pquispea_m5

Hi,
I'd like to estimate the stall speed of the model i am building. it use NACA64A204 (root and Tip, washout 3º), W/S=22.4, Weigth=25kg, aspect ratio=3, t/c=0.04. Need to know the procedure to estimate it. Thanks
Grts,
Pablo

beepee

Wow, good luck Pablo. I suspect there are a number of aspects impacting stall speed that you have not covered. I think it would be quite hard and in the end, how will you know when you are flying at that speed?

Bedford

CloudyIFR

http://www.rcgroups.com/forums/showt...57#post1391393

Curtis

Lnewqban

Pablo,

You need to determine the maximum coefficient of lift (CL) of that airfoil for the worst Reynolds number of your wing (tip if tapered wing).

Then you can use this calculator:

http://adamone.rchomepage.com/calc_stallspeed.htm

BMatthews

Another simple tool I like to use for this sort of ball park guestimate is Foilsim. But as Lnewqban says you need to look at a set of lift polars for the reynolds number you'll be using for this airfoil to find out what the best lift coefficient will be for your airfoil. That would likely mean inputing the coordinates into Xfoil using either the command line freebie version or getting Profili 2 and buying the unlock fob for the Xfoil and other advanced features.

Or since this is a really flat airfoil used on such full size jets as the F16 I'd just "guess" at the max Cl being in the .7 range.

Go to http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html and input your span and average chord and set the wing's angle of attack to 7 degrees (a likely stall angle for a model of this size and reynolds number). Then slow the speed down until it matches the weight of your model. The resulting flying speed will be about what the stall speed would be.

The taper and use of any flaps will greatly alter this value. Especially the flaps.

I would enter the numbers to show you but your wingspan of "22.4" doesn't make any sense. What units are these? It's obviously not meters and it's obviously not cm's. So I'm not sure what the span would be. If it's decimeters then that's 224 cm span so that would give us numbers for Foilsim of 7.34 feet span and a chord of 2.45 feet. Putting those values into Foilsim I get an expected stall speed of around 37 mph or 59 kph with your expected weight of 25Kg/55lbs. With an angle of attack of 7 degrees Foilsim predicts a Cl of 0.803. And that is not an unlikely number for such a flat low camber airfoil operating at what will be quite a high reynolds number thanks to the size and weight keeping the speed up like this.

As I said, I'm assuming you would be using flaps and possibly even a drooping leading edge. If so then your airfoil is no longer a NACA 64A204. It becomes something all together different and you would need to find a way to see what the lift polars are for the shape with the flaps and leading edge deployed. And that would be tough unless a program such as Xfoil can deal with an airfoil with such a kink in the camber line. As a guess if I increase the camber of the airfoil in Foilsim to 7% which would not be an unreasonable camber value increase for fairly wide flaps then the stall speed goes down to 25 mph/40kph. But this assumes that the airfoil with flaps down can achieve a Cl of around 1.66 without stalling before getting to that value. But as it sits now somewhere around 59kph would be your expected "clean" stalling speed.

pquispea_m5

Thanks for your answers friends. BMatthews, sorry i missed the units. I attached a sketch and NACA64A204 data in *.dat format (i change into *.jpg to cheat the system, just return to *.dat format when you downloaded). we'll keep in contact for more questions. Now I need to study this information for what i need. thanks again.
Grts,
Pablo

BMatthews

Best of luck with the model. If you can try to get the weight down by quite a bit. 25kg is heavy for a model of this size and that is why the stall speed is fairly high. But heavy wing loadings produce problems in more ways than just higher stall speeds for landing. In a tight turn the high wing loading is multiplied by the G load and with such a heavy weight you will find that the model wants to stall even at high speeds when pulling too tight a turn. I would suggest that you work hard to get the weight down to less than 20kg. And getting to around 15 to 17 would be even better. The F16 has a lot of wing area but you can only ask the wing to do so much. A lighter model will just always fly better and produce less problems and be more fun than flying a heavy one where you need to be careful to avoid breaking through the limits.

