Finding Re with motor and prop

[B1Z4RR0]

Hello

I want to know if can find the Re (Reynolds Number) If I know the R.P.M motor (.51) and propeller dimensions (10x6)?

When i try to use this formula:

Re = Speed (kms x h) x wing chord (cms) x 189

I don't know the model speed. I try to do it this:

propeller= 10 x 6 (inches)
Propeller= 25,4 x 15,24 (cms) (advance 15,24 cms in one revolution)

The .51 motor
Min RPM= 2.200 (2.200 revolutions in one minute)
Max RPM= 18.000

Using the min RPM to do this calculations (2.200 R.P.M)

15,24 x 2.200 = 33.528 (Cms in one minute) or 0,33 Kms in one minute or 19,8 Kms in one hour

Speed=Space/time Speed=19,8 Km/1 hour Speed= 19,8 Kms/h

Then,

Re = 19,8 (kms x h) x 26 (cms) x 189
**** Re(min)= 97.000 *****

The same to max RPM (18.000 R.P.M)
Speed = 164.58 Km/h

**** Re(max) = 808.000 *****

Considering the wingspan = 1,53 mt this Re values are too low and too high?? the high and low speeds are correct??? this calculations are wrong or not????

Tks for your help, and excuse my bad english!!!

[slipstick]

Well the formula I use