Stall Speeds - Fact or Fiction?


What is stall speed?

The speed at which the maximum lift equals the weight is the stall speed. (We will ignore the lift component required to offset trim.)

How can we evaluate claimed stall speeds?

By calculating the coefficient of lift required to produce the claimed stall speed, and comparing it to known parameters, such as the airfoil’s ideal properties and the known performance of similar airplanes.



How can we calculate the Coefficient of lift?

The lift equation: L = Cl q S

L = Lift, in Lbs.

Cl = Wing Coefficient of lift

q = Dynamic Pressure, in Lbs. per Ft.²

S = Wing area, in Ft.²

W = Weight, in Lbs.



At stall in steady state flight, maximum Lift equals Weight:

Lmax = W = (Clmax) (q) (S)

Clmax = W / q S

q = (Vi) (Vi) (0.00256) @ Std conditions, in MPH

Clmax = W / (Vstall)² (0.00256) (S)

Note: This is the Clmax for the wing as a whole. It will always be less than the airfoil can generate in 2-D flow.



What parameters affect a wing’s Clmax?

Basic airfoil section design

Higher speed airfoils are designed for delayed separation (more laminar flow) => lower Clmax at speeds less than cruise


Stall Speeds - Fact or Fiction?Stall Speeds - Fact or Fiction?



Reynolds number = VL/

Lower Rn = lower Clmax

Hard to find test data between Rn of 500,000 and 3,000,000

Most airfoil data is at Rn higher than most homebuilt Rn at stall speed

Example: 48” chord, 2000 Ft altitude, 50MPH => Rn of 1,800,000




Lift Distribution - Elliptical is best

Wing Plan form

Elliptical is best

Tapered - Tips stall first

Wing Twist - Washout

Most Certified Light A/C use it

Airfoil section verses Span




Aspect Ratio - Span² / S

Higher AR => Steeper lift curve slope

Higher AR => Slightly increased Clmax



Lift Curve



High Lift Devices

Plain Flaps

Slotted Flaps

Fowler Flaps

Leading Edge Devices

Boundary Layer Control Devices

Vortex Generators

etc.



Plain Flaps

Simple to build

Less costly

Add lift

Used to create drag

Potential to increase Clmax by approximately 60%



Slotted Flaps

More complicated to build

More costly

Add lift

Typically => greater change in => more nose down

Potential to increase Clmax by 80% or more

Wide variation in performance based on flap shape and slot geometry



Fowler Flaps

Most complicated trailing edge flaps to build

Most costly

Potential to increase section Clmax by more than 100%

Also add effective wing area

Wide variation in performance based on configuration



Leading Edge Devices

Complex

Costly

Used for special purpose and complex aircraft

Add lift

Raise the nose / offset the nose down effect of T.E. flaps



High Lift Devices

Effect on a Clark Y airfoil

Clark Y Clmax

Basic section 1.3

Plain flap 1.8

Slotted flap, w optimum slot 2.0

Slotted + Leading Edge Slat 2.2



High Lift Devices

Effect on a GAW-1 airfoil, 2-D in wind tunnel

18%, design Cl = 0.4, Rn ~ 2,500,000

GAW-1 Clmax stall

Basic section 1.55 16°

Flower, 30% C, 40° 3.1 8°

L.E. + Fowler 3.8 15°

L.E. + Fowler + VGs 4.0 15°




Parameter Parameter Clmax

Design Speed

Rn

Lift Dist. Less than elliptical

Taper Ratio

Aspect Ratio

Flaps, etc.



Circulation



Bernoulli or Newton?

a.k.a.

Pressures or Momentum?



AirplaneModelHP 180 310 230Gross Weight 2450 3400 3100Stall Speed 59 70 56Max. Speed 147 228 173Wing Span 32.75 38.50 36.00Wing Area 146.0 149.9 174.0Airfoil 63/2 A415 Roncz 2412Airfoil Clmax 1.60 1.50 1.60Plan form Rectangular Tapered TaperedFlaps Plain Slotted FowlerAspect Ratio 7.3 9.9 7.4Wing Clmax 1.88 1.81 2.22

Fact or Fiction?



