How to predict the stall speed of an airplane?

[Twmaster]

I know some of the software out there predicts all sorts of useful info but do any of them also predict the stall speed of the airframe? It would be useful (to me at least! :)) to be able to get a grip on this.

Anybody?

:cool:

[jonnyjetprop]

In a round about way, motocalc predicts the stall speed. How accurate:confused:

John

[fdix]

You can predict the stall speed. Stall occurs when the lift generated by the airfoil is no longer sufficient to support the aircraft weight.

You need the following data:
Airfoil
Wing area in m^2 [S]
Weight of aircraft in N [L]
Density of air in kg/m^3 [ρ]

The procedure is basically like this: you calculate Clmax (maximum lift coefficient) with xfoil for your airfoil then put this Cl in the following formula
http://infinityinfinity.com/x/cl.jpg

You now have all the data to calculate v

If you like we can go step by step trough the whole procedure... Just give me the necessary data.

The prediction is fairly accurate for a rectangular planform for more complicate planfors things get more difficult.

[fdix]

I thought I'll give an example for anybody interested.

Aircraft is a 2m slope racing machine.

HN-354 airfoil
0,39m^2 wing area
12,6N aircraft weight
1,2 kg/m^3 air density (sea level 20°C)

From the polars we can see that we won't get much more than Cl = 0,85 without a very significant increase in drag. So let's take Clmax = 0,85

Doing the maths we have V = 2,49 m/s

This is not a bad result, even if it is lower than the actual stall speed. This because with xfoil we assume a wing with infinite span. A real wing's Clmax is always lower because of induced drag.
From experience with the model I would have said 3-4m/s stall speed so we can be happy with the result!

If anybody would like more information or didn't understand something feel free to ask! ;)

[rebell]

A bit of life in a old thread.

The average modeler can not see the m/s speed in flight. You don't have a airspeed gauge in front of you. One must learn to know how a model is flying. You will see when it is flying "happy" and when it is going to stall. You learn to feel the model.

[watt_the?!]

great thread guys!

these comments are very valuable!

Tim

[flyranger]

Or if you are math challenged like me, click on the following link and enter the data at the bottom. Only uses wing area and model weight.

http://sky.prohosting.com/air2/cg_calc.htm

[skiman762]

A bit of life in a old thread.

The average modeler can not see the m/s speed in flight. You don't have a airspeed gauge in front of you. One must learn to know how a model is flying. You will see when it is flying "happy" and when it is going to stall. You learn to feel the model.

I agree plus you would need to know the wind speed and direction at all times as this effects true airspeed things we can't do with models. When I get a new plane after it's maiden I get it up to a safe altitude and stall it to see what signals it sends out some planes don't warn you they just stall my yak-54 is like that one sec it's fine next it stalled I'm sure lots of other 3d palnes are that way too

[Jurgen Heilig]

I thought I'll give an example for anybody interested.

Aircraft is a 2m slope racing machine.

HN-354 airfoil
0,39m^2 wing area
12,6N aircraft weight
1,2 kg/m^3 air density (sea level 20°C)

From the polars we can see that we won't get much more than Cl = 0,85 without a very significant increase in drag. So let's take Clmax = 0,85

Doing the maths we have V = 2,49 m/s

This is not a bad result, even if it is lower than the actual stall speed. This because with xfoil we assume a wing with infinite span. A real wing's Clmax is always lower because of induced drag.
From experience with the model I would have said 3-4m/s stall speed so we can be happy with the result!

If anybody would like more information or didn't understand something feel free to ask! ;)

Your formula is correct, so obviously your maths are wrong ;):

Try again, and you should get v=7.96m/s

Of course you can also increase cl before touch down to reduce this speed even further.

:) Jürgen

[fdix]

ooops :D

I'll recalculate then when I have a moment of spare time...

Anyway this shows that you really read my post ;)

[jb48]

A fairly good shortcut formula for predicting stall speed of any monoplane with rectangular wing is: Wing loading square root multiplied by 4. example: my Superstar EP has a wing loading of 20 0z/sq ft when powered with a 8 cell SC 2400 Nicad, so the square root of 20 is 4.4721359, 4 times that number gives 17.88 mph. Accurate enough for everyday flying needs.

