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Calculators for Aero-Modeling - 9/13/2004 1:35:03 AM   
PylonWorld



Posts: 1346
Joined: 12/29/2001
From: Monroe, NC, USA
Status: offline 		Every time the issue of props, rpms, and maximum theoretical speed comes up, I have to locate my spreadsheet with various calculators for modeling and aerodynamics.

I decided to create a quick little web application so that I can get to my calculations from the web.

The first calculator I have implemented is for Pitch Speed. It is at http://www.rcpro.org/rccalc. It allows you to input a propellor pitch in inches and engine RPM to see the theoretical maximum speed in miles per hour. It does not account for pitch measurement imprecision or unloading in the air. Nor does it account for propellor inefficiencies. But it is convenient for reality checks, or just messing around. I will be adding the ability to calculate using different variables.

I will be adding calculators for things like Miles Per Hour to Feet Per Second, G's in a turn of a specified radius, MAC, tail to moment ratios, etc.

If you would like to see a specific calculator or if you have calculations that you think other modelers would be interested in, please let me know.

I will also be enabling the calculators for phone and PDA browsers.

_____________________________

Don Stegall
RCPRO Chairman of the Board
       Post #: 1

RE: Calculators for Aero-Modeling - 9/13/2004 1:26:39 PM   
Ed Smith


 

Posts: 2875
Joined: 12/29/2001
From: Brantford, ON, CANADA
Status: offline 		Nicely done Don, nicely done.

Maybe, only maybe, we will see some more realistic speed claims posted now.

Ed S

(in reply to PylonWorld)
       Post #: 2

RE: Calculators for Aero-Modeling - 9/13/2004 5:02:10 PM   
DHG


 

Posts: 896
Joined: 4/24/2002
From: Arvada, CO, USA
Status: offline 		Don,

Here's a suggestion for something to do last of all, if time permits.

How about a plot of Reynolds numbers at various stations on a propeller?

To make it easy (easier, that is), I would suggest a fixed number for rpm -- call it 20,000 -- and a fixed number for blade width -- call it 1/2 inch, or perhaps 1 cm. Then it would be one simple calculation to adjust the Reynolds number for a different rpm, and one simple calculation to adjust it for blade width.

Three or four points is all you'd need; call it stations 3, 5, 7, and 9 on the Prather or Edmunds gauge (that's at a diameter of 4 cm, 6 cm, 8 cm, and 1 dm out from the hub).

The idea here is to allow us diehard prop-carvers to select a good airfoil for the hub, the tip, and in between based on NACA charts or whatever else we might be able to scrounge up. In particular, I'd be interested to know whether an undercambered airfoil helps at all, and if so, whether it should be at the hub, the tip, or in between. And then of course there's the question of how thick to make it at the various stations. Knowing the Reynolds number would give a clue as to what's likely to work best.

No good deed goes unpunished, does it, Don?

Duane Gall
RCPRO

(in reply to Ed Smith)
       Post #: 3

RE: Calculators for Aero-Modeling - 9/14/2004 12:04:14 AM   
PylonWorld



Posts: 1346
Joined: 12/29/2001
From: Monroe, NC, USA
Status: offline 		Ed,

Thanks. I thought you might like it.

I have now added the G-Force Calculator. Input some of your favorite hypothetical speed and turning radius info to see why carbon fiber is such a miracle material for pylon racing airplane wings.

DG,

We don't have to make it simple. Computers are good for complex numbers and scenarios. Now if you can explain to me how to do the calculations, I'll be glad to create the calculator for you propeller heads.

< Message edited by PylonWorld -- 9/14/2004 12:37:33 AM >

_____________________________

Don Stegall
RCPRO Chairman of the Board

(in reply to DHG)
       Post #: 4

RE: Calculators for Aero-Modeling - 9/14/2004 4:29:33 AM   
Duane-RCU



Posts: 440
Joined: 5/5/2002
From: Arlington, TX, USA
Status: offline 		Fantastic! I've been having all kinds of fun with this. My Rookie (basically an Ugly Stick) has been clocked at 95MPH with a Magum .46 Pro. So I guess with a 10x6 MA it's around 16,800 RPM, I think it had somewhere around 14,000 on the ground(I forget). I wonder what my Viper with the ST .45, and a 9x7APC that has 16,000 on the ground, unloads in the air? It sure seems a whole lot faster the the Rookie. To be able to go 120, it would have to turning over 18,000(could it?) Keep up the good work, you guys here don't know how much you help some of us "small time" racers! Thanks for sharing the info.

< Message edited by Duane-RCU -- 9/14/2004 12:50:53 PM >

(in reply to PylonWorld)
       Post #: 5

RE: Calculators for Aero-Modeling - 9/14/2004 6:44:39 PM   
DHG


 

Posts: 896
Joined: 4/24/2002
From: Arvada, CO, USA
Status: offline 		Don (and our studio audience, if any),

We all know big airplanes fly more efficiently than little airplanes, and airplanes fly better when they're going faster. That's because air is sticky. It sticks especially well to small objects that are moving slowly. Size and speed are equal factors. The "Reynolds number" measures both at the same time. What you do is multiply the chord length of the airfoil by the speed it's traveling, then multiply those by some constant, and you get a handy number representing the stickiness of the air molecules. (Actually it's the number of air molecules contacting the wing surface during any given time period, but the main thing we care about is their collective stickiness & the way they flow.) This, in turn, can be used to predict the behavior of an airfoil.

Until recently, all the available performance charts were for full-scale airplane airfoils (3- to 10-foot chords) at full-scale airplane speeds (50 to 500 mph). We modelers had to use trial and error to find out what worked for us. Model-oriented, "low Reynolds number airfoil performance data" didn't exist until Mike Selig enrolled at the University of Illinois in the late 1980s and decided to parlay his love of model sailplanes into the basis for his aero engineering degree thesis. Now we've got some pretty good stuff, including even laminar-flow airfoils that seem to work on a 10-inch-chord model traveling at under 200 mph. That's pretty radical. But not much has been done on tiny propellers, as far as I know.

Figuring out the Reynolds number of a propeller would involve figuring out the airspeed at a given distance from the center (which you'd derive by multiplying the circumference of the circle by the rpm), then plugging that and the chord of the prop into the Reynolds number formula. Fairly simple math, but a lot of numbers to crunch ... that's why I haven't got off my butt to do it myself.

I'm at work right now (OK, I'm at my office) and I don't have the Reynolds number formula handy. It should be fairly easy to find, perhaps on Martin Hepperle's site. If you run into any trouble, contact me off-line and I'll scrounge around in my filing cabinet of precious airplane design data.

Duane Gall
RCPRO

(in reply to PylonWorld)
       Post #: 6

RE: Calculators for Aero-Modeling - 10/1/2004 8:19:22 PM   
PylonWorld



Posts: 1346
Joined: 12/29/2001
From: Monroe, NC, USA
Status: offline 		The Pitch Speed calculator is now available on internet connected PDA's and phones with web browsers. More will be coming now that I've got a better understanding of the mobile connectivity technical issues.

Just point your mobile device to www.rcpro.org or www.rcpylon.com.

You can point PDA (or desktop) browsers directly to mobile.rcpylon.com and phone (or desktop) browsers to wap.rcpylon.com.

If you can, please try the calculators out and send me feedback good or bad.

If you want to see what the phone access looks like without using a web enabled phone, go to RCPRO WAP via Wapsilon and give it a whirl. Click on links with your mouse and input numbers using your PC keyboard. Wapsilon is slow sometimes, so please be patient.

_____________________________

Don Stegall
RCPRO Chairman of the Board

(in reply to DHG)
       Post #: 7

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