Airplane Sundowner Cirrus CessnaModel C23 SR-22 Skylane 182THP 180 310 230Gross Weight 2450 3400 3100Stall Speed 59 70 56Max. Speed 147 228 173Wing Span 32.75 38.50 36.00Wing Area 146.0 149.9 174.0Airfoil 63(2) A415 Roncz 2412Airfoil Clmax 1.60 1.50 1.60Plan form Rectangular Tapered TaperedFlaps Plain ? Slotted ? FowlerAspect Ratio 7.3 9.9 7.4Wing Clmax 1.88 1.81 2.22

Fact or Fiction? Fact Fact Fact



AirplaneModelHP 450 450 80Gross Weight 5100 3500 1050Stall Speed 45 50 44Max. Speed 158 158 125Wing Span 48.00 48.00 27.00Wing Area 250.0 250.0 130.0Airfoil 64-415 64-415 65-018 mod ?Airfoil Clmax 1.5 1.5 ?Plan form Rectangular Rectangular RectangularFlaps Slotted? Slotted? NoneAspect Ratio 9.2 9.2 5.6Wing Clmax 3.94 2.19 1.63Fact or Fiction?



Airplane Beaver Beaver ZodiacModel CH 601 HDHP 450 450 light 80Gross Weight 5100 3500 1050 (under)Stall Speed 45 50 44Max. Speed 158 158 125Wing Span 48.00 48.00 27.00Wing Area 250.0 250.0 130.0Airfoil 64-415 64-415 65-018 modAirfoil Clmax 1.5 1.5 Plan form Rectangular Rectangular RectangularFlaps Slotted? Slotted? NoneAspect Ratio 9.2 9.2 5.6Wing Clmax 3.94 2.19 1.63Fact or Fiction? Fiction Fact Fact



AirplaneModelHP 100 160 180Gross Weight 1320 1483 1600Stall Speed 38 64 59Max. Speed 95 211 207 est..Wing Spa 35.50 24.34 20.83Wing Area 180.0 97.1 86Airfoil USA35B 64A-212 62(1)-412Airfoil Clmax 1.50 1.50 1.5Plan form Rectangular Tapered RectangularFlaps None Plain PlainAspect Ratio 7.0 6.1 5.0Wing Clmax 1.98 1.46 2.09 some powerFact or Fiction?



Airplane Legend Cub Bushby ThorpModel -100 LSA Mustang II T-18 CAFEHP 100 160 180Gross Weight 1320 1483 1572Stall Speed 38 64 59Max. Speed 95 211 207 est.Wing Spa 35.50 24.34 20.83Wing Area 180.0 97.1 86Airfoil USA35B 64A-212 62(1)-412 modAirfoil Clmax 1.50 1.50 1.5Plan form Rectangular Tapered RectangularFlaps None Plain Plain ?Aspect Ratio 7.0 6.1 5.0Wing Clmax 1.98 1.46 2.03 some powerFact or Fiction? Fiction Fact Fact



AirplaneModelHP 260 80 180Gross Weight 2000 1300 1500Stall Speed 54 50 54Max. Speed 250 172 212Wing Span 21.83 26.10 23.00Wing Area 104.0 95.0 110.0Airfoil 23013.5 Dykins 23013.5Airfoil Clmax 1.50 1.50est 1.50Plan form Rectangular Tapered+WO RectangularFlaps Plain ? PlainAspect Ratio 4.6 7.2 4.8Wing Clmax 2.58 2.14 1.83Fact or Fiction?



Airplane Team Rocket Europa Van'sModel F-1 Std Classic CAFE RV-4HP 260 80 180Gross Weight 2000 1300 1500Stall Speed 54 50 54Max. Speed 250 172 212Wing Span 21.83 26.10 23.00Wing Area 104.0 95.0 110.0Airfoil 23013.5 Dykins 23013.5Airfoil Clmax 1.50 ? 1.50Plan form Rectangular Tapered+WO RectangularFlaps Plain ? PlainAspect Ratio 4.6 7.2 4.8Wing Clmax 2.58 2.14 1.83Fact or Fiction? Fiction Fact Fact



AirplaneModelHP 260 300 295Gross Weight 2300 2400 2100Stall Speed 65 86 50Max. Speed 225 295 265Wing Span 24.75 24.40 24.83Wing Area 120.0 71.5 104.0Airfoil 64(2)-415 NLF(1) 0416 MS(1)-313Airfoil Clmax 1.50 1.40 1.50Plan form Split Tapered Tapered Flaps Plain? Fowler SlottedAspect Ratio 5.1 8.3 6.0Wing Clmax 1.77 1.77 3.22Fact or Fiction?