[skiman762]

All the math is good stuff but without instruments to tell you TAS AOA etc it does no good with models it still more of an art.
I just follow the advice I was given by the ole boy then got me started
''When in doubt throttle out''
It has saved more the one plane for me for sure
what works best for me is learning to see what the plane is telling me
most will let you know when they don't want to fly anymore. It's up to us to listen

[Jurgen Heilig]

A fairly good shortcut formula for predicting stall speed of any monoplane with rectangular wing is: Wing loading square root multiplied by 4. example: my Superstar EP has a wing loading of 20 0z/sq ft when powered with a 8 cell SC 2400 Nicad, so the square root of 20 is 4.4721359, 4 times that number gives 17.88 mph. Accurate enough for everyday flying needs.

Your "shortcut formula" is based on a lift coefficient (cl) of 1.6 - which is rather high. If you use the factor 5 instead of 4, you get the speed for cl=1.0

:) Jürgen

[Jurgen Heilig]

All the math is good stuff but without instruments to tell you TAS AOL
etc it does no good with models it still more of an art.
...

You are absolutely right, the math normally doesn't help you in the real world, except if you have an expensive model with onboard telemetry.
I assume you meant AOA (angle of attack) and not AOL. AOA is even more important than the stall speed, as you can stall the wing well above stall speed.

:) Jürgen

[jb48]

Your "shortcut formula" is based on a lift coefficient (cl) of 1.6 - which is rather high. If you use the factor 5 instead of 4, you get the speed for cl=1.0

:) Jürgen

You are wright Jurgen, the shortcut formula gives good prediction for high lift straight wing, undercambered and/or thick flat bottom section. For symetric or thin airfoil, factor 5 is closer to reality.

[jrb]

Simple, but effective: WL^.5 * 3.7; oz / sq ft * 3.67 = mph.

Corrected!

[Jurgen Heilig]

Simple, but effective: WL * 3.7; oz / sq ft * 3.67 = mph.

Simple - but wrong. ;)

The sample model with a wing loading of 20 oz/sq ft would have a stalling speed of 73.4 mph according to this "formula".

:) Jürgen

[watt_the?!]

if i am anticipating a stall- due to airspeed and AoA i quickly revert to at least a level attitude...if not slightly nose down.

on landing its always slightly nose down until i know the model's characteristics.

one thing though, it dont help with a deep stall....that happened to me once in my huckebein...at decent speed and shallow ascent...never seen it before or since.

best of all i got it on video!

Tim

[jrb]

Simple, but effective: WL^.5 * 3.7; oz / sq ft * 3.67 = mph.

20^.5 * 3.67 = 16.4mph

[adam_one]

Your "shortcut formula" is based on a lift coefficient (cl) of 1.6 - which is rather high. If you use the factor 5 instead of 4, you get the speed for cl=1.0

:) Jürgen
Hi Jürgen,
How do you know that the shortcut formula is based on Cl of 1.6? could you give us some more details or references to your source?
Thanks.

[Jurgen Heilig]

Hi Jürgen,
How do you know that the shortcut formula is based on Cl of 1.6? could you give us some more details or references to your source?
Thanks.

Hi Adam,

The formula in post #3 is correct:

http://www.wattflyer.com/forums/showpost.php?p=14235&postcount=3

You just convert it so it says v=... and compare it with the simplified formula. The orginal formula delivers the same figures as the simplified formula when you use cl=1.6.

:) Jürgen

[adam_one]

Thanks Jürgen, now I see where you got Cl 1.6 from... :)

Fdix used 1,2 kg/m^3 for the air density, so I assume that's the standard value at see level.
Another detail that we may also consider is that the max Cl of the whole wing is lower than the airfoil's max Cl due to the wing's induced drag...

Stall speed (m/s) = [2*Weight / (Clmax*1.2*Area)]^0.5

[Bill G]

I have an issue of Quiet Flyer, something like maybe APR 2004, can't remember exactly, that had this info in it. After getting all the values into the formula, the result actually looked reasonable.
The article also touched on cubic wing loading. As planes get larger, the wing is more efficient. The info that stuck in my head was that a full size high wing trainer has something like 13lbs/sq-ft wing loading. That's right, pounds, not ounces.