Granted I don't know jets all that well I know that the size of wing you're working with will not be happy with a 25Kg weight. I'd suggest you look around at similar size wing area and span models and watch how they perform and then find out the weight and wing loadings and use these performace clues as examples of what you should be aiming for as to model weight and wing loading.

pquispea_m5

thanks BMatthews, you are rigth. A model built by Gerald Rutten weight 15-19kg ( http://members.tele2.nl/geraldensuzanne/F16%20XL.htm ). Just wanted to analyze asumming extra weigth, but the goal is that <20kg. Wich software do you use to see the aerodynamics of your models?. Do you use finite elements softwares like ANSYS?.
Grts,
Pablo

Rob2160

Not trying to be smart.. but the stall speed is any speed you want it to be..

That is the first thing we teach pilots when they do a full size aerobatic rating..

Stall is a function of angle of attack.. and if you exceed the critical angle of attack the wing will stall, irrespective of speed.

Now.... if you are asking.. What speed will my aircraft stall at when experiencing 1G of lift... That question can have a precise numerical answer...

A much more useful thing to know is given your actual control link set up.. where will my elevator stick be when I stall?

That position will not change in your jet..

You can set it up to stall at say 85% elevator deflection.. the final 15% of elevator movement will only deepen the stall..

Your plane will stall at this stick position every time regardless of speed or attitude..


And yes, a heavier aircraft will always stall at a higher speed.. the best example of this is to imagine two 747s.

One is full of fuel and passenger and weighs over 400 Tons.

The 2nd is almost empty, minimum fuel.. and weighs 200 Tons (just for example)

Both are flying in formation at the same speed.. which one will have the higher nose attitude? IE angle of attack?

Now assume the wing stalls at 16 degrees angle of attack..

They both reduce speed steadily, remaining in formation... which one will reach 16 Degrees A of A first..??

That is an easy way to visualise how increasing weight will increase the stall speed...

Some great links from the other posts.. I'll check them out.. they look interesting!.

Good luck with your model.. It looks very impressive!

BMatthews

The only computational tools I use these days is the online CG calculator and that Foilsim applicaition. For the rest I've been designing, building and flying long enough that I use the TLAR method (That Looks About Right) If I'm doing an extreme design that I have doubts about for either pitch or yaw stability I'll make a simple all sheet profile glider to test for stability and to find the CG location through testing.

charlie111

I am talking about typical departure and approach stalls.Not to be smart but, I knew that already you did'nt have to go into so much de tail .I'm just trying to help this guy and thought with a it would assist in lowering the nose to the horizon if his control surfaces were'nt working at to slow of an airspeed.This is not an acrobatic aircraft he is building .Let him get it of the ground first before you start telling him about all the manuevers he can' do!

foutsjoe

A basic stall speed calculator (read: estimator) I have had good luck with is the following calculation (from the Pamadi dynamics book):

Vstall=Square root of (weight / (0.5*Max Cl*0.9*Wing area*Air Density))

It's just the general lift equation solved in terms of Velocity.

Weight is in lbs
Area is in ft^2
Density is in lb/ft^3  (usually around 0.002 for air)
Vstall is in ft/sec

As many have already said, the Max Cl is obviously the tricky part. I use Profili to estimate it for our Reynold's Numbers, but it won't do complex wings.

pquispea_m5

hi foutsjoe,
which velocity do you use to calculate Reynold's Number? is it Vstall???
grts,
Pablo

foutsjoe

Pablo,

Igenerally estimate a slightly lower than expected stall speed, and run the Reynold's calculations with that in mind. Most low cambered airfoils on our scale seem to increase in efficiency (a higher Cl at a given angle of attack) as the reynold's number increases, so choosing a speed on the low end adds a bit of a safety factor in the max Cl. If that reynold's number leads to a final stall speed that was vastly different from your estimation, you can re-run the Re calculation with a better estimation. It's a rather iterative process.

Because of the sensitivity to the Reynold's number, the Max Cl is usually the number that may not be realistic (depends on manufacturing, surface finish, tip losses, etc.); but that calculation will usually give you a good idea.