Airplane Barracuda Swearingen Team RocketModel SX-300 F-1 EvoHP 260 300 295Gross Weight 2300 2400 2100Stall Speed 65 86 50Max. Speed 225 295 265Wing Span 24.75 24.40 24.83Wing Area 120.0 71.5 104.0Airfoil 64(2)-415 NLF(1) 0416 MS(1)-313Airfoil Clmax 1.50 1.40 1.50Plan form Split Tapered Tapered Flaps Plain? Fowler SlottedAspect Ratio 5.1 8.3 6.0Wing Clmax 1.77 1.77 3.22Fact or Fiction? Fact Fact Fiction



AirplaneModelHP 170 310 310Gross Weight 1775 2200 2217Stall Speed 71 65 68Max. Speed 233 285 292Wing Span 23.70 25.50 25.50Wing Area 76.0 82.5 82.5Airfoil NLF1-0215F GC10/11GC10/11Airfoil Clmax 1.50 est. ? ?Plan form Tapered Tapered TaperedFlaps Plain Fowler FowlerAspect Ratio 7.4 7.9 7.9Wing Clmax 1.81 2.47 2.27 Fact or Fiction?



Airplane Lancair Lancair LancairModel 320 CAFE Legacy Legacy CAFEHP 170 310 310Gross Weight 1775 2200 2217Stall Speed 71 65 68Max. Speed 233 285 292Wing Span 23.70 25.50 25.50Wing Area 76.0 82.5 82.5Airfoil NLF1-0215F GC10/11GC10/11Airfoil Clmax 1.50 est. ? ?Plan form Tapered Tapered TaperedFlaps Plain Fowler FowlerAspect Ratio 7.4 7.9 7.9Wing Clmax 1.81 2.47 2.27Fact or Fiction? Fact LoC Fact



AirplaneModelHP 160 160 160Gross Weight 1750 1758 1960Stall Speed 50 49 49Max. Speed 194 193 167Wing Span 28.00 28.00 35.00Wing Area 124.0 124.0 128.0Airfoil Roncz Roncz LS(1)-0413 modAirfoil Clmax ? ? 1.60Plan form Rectangular Rectangular RectangularFlaps Slotted Slotted Fowler + Vortex Aspect Ratio 6.3 6.3 9.6Wing Clmax 2.21 2.31 2.49Fact or Fiction?



Airplane Van’s Van’s GlastarModel RV-9A RV-9A CAFE GlastarHP 160 160 160Gross Weight 1750 1758 1960Stall Speed 50 49 49Max. Speed 194 193 167Wing Span 28.00 28.00 35.00Wing Area 124.0 124.0 128.0Airfoil Roncz Roncz LS(1)-0413 modAirfoil Clmax ? ? 1.60Plan form Rectangular Rectangular RectangularFlaps Slotted Slotted Fowler + Vortex Aspect Ratio 6.3 6.3 9.6Wing Clmax 2.21 2.31 2.49Fact or Fiction? Fact Fact Fact



AirplaneModelHP 160 180 300Gross Weight 1950 2100 2400Stall Speed 47 68 73Max. Speed 158 228 290Wing Span 35.67 23.30 23.30Wing Area 128.0 81.3 81.3Airfoil LS(1)-0413 mod LS(1)-0413 mod LS(1)-0413 modAirfoil Clmax 1.60 1.60 1.60Plan form Rectangular Tapered TaperedFlaps Fowler + Vortex Slotted SlottedAspect Ratio 9.9 6.7 6.7Wing Clmax 2.69 2.18 2.16Fact or Fiction?



Airplane Glastar Glasair GlasairModel Glastar CAFE SII- FT short III shortHP 160 180 300Gross Weight 1950 2100 2400Stall Speed 47 68 73Max. Speed 158 228 290Wing Span 35.67 23.30 23.30Wing Area 128.0 81.3 81.3Airfoil LS(1)-0413 mod LS(1)-0413 mod LS(1)-0413 modAirfoil Clmax 1.60 1.60 1.60Plan form Rectangular Tapered TaperedFlaps Fowler + Vortex Slotted SlottedAspect Ratio 9.9 6.7 6.7Wing Clmax 2.69 2.18 2.16Fact or Fiction? Fact Fact Fact



AirplaneModelHP 300 300 180Gross Weight 2400 2500 1800Stall Speed 80 74 58Max. Speed 282 281 209Wing Span 23.30 27.00 25.00Wing Area 88.0 97.2 121.0Airfoil LS(1)-0413 mod LS(1)-0413 mod 23013.5Airfoil Clmax 1.60 1.60 1.50Plan form Tapered Tapered RectangularFlaps Plain Plain PlainAspect Ratio 6.2 7.5 5.2Wing Clmax 1.66 1.83 1.73Fact or Fiction?