[adam_one]

I guess you're referring to August 04 issue of Quiet Flyer, as I've got it too.
They refer to a full scale Cessna 152 having 160 sq ft of wing area and weighting 1,670 lb fully loaded giving a wingloading of 167 oz/sq ft.
They use the following rule of thumb:
An airplane's stall speed in mph is approximately equal to four times the square root of the wing loading in ounces per square foot.

But I think that the formula shown in the previous post above is more accurate.

[tillmas]

The reason that aircraft wings get more efficient as they get larger is that they are flying at much higher Reynolds numbers. Take a look at a polar plot sometime and you'll see that the higher you go in Re, the better off you are. Real airplanes have Re's well into the millions, where as we operate in the hundreds of thousands with the models.

The bottom line with stall speed is that there is no way to use a simple calculation and calculate it accurately. You all are getting very close to wanting real aerodynamic answers without being willing to live with the math involved. Just be careful, because that can lead to some dangerous miscalculations.

And to answer an earlier post, you CAN tell how fast your model is going while it is flying. As a competitive sailplane pilot, I know the exact speeds at which my wing is most efficient (the drag bucket), and I have calibrated sticks that provide me with a very good estimate of whether I am in the bucket or not. However, you are correct in that you have to learn to feel the plane (especially on base and final).

Matt

[rebell]

The experienced pilot can tell the ballpark figure for speed, but like I said, the beginner to average pilot can not.

[Jurgen Heilig]

...
I have calibrated sticks that provide me with a very good estimate of whether I am in the bucket or not.
...
Matt

Hi Matt,
What do you mean with calibrated sticks? :confused:

:) Jürgen

[Twmaster]

Wow. Talk about thread drift...

I'll stop asking simple questions now.

[fdix]

To bring things back on track a bit...

My first posts were a year ago. In the meantime I've learned a lot. Getting a Cl from a polar and assuming this Cl is constant over the entire wing area is not such a good idea.

A by far more accurate approach is using for example the program Nurflügel by Frank Rains, Michael Möller and company. Not only it saves you a lot of calculation but also the results are rather accurate, the calculated values came very close to real world data in many cases.

So here goes an other example. Aircraft is a Tigger, a 1.9m span, full carbon all out slope glider.

The procedure is like this

- Draw the wing in Nurflügel
- Insert a low speed value (I usually start with 8m/s)
- You get a Cl value, an angle of attack and the program also gives you the Re number for the entire wing (the blue curve)
- Start xfoil
- Plot a polar at the Re number you get form Nurflügel (be sure to go at least to the angle of attack you obtained earlier)
- Now comes the interesting part: can you reach the calculated Cl?
-> Yes. Ok it's time to party, after the party start over again with a lower speed value
-> No. Sorry no party, start over and try a higher speed value.

To follow the procedure you must be familiar with Nurflügel and xfoil and have a basic understanding of aerodynamics, so to know what you are calculating.

In my example I got 9m/s at Cl=1,086, alfa=9,8.
As you can see from the polar the RG-14 can just barley reach Cl=1,01 at 100k Re. So this is really the limit.

Ok now I've calculated all this and of course I'm exceptionally happy. But stall speed is interesting on landing.... And how do you land with a quadroflap wing? With butterfly of course...
You can consider flaps in Nurflügel but then things get complicated. You have a very high Cl where the flaps are positive and a very low Cl where the flaps are negative. You can't use the average Cl in xfoil.

It is not impossible to calculate stall speed when using butterfly but it is a rather long procedure.

Instead of spending lots of time in front of the computer just go fly and leave this dull task to the engineers ;)

Uups this is a long post, hope nobody is shocked :D

Edit: added the picture of the Tigger

[Lip84]

I just follow the advice I was given by the ole boy then got me started
''When in doubt throttle out''
It has saved more the one plane for me for sure
I have never heard this but it is gneious...absolute GENIOUS! And it really makes a lot of sense too...lol


what works best for me is learning to see what the plane is telling me
most will let you know when they don't want to fly anymore. It's up to us to listen

I like your, im probably going to biff this one, Zen?? approach to flying. Math is a wonderful tool but all the math in the world wont save your arse if you dont pay attention and love your bird.