Hope this helps,

Joe

pquispea_m5

thanks Joe,
I used profili to estimate stall speed. But I had problems to generate the graphs. Profili show me an error in the XFoil calculation, so i followed the instructions in the message but the airfoil thickness changed from 4% to 4.23%. I attached some results. could you check this please.
Grts,
Pablo

Hossfly

Charlie111 you are displaying a characteristic that you may well not care to display. Rob2169 was providing you with some excellent information that far too many 1:1 scale pilots do not understand let alone RC "pilots". There is no such thing as "stall speed." Is an aircraft sitting on the tarmac "stalled"? Of course not, Can an airplane obtain ZERO airspeed in the air and not be stalled? Yep, it can. Done it many times. Most RCers have done same, just did not know it. Example: Performing a straight-up flight maneuver at a given power setting, when it runs out of steam, and drops back, for an instant the airplane was at ZERO airspeed, but NOT "stalled".

The term "stall" in the aeronautical term has to do when the airflow over a surface becomes deflected to the point where the free-air flow separates from said surface and creates separation. That happens at certain applied forces.
Most civil use airplanes in sub-sonic convergent airflow can reach this situation reference the wing airflow. When said separation happens, then there is no longer a lift force from that wing. Gravity takes over. For a given wing form, that separation happens at some specific angle-of-attack, the difference between the wing chord and the oncoming free air stream.

As far as maneuvers think on this one. My example will be the old USAF Lockheed T-33, trainer version of the F-80. We use to do this for fun and to show the student pilots some reality as at that time in their academic schedules they were into Applied Aeronautics for Pilots. It took IIRC (in the 1965 era) some 320 knots IAS to attain the limit of 7.33 G wing loading without attaining the critical angle of attack and experiencing an airflow separation (stall), that is one could not obtain the wing-loading limit angle of attack at the lesser speeds. So the guy doing 350 IAS (Indicated Air Speed) could turn inside the slower machine simply because he could pull more "Gs". BTDT.

Therefore Be aware that the old "stall speed" is a myth. The formula is Lift is equal to the coeficient of lift (angle of attack) times 1/2 of air mass density, times the airspeed squared, times the wing area. As a pilot you can control airspeed, angle of attack and to a minor degree the wing area (Fowler Flaps). All the rest is just wishful thinking.

blueskybud

There is vectoring to consider as well as newton's second law. Temp. also plays a role.

Shoe

I'll buy that the airspeed where you can just reach the g-limit ("corner airspeed") will give you a better turn rate, but it will NOT give you a tighter turn radius than a slower airplane of the same type.

Turn Radius = (V*V)/(n*g) (true airspeed squared divided by load factor times g)

n*g = Lift/mass = q*S*CL/mass where q=(1/2)*density*V*V, and S is reference wing area so...

Turn Radius = (2*mass)/(density*S*CL)

Minimum Turn Radius = (2*mass)/(density*S*CLmax)

For a given altitude and gross weight, the minimum turn radius is about constant (as long as you are below the airspeed where you can just reach the g-limit). A faster airplane will obviously get around the circle faster, but it won't have a smaller radius. In practice, CLmax varies with Mach number, and the airspeed band for minimum radius tends to be below corner airspeed.

Rick.

The best way to estimate your models minimum speed before the increasing angle of attack stalls the wing is to FLY it, slow it down at a safe height and let it stall - you can then SEE how fast its flying for this to happen. You'll find things simply do not work according to the number crunching, for example, despite the washout, that narrow wing tip may just decide to stall on one side, well before a nice clean straight stall can develop, if it does that'll chuck all the carefully crafted math out of the window.

Don't get me wrong, the math IS fun, I calculate my models expected stall speeds (or more correctly speed at the stalling angle of attack) just to get an idea of it, but for flying models with low Re and the unpredictable effects of gusts cross winds and even low level windshear, the math will not tell you how to fly. I assume that as this is an ambitious large model compared to typical sunday club flyers machines, you must be a well experienced model flyer already - in which case you'll already know that calculating a theoretical stall speed ain;t gonna count for anything when your out on the grass strip flying the maiden by the seat of your pants.

It does look like its shaping into an impressiive plane and when its ready for flight it would be great to see some pics of it in action. Good luck and don't worry too much about the numbers

Oh, one final thing - what are you powering it with - i guess its too big for electric, so is it glow DF or are we talking real jet turbine????

Lnewqban

It is not a myth, it is just an indirect reference point that full scale pilots have used due to lack of better instruments.

Applicable for radio piloting?

Not so much:

http://www.rcuniverse.com/forum/m_10...m.htm#10119857