Aerofoil Tradeoff
Option 1a:
The N60 (or NACA 25012)
Thickness: 0.12c α0 = -1.59 Deg
Camber Max: 0.02c Cl = 4.282Alpha +0.242
@: 0.25c Cd = 0.053Cl^2 +0.003
APPLICATION:
This aerofoil and the aerofoils of similar shape and thickness are used in several STOL aircraft due to
their high lifting abilities, and relatively low drag characteristics in cruise conditions.
ADVANTAGES/DISADVANTAGES:
Although these (Option 1) aerofoils exhibit high lifting capabilities, at higher positive/negative angles of
attack they are prone to sharp flow separation and stalling – which can be dangerous in low altitude flight
or in gusty conditions. Drag is small in cruise. Manufacture could be difficult as the aerofoil is slightly
complex. Experimental data available.
Option 1b:
NACA 23012
Thickness: 0.12c α0 = -1.18 Deg
Camber Max: 0.02c Cl =4.282 Alpha +0.179
@: 0.15c Cd =0.053 Cl^2 +0.002
APPLICATION:
Civil transport (used on Beech KingAir for eg.), medium velocity flight with ability to support slow flight
in takeoff and landing.
ADVANTAGES/DISADVANTAGES:
Can sustain high lift characteristics up to relatively high AoA’s, but has a sharp stall around 16 degrees.
Drag is small in cruise. Manufacture could be difficult as the aerofoil is complex. Experimental data
available.
Option 2a:
The CH 701’s aerofoil is said to be a modified version of the NACA 6515 (shown below):
Thickness: 0.15c α0 = -7.22
Camber Max: 0.06c Cl = 4.282Alpha +1.097
@: 0.5c Cd = 0.053Cl^2 +0.065
APPLICATION: This is an old school aerofoil, used on the Central-Lamson 101 (its a 1950’s agricultural
biplane). Slow and high manoeuvre flight.
ADVANTAGES/DISADVANTAGES: High lift with low stall. Experimental data not readily available.
Very difficult to manufacture.
Option 2b:
NACA 6515 Mod:
Thickness: 0.15-17c α0 = -6.1 Deg
Camber Max: 0.02c Cl = 4.282Alpha +0.927
@: 0.5c Cd = 0.053Cl^2 +0.046
APPLICATION: This is the modified version of the NACA 6515 that is used on the CH 701. The
modification is the flat lower region of the aerofoil, which also ads a bit of thickness.
ADVANTAGES/DISADVANTAGES: We know this works very well – however we don’t have
experimental data to back it up (just yet). High lift, low stall speed, able to obtain massive AoA’s and has
a benign stall – which means when the plane stalls, it doesn’t lose much control. This is the major
advantage over the thinner 5 & 6 series aerofoils. High drag in cruise conditions. Manufacture is easy, as
they are designed for kit planes.
Option 2c:
Gottingen 625:
Thickness: 0.20c α0 = -5.61
Camber Max: 0.06c Cl = 4.282Alpha +0.853
@: 0.5c Cd = 0.053Cl^2 +0.039
APPLICATION: Old school (1920-30’s, Germany) aerofoil again, used in high lift slow flight.
ADVANTAGES/DISADVANTAGES: Although this is an old school aerofoil section, it has almost
identical lifting characteristics as the NACA 6515 Modified. So it has High lift, low stall speed, able to
obtain massive AoA’s and has a benign stall, all of the positives of the NACA 6515 Mod, but with
experimental results to back it up. This has particularly high drag in most conditions.
Option 2d:
NACA 4221
Thickness: 0.21c α0 = -4.48
Camber Max: 0.04c Cl = 4.282Alpha +0.681
@: 0.20c Cd = 0.053Cl^2 +0.025
APPLICATION: Slow flying civil aviation.
ADVANTAGES/DISADVANTAGES: This has the ability to produce a lot of lift. Can achieve high
AoA’s and has a relatively benign stall. Experimental data is available. Difficult to manufacture because
it is complex.
Preliminary Aerodynamic Analysis:
To see if we can get off the ground.
Assumptions:
- Trailing edge flaps at full extension effectively produce 50% more lift.
- Leading edge slots enable the wing to reach higher AoA.
For a 500kg Aircraft,
Wing span of 9m (allowing 1m for cabin) and a chord of 1.5m.
Wing Area: 13.5m2
Aspect Ratio: 6.0
@SL
Cruise speed of 70kts* (35m/s) (Cl = 0.544)
Stall Speed of 25kts (12.5m/s) (Cl = 4.267)
Lets say that cruise is @ 0 Deg AoA, and stall is @ 18 Deg AoA(based on standard aerofoil data), clean
aerofoil can produce ...% of lift required:
1a) Cruise – 100% Stall - 37%
1b) Cruise – 100% Stall – 36%
2a) Cruise - 100% Stall – 57%
2b) Cruise – 100% Stall – 53%
2c) Cruise – 100% Stall – 51.5%
2d) Cruise – 100% Stall – 47.5%
Alpha 0’s found using 2D Panel Method.
Coefficients found using Vortex Lattice Method assuming CG @ 0.25c.
All of the aerofoils are able to produce 100% of the lifting force required to keep the aircraft in the air at
cruising condition. Most require a negative angle of attack.
For STOL condition, the take-off rotation needs to occur in the vicinity of 18-20 degrees AoA.
Experimental data shows that only the Gottingen 625 is able to reach this AoA safely as a clean aerofoil.
With flaps and leading edge devices (slats) however, each of the aerofoils is able to obtain at least the
required AoA and maybe even achieve the 25-30 degrees AoA the CH701 does.
In clean configuration (no flaps or slats), provided that the aerofoils were able to obtain an AoA of 18
degrees without stalling, only Options 2a) 2b) and 2c) would be able to produce enough lift to keep our
A/C off the ground around an airspeed of 25Kts. Leading edge slots may enable the other aerofoils to
reach beyond this angle of attack, but we don’t yet know this.