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Old Dec 22, 2007, 10:11 PM
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Equation for stall speed, wing loading, speed?

Thanks. I'm reading an article

http://www.masportaviator.com/ah.asp...&ID=72&index=1

discussing prop selection, and he mentions an equation for calculating stall speed as a function of speed and wing loading, then he does not include the equation. Does anybody know of such/

Thanks
Jim
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Old Dec 23, 2007, 02:39 AM
hul
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this is an approximation by Keith Shaw:

stall speed = 3.7 * square root of wing loading

speed in mph
wing loading in oz/sqft

Hans
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Old Dec 23, 2007, 03:07 AM
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Quote:
Originally Posted by hul
this is an approximation by Keith Shaw:

stall speed = 3.7 * square root of wing loading

speed in mph
wing loading in oz/sqft

Hans
Thanks very much, Hans.

Jim
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Old Dec 23, 2007, 04:45 AM
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The 'full' formula (in S.I. units) is:

V = sq root (W x 9.81/(1/2p x S x Cl_max))

where:
V = Stall speed M/s
p (rho) = air density KG/M^3
S = wing area M^2
Cl_max = Coefficient of lift at stall
W = weight KG

The simplified 'rule of thumb' version posted above assumes that you are flying at sea level and uses an assumed Cl max.

Steve
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Old Dec 23, 2007, 06:10 AM
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Or more readably if your computer does all these characters:

V = √( 2 W g / ρ S Clmax )

g = 9.81 m s^-2
ρ is rho

The rest as above.
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Old Dec 23, 2007, 09:12 AM
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Quote:
Originally Posted by JetPlaneFlyer
The 'full' formula (in S.I. units) is:

V = sq root (W x 9.81/(1/2p x S x Cl_max))

where:
V = Stall speed M/s
p (rho) = air density KG/M^3
S = wing area M^2
Cl_max = Coefficient of lift at stall
W = weight KG

The simplified 'rule of thumb' version posted above assumes that you are flying at sea level and uses an assumed Cl max.

Steve

Thanks Steve and Andrew. Not sure how I'd get air density or coefficient of lift at stall?

Thanks a lot.
Jim
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Old Dec 23, 2007, 10:42 AM
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air density is about 1.2 kg/m^3 at sea level, max airfoil Cl must be determined from airfoil polar graphs ( Cl/AOA or Cl/Cd, typically).
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Old Dec 23, 2007, 10:51 AM
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Quote:
Originally Posted by cloud-9
Thanks Steve and Andrew. Not sure how I'd get air density or coefficient of lift at stall?

Thanks a lot.
Jim
Air density is about 1.25 KG/M^3 at sea level, less as you get higher.

Cl max varies according to airfoil used, wing aspect ratio and the size and speed of the model... Somewhere in the range 1 to 1.5 would be reasonable, small slow flying models being closer to 1 and larger and faster models closer to 1.5

Steve
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Old Mar 18, 2008, 12:36 PM
jeppe
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Is it Possible that The stall Speed Is Lower than the speed i used for Carculate the Lift?? The wight in the stall speed formula i used the total Lift from the lift Carculation.. but even the stall speed is about 1/5 Lower than i think it shuld be
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Old Mar 18, 2008, 12:57 PM
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I don't understand the question… the equation above is for stall speed in terms of the actual weight of the aircraft, you don't need to calculate any lift forces because they are already rolled into the formula above.
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Old Mar 18, 2008, 01:28 PM
jeppe
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Quote:
Originally Posted by Andrew McGregor
I don't understand the question… the equation above is for stall speed in terms of the actual weight of the aircraft, you don't need to calculate any lift forces because they are already rolled into the formula above.
It a bitt hard to explane the question. but im well aware that is no need of carulate lift force in for stall speed.
I Thougt, If I take the Lift Created From the wing(Carculated by formula) Insteed of the actual weight of the aircraft, In that way i can check if the formula is right. For example if i Carculate Lift For 100 Km/h and use the same Data in the stall speed carculation, i would come up with the same Speed (100km/h) but insteed I came up with a stall speed of 72 Km/h. i just wondred if the stall speed use to be lie lower than the speed I carculated lift with?
Im about to do a Design Templet, For making the design better and also adjust
it after the requerments i want.

Thanx
// Luft 46' Design
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Old Mar 18, 2008, 02:04 PM
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Quote:
Originally Posted by Horten_XVIII
In that way i can check if the formula is right. For example if i Carculate Lift For 100 Km/h and use the same Data in the stall speed carculation, i would come up with the same Speed (100km/h)
But you would be checking the formula against itself ... The 'lift formula' is the same formula as the 'stall speed formula', just transposed to find the different 'unknown' quantities.
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Old Mar 18, 2008, 02:32 PM
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some additions:

you have to separate cl and cl_max.

for stall speed you have to use cl_max, for all other flight conditions you have to use cl.


cl depends from alpha (AOA) and has a maximum with cl_max

--> cl < cl_max

for stallspeed the lift is the weight of the model. for climbing lift > wheigt!.

if you are flying faster than stallspeed and NOT climbing (--> cruise), then cl < cl_max and alpha < alpha_max.
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Old Mar 18, 2008, 03:42 PM
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Quote:
Originally Posted by HugePanic
for climbing lift > wheigt!.
No for a steady state climb; lift = weight.
If the lift was greater than the weight the aircraft would be accelerating upward, i.e. the rate of ascent would be constantly increasing. It's 'excess' thrust that makes an aircraft climb, not 'excess' lift.

Stall speed itself is a somewhat missleading term because it's only applicable if the aircraft is flying straight and level. If the aircraft is turning then stall speed will increase, in fact try to turn too hard and it's quite possible to stall when flying at flat out speed (or pull the wings off, whichever comes first .

Steve
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Last edited by JetPlaneFlyer; Mar 18, 2008 at 03:48 PM.
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Old Mar 18, 2008, 03:53 PM
German Engineering.......
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Quote:
Originally Posted by JetPlaneFlyer
No for a steady state climb; lift = weight.
Steve
you are right!
for unaccelerated climb lift = weight *fullstop*
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Last edited by HugePanic; Mar 18, 2008 at 04:08 PM.
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