

e) Speed = Average+

Option 2 - Lower Displaced

This option is used very successfully on the SPAD "DPS" (DasPlasStick) and the "Debonaire" where the Aileron and Wingwash are 65mm away from the Tailplane and Elevator. I used this option on KT4 and the design needs little elevator trim to fly straight and level.

Attributes

1) Short MA.

2) No wingwash.

3) Full strip Ailerons.

4) Airfoil depth = 34mm  (26mm spar plus 8mm Correx)

Side View

Rear View

Plan View

Characteristics

a) Laterally very stable.

b) Pitch stable.

c) Actual MA is 4.75".

d) Speed = depends on aerofoil thickness.

Option 3 - Higher Displaced

I used this option on Vlad-2 very with only partial success. The Tailplane/Elevator height is 65mm above the Fuselage. MA is the same as the original "Dagger". 

Attributes

1) Short MA.

2) Full strip Ailerons.

3) Wingwash lessened but still there, so down elevator still an issue.

4) Airfoil depth = 28 (20 +8mm Correx)

Side View

Rear View

Plan View

Characteristics

a) Laterally stable.

b) Pitch stable.

c) Full strip Ailerons.

d) MA = 6".

e) Speed = Dependent on aerofoil thickness.

Possible Modification

With 32.5mm (as per diagram below) between Ailerons and Tailplane the following is an option. This was tested on "Testbed-1"  but was still prone to elevator compensation. 32.5mm is not enough distance to avoid wingwash.

Side View

Rear View

Plan View

Characteristics

a) Laterally stable.

b) Pitch not quite stable.

c) Full strip Ailerons.

d) MA = 6".

e) Speed = Dependent on aerofoil thickness.

Option 4 - Wing and Elevons (Elevator/Ailerons) on same datum line.

"Dracula" uses this layout and is very sensitive to both CG placement and control movements.

Attributes

1) Full strip Elevons.

2) No Wingwash.

3) No MA.

4) Airfoil depth = 26mm (18mm + 8mm Correx)

Side View

Rear View

Plan View

Characteristics

a) Laterally almost unstable.

b) Pitch almost unstable.

c) Pivots on a sixpence.

d) Speed =  Very Fast

e) No Combat "groove".

Option 5 - Elevator Higher (flat)

Tried this configuration on "Ironmonger" and it works well. Wing chord was a little too wide so loop radius was a little larger than expected. Probably should sweep wing as per "Dracula".

Attributes

1) Full strip Ailerons.

2) No Wingwash.

3) No MA.

4) Wing depth = 26mm  (18mm spar + 8mm Correx)

5) Lack of Combat "groove".

6) Very fast.

Side View

Rear View

Plan View

Characteristics

a) Laterally more stable than "Dracula".

b) Pitch more stable than "Dracula".

c) Pivots more slowly than "Dracula".

d) Speed =  Fast

e) No Combat "groove".

Option 6 - "V" Elevator Higher still

"V" tailplane and Elevator has the effect of raising the Elevator even higher out of the wingwash, but at the possible expense of elevator authority.

Tried first on the "Viper" but the V angle is critical and the mechanics were sloppy.

Configuration works ok.

Attributes

1) Full strip Ailerons.

2) No Wingwash.

3) short MA.

4) Airfoil depth = 18mm spar + 8mm Correx.

Side View

Rear View

Plan View

Characteristics

Stable if external V angle is 60/60/60 degrees.

No sign of the "down elevator syndrome".

Has potential if elevator size increased.

Option 7 - Elevator and Partial Ailerons on wing datum line.

Partial ailerons are inset into wing and avoid the airflow over the Elevator. There is no Tailplane. Elevator is directly attached to wing.

"Anduril" used this configuration but had no aileron authority on take-off, due to lack of speed. Additionally was very unstable in pitch due to no MA.

Attributes

1) Partial Ailerons.

2) No Wingwash.

3) No MA.

4) Airfoil depth = 26mm (18mm spar + 8mm Correx)

Side View

Rear View

Plan View

Characteristics

1) Unstable in pitch.

2) Ailerons ineffective at low speeds.

3) CG ultra critical  aound 15%.

So which is the best configuration ?

Depends on your own priorities as regards Combat Characteristics.

1) KT4

A compromise as usual but if stability and control are paramount, then KT4 is ahead.

2) KT4 layout with high mounted V Tail

Works but problems with synchronising elevator halves at high speed - causes roll.

3) Viper

If speed, looping and turning ability are paramount with slightly less stability, then Viper is ahead.

4) Venom

Viper with "V" tail. - too problematical getting elevator halves movement synchronised at high speed - causes roll.

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