Airplane Glasair Glasair Van'sModel III short CAFE III long CAFE RV-7HP 300 300 180Gross Weight 2400 2500 1800Stall Speed 80 74 58Max. Speed 282 281 209Wing Span 23.30 27.00 25.00Wing Area 81.3 97.2 121.0Airfoil LS(1)-0413 mod LS(1)-0413 mod 23013.5Airfoil Clmax 1.60 1.60 1.50Plan form Tapered Tapered RectangularFlaps Plain Plain PlainAspect Ratio 6.2 7.5 5.2Wing Clmax 1.80 1.95 1.73Fact or Fiction? Fact Fact Fact



AirplaneModelHP 160 160 200Gross Weight 1750 1758 1800Stall Speed 50 49 58Max. Speed 194 193 219Wing Span 28.00 28.00 24.00Wing Area 124.0 124.0 116.0Airfoil Roncz Roncz 23013.5Airfoil Clmax ? ? 1.50Plan form Rectangular Rectangular RectangularFlaps Slotted Slotted PlainAspect Ratio 6.3 6.3 5.0Wing Clmax 2.21 2.31 1.80Fact or Fiction?



Airplane Van's Van's Van'sModel RV-9A RV-9A CAFE RV-8AHP 160 160 200Gross Weight 1750 1758 1800Stall Speed 50 49 58Max. Speed 194 193 219Wing Span 28.00 28.00 24.00Wing Area 124.0 124.0 116.0Airfoil Roncz Roncz 23013.5Airfoil Clmax ? ? 1.50Plan form Rectangular Rectangular RectangularFlaps Slotted Slotted PlainAspect Ratio 6.3 6.3 5.0Wing Clmax 2.21 2.31 1.80Fact or Fiction? Fact Fact LoC



AirplaneModelHP 200 180 260Gross Weight 1668 1800 2700Stall Speed 58 58 63Max. Speed 220 207 208Wing Span 24.00 25.00 31.75Wing Area 116.0 121.0 148.0Airfoil 23013.5 23013.5 23013.5Airfoil Clmax 1.50 1.50 1.50Plan form Rectangular Rectangular RectangularFlaps Plain Plain PlainAspect Ratio 5.0 5.2 6.8Wing Clmax 1.67 1.73 1.80Fact or Fiction?



Airplane Van's Van's Van'sModel RV-8A CAFE RV-7A RV-10HP 200 180 260Gross Weight 1668 1800 2700Stall Speed 58 58 63Max. Speed 220 207 208Wing Span 24.00 25.00 31.75Wing Area 116.0 121.0 148.0Airfoil 23013.5 23013.5 23013.5Airfoil Clmax 1.50 1.50 1.50Plan form Rectangular Rectangular RectangularFlaps Plain Plain PlainAspect Ratio 5.0 5.2 6.8Wing Clmax 1.67 1.73 1.80Fact or Fiction? Fact Fact Fact



AirplaneModelHP 180 310 300Gross Weight 2100 3400 3650Stall Speed 68 66 68Max. Speed 238 219+ ?Wing Span 23.30 35.80 33.50Wing Area 81.3 141.2 180.7Airfoil LS(1)-0413 mod ? 23016.5 / 12Airfoil Clmax 1.60 ? 1.60Plan form Tapered Tapered TaperedFlaps Slotted Slotted PlainAspect Ratio 6.7 9.1 6.2Wing Clmax 2.18 2.16 1.71Fact or Fiction?



Airplane Glasair Columbia BeechcraftModel SII - RG short 350 G36HP 180 310 300Gross Weight 2100 3400 3650Stall Speed 68 66 68Max. Speed 238 219+ ?Wing Span 23.30 35.80 33.50Wing Area 81.3 141.2 180.7Airfoil LS(1)-0413 mod ? 23016.5 / 12Airfoil Clmax 1.60 ? 1.60Plan form Tapered Tapered TaperedFlaps Slotted Slotted PlainAspect Ratio 6.7 9.1 6.2Wing Clmax 2.18 2.16 1.71Fact or Fiction? Fact Fact Fact



Possible reasons for the unsubstantiated claims:

Equating indicated airspeed to calibrated airspeed

Too rapid deceleration into stall

Testing with some power on

Testing at other than standard conditions

Testing at less than gross weight

Self-certify = right to claim ignorance



Observations and Conclusions:

Several kit manufacturers list neither maximum speed nor red line speed.

It is not clear that all manufacturers list stall speeds at gross weight.

It is not clear that all manufacturers use calibrated airspeed.

Some manufacturers quote stall speeds with power above idle.

Most pilot reports do not include measured, calibrated, performance data.

There are many misperceptions about airfoils.

There is no magical airfoil.

CAFE performs an excellent service.

Stall Speeds - Fact or Fiction?

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