Equation for stall speed, wing loading, speed?

[cloud-9]

Thanks. I'm reading an article

http://www.masportaviator.com/ah.asp?CatID=2&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

[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

[cloud-9]

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

[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

[Andrew McGregor]

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.

[cloud-9]

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

[Flyingwingbat1]

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).

[JetPlaneFlyer]

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

[Horten_XVIII]

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:confused:

[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.

[Horten_XVIII]

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

[JetPlaneFlyer]

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 :confused: ... The 'lift formula' is the same formula as the 'stall speed formula', just transposed to find the different 'unknown' quantities.

[HugePanic]

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.

[JetPlaneFlyer]

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 :rolleyes: .

Steve

[HugePanic]

No for a steady state climb; lift = weight.
Steve

you are right!
for unaccelerated climb lift = weight *fullstop*

[Horten_XVIII]

But you would be checking the formula against itself :confused: ... The 'lift formula' is the same formula as the 'stall speed formula', just transposed to find the different 'unknown' quantities.

I Know its the same Formula, But i Would test it against it self. I Would Get
the same speed.. but i doesnt. Here is my Convertet Formula i used..
<wight in gram>/1000*9,81/(0,5*<rho in Kg/m^3>*(<wing area in Cm^2>/10000)*<CL>))^(1/2)*60*60/1000

Fat Test= To get it in Km/h

[Horten_XVIII]

A preview of my work.
Orignaly Made For Tandem wings but Works to other to..

[JetPlaneFlyer]

Horten,
to be honest you have lost me :confused: ... Stall speed is one thing. Stall is reached when the wing reaches it's Clmax and that can be calculated for level (unaccelerated) flight by the formula posted previously and which you show above.

What are you trying to achieve by substituting in arbitrary airspeed values?

BTW... the aircraft you are using as an example is unfeasibly heavy for it's size... are you proposing to carve it from solid teak?

[Horten_XVIII]

Horten,
to be honest you have lost me :confused: ... Stall speed is one thing. Stall is reached when the wing reaches it's Clmax and that can be calculated for level (unaccelerated) flight by the formula posted previously and which you show above.

What are you trying to achieve by substituting in arbitrary airspeed values?

BTW... the aircraft you are using as an example is unfeasibly heavy for it's size... are you proposing to carve it from solid teak?


CLmax is the Top of the Cl courve, The critical angel of attack?
so its not for level fligt?
Haha, Solid teak is indestrutiable... No, thats just the templet :rolleyes:

[JetPlaneFlyer]

Clmax is the peak of the Cl curve... you would not fly at this value in normal level flight, cruise speed would be well below Clmax.

[Horten_XVIII]

Clmax is the peak of the Cl curve... you would not fly at this value in normal level flight, cruise speed would be well below Clmax.
Thanx so much jetPlane:P
You just help me a bit on my progress of that templet :)

[Flyingwingbat1]

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 :rolleyes: .

Steve

Well, I'm being nit-picky here, but if you draw a force diagram with lift, drag, weight, and thrust vectors, the total lift (defined as the force component perpendicular to the flight path) ends up being LESS than aircraft weight. Think of the most extreme examples, like a 3D plane or full-scale fighter jet in a vertical climb; the lift from the wings effectively goes to zero. A "regular" plane in a climb is simply a less extreme example of that.

[HugePanic]

it's not soo easy!

if i am picky, then you are wrong too! ;)
at least in some flight conditions..

imagine an trimmed aircraft in unaccelerated level flight. (longitudinal axis is horizontal)

now you increase thrust, but you keep the nose down/horizontal.
the aircraft will increase speed and as a result of this climb (Lift is proportional to v²).

but the thrust here has no vertical component.
here the lenght of the lift-vector is larger than the weight-vector.
the vertical component of the lift-vector is equal to the aircrafts' weight...
due to its climb the airflow now has an angle to the longitudinal axis of the plane, so the lift vector is angeld to the back of the AC. the x/horizontal component is compensated by the added thrust.

is anybody more picky and find more mistakes??

[JetPlaneFlyer]

Well, I'm being nit-picky here, but if you draw a force diagram with lift, drag, weight, and thrust vectors, the total lift (defined as the force component perpendicular to the flight path) ends up being LESS than aircraft weight. Think of the most extreme examples, like a 3D plane or full-scale fighter jet in a vertical climb; the lift from the wings effectively goes to zero. A "regular" plane in a climb is simply a less extreme example of that.

Yes, i was over simplifying a little, in reality as the plane puts it's nose up into a climb the thrust from the engine does some of the 'lifting' and the wing does less... the point I was trying to make was that the wing does not have to lift more in a climb than it does in level flight.