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Aug 23, 2010, 07:22 PM  
Joined Sep 2006
585 Posts

Propellor Stall Thrust
I recently ran into the term "Propellor Stall Thrust" when using the calculator program on the Castle Creations web site. It's the first time I have seen the term and I was wondering if anyone could define it for me.
Regardless of the definition, the results presented are interesting. The program predicts a stall thrust which is typically much lower than the static thrust. It provides a warning for certain high pitch props that it is likely that the static thrust will not be reached; the prop is in a stalled state at the initial stage of takeoff. For something like an APC 12x12, the stall thrust can be as little as 40% of the static thrust prediction. For me it makes a tremendous difference in my motor/battery selections for the airplanes that I am flying. Bob 
Aug 24, 2010, 02:31 AM  

Hi Bob,
your propeller runs into the prop stall when Pitch : Diameter > 0.66 (you have 12:12 = 1!). with high pitch propellers the air looses contact to the propeller blade. this result in significant less thrust (you can even hear that as the propeller gets suddenly louder.) in this case you will not reach the static thrust (and calculated current) on a static measurement  you rech only the Stall Thrust figure. high pitched propeller makes only sence for speed models (like pylon). If you want to use this setup in a none speed model just stay below 0.66 pitchdiameterratio. so, the question now is what type of plane you want to fly....? kind regards eJet 
Aug 24, 2010, 11:23 AM  
Joined Sep 2006
585 Posts

ejet,
Thanks for the concise explanation. I started in electrics with the large, 71", Seawind. With a limited prop diameter, 13", and a motor with an upper limit on number of cells, I was trying to get thrust with prop pitch. Flying the Seawind off snow was not a problem, but getting it off the water, at high altitude has been a problem. Recently I switched to a higher kv motor with a higher cell count and a lower pitch prop with much better results. This is the first time I had seen the terminology and I am trying to correlate my emperical data with what I calculate. Thanks again, Bob 
Oct 04, 2010, 04:31 AM  
Joined Oct 2007
65 Posts

Prop Size
Hello All,
I have a 1400Kv motor with a 18 A ESC and 11.1 Volt 1350 mAh battery. My plane weighs about 650 gms and I use a 6x4 prop. Even after a flight of about 56 minutes the motor and battery are cool. Although a bit sluggish, the overall performance is satisfactory. I now plan to build another plane but atleast 150g lighter. Can I use a 5x5 prop, now that the plane would be lighter ? Intention is to have a longer flight with a more efficient plane. Thanks, b 
Nov 09, 2010, 10:33 AM  

About Web o Calc and CWL
Hi
It seems I cannot reply in the WOC thread (too old) http://www.rcgroups.com/forums/showthread.php?t=930018 So if Flieslikeabeagle (John) is listening : Data in metric Weight 700g Span 1370mm Wing area 24 dm2 WOC leads to CWL = 4.4 g/dm3 I find CWL = 700 / (24^1.5) = 5.95 g/dm3 WL = 29.2 g/dm2 is correct Regards Louis 
Nov 09, 2010, 11:14 AM  

Quote:


Nov 13, 2010, 03:31 PM  

Quote:
However, in this particular case, what you found is not a mistake. There is in fact more than one way to calculate cubic wing loading, and some are better than others. I used a better way in WebOCalc. Let me explain what I mean. Imagine two RC models, a powered glider and a scale Extra 260, both with the same total weight and same wing area. The glider will have far more wing span, with its long narrow wings, while the Extra has stubby broad wings typical for highly aerobatic aircraft. From practical experience, we know that these two models fly quite differently  the glider will feel much lighter and floatier than the Extra. But if you calculate the wing loading or traditional cubic wing loading for both models, you get the same answer. In other words, the CWL says the glider and Extra will fly similarly  but in the real world, they do not. The CWL fails to do its job  it's supposed to predict how the model will fly, but it fails to do this accurately. So, the traditional way of calculating the CWL is not very good. It's better than plain old wing loading, but it's still not very good. We can do better! Here's what's going on. The goal of CWL is to allow for the fact that the airflow around the wing is threedimensional, so that changes in wing span and wing chord also affect the airflow above and below the wing. Therefore, we need a formula that transforms the wing dimensions (dimensions of length squared) into a volume (dimensions of length cubed). The simplest CWL formula is to simply raise the area to the power 1.5: Code:
Effective volume = wing area^1.5 = wing area x square root of wing area Therefore CWL = model weight/effective volume = weight/(wing area^1.5) Here's why: if you look at calculations or observations of the actual 3D airflow around a real wing, you'll find that changes in wing span cause a big change in the amount of airflow over the wing. The influence of the wing on airflow extends upwards above it to about half the wing span, so a wing with a 40 foot span substantially affects the airflow a good 20 feet above its center point. Also, increasing the span reduces the effect of wing tip losses, and this also produces a significant increase in lift  we all know that gliders have long, skinny, high aspectratio wings because they are more efficient at producing lift with less drag. However, changes in wing chord do not have as big an effect on a wings efficiency. They do not affect the airflow around the wing and aerodynamic losses in the same way as increases in wing span. They do not help with the wing tip losses either. Adding more wing area by increasing the chord does not benefit the wing as much as adding area by increasing the span. Now, the wing area is simply average span x average chord, but as we've just discovered, the two things (span and chord) are NOT equal when it comes to their effect on lift. Therefore, a better formula for cubic wing loading would take this difference between span and chord into account. The wing span should be weighted more heavily than the wing chord when calculating the effective volume I referred to in the first paragraph. We need an algebraic quantity with the dimensions of length cubed, but with wing span given more weight than wing chord. The simplest answer is to use (wing span x wing span x wing chord)  because the span appears twice in the formula, changes in span affect the product more than changes in chord, which is exactly what we want. Using this, here's how we arrive at a better CWL formula: Code:
Effective volume = wing span x wing span x wing chord Therefore, Cubic Wing Loading = model weight/(wing span x wing span x wing chord) At this point, WebOCalc has been successfully used to predict, with reasonable accuracy, the performance for everything from subounce indoor models to giant 20 lb and 30 lb models, everything from vintage biplanes to powered gliders to scale commercial jets. I myself have been surprised at how well the program handles this very wide range of airframe sizes and types, given how extremely simple the program is at heart. Using the more accurate calculation for CWL described above was a significant factor in getting the program to work as well as it does. Incidentally, Ken Myers contacted me about this same issue a year or two ago, and he and I had almost this same email conversation. By the way, I also found an error in the lift calculation used in a large number of RC calculators popular here in the USA. Everyone knows the basic formula for wing lift (L = 1/2 * Cl * rho * V^2 * S). Years ago someone plugged in Cl=1.0 and rearranged the formula to calculate V, the stall speed. So far so good. But something went wrong numerically in converting that answer to miles per hour, given wing loading in ounces per square feet  I've forgotten the details now, but I remember finding the wrong formula all over the Internet, and that same wrong formula is also incorporated into a number of RC calculator programs I tried. I found the error because I started out working out the formulae for WebOCalc in SI units (I didn't grow up in the USA). I worked out the formula for stall speed using the density of sealevel air at room temperature, and when I converted my answer to miles per hour, it didn't match the other calculators I compared it with. The stall speed I calculated is higher, for the same wing loading. I went over my math several times and tried several sanity checks (wing loading of one newton per square metre, for instance), and eventually convinced myself that the mistake wasn't mine, but was in the formula used in so many other calc programs. So keep your eye open for inaccurate stall speed numbers floating around out there! Flieslikeabeagle 

Mar 26, 2011, 03:28 PM  
United States, IA, Mt Pleasant
Joined Nov 2007
10 Posts

I'm trying to convert a Slinger 380 brushed motor over to a brushless motor and I'm about as lost as you can get. I know the prop, esc, and motor have to match up but there are so many that I'm having a hard time trying to figure out a good combination. What combination (esc, battery, motor, and prop) could I install that would get me in the air? wdretired. ..

Mar 27, 2011, 07:29 AM  
USA, ME, Ellsworth
Joined May 2008
16,806 Posts

Mention the prop size, the battery you were using before, and the amount of power you were using (volts x amps = watts) if you know any of that. All of that goes to getting a Kv that will work with the prop and a motor of a weight that will have enough power.
Jack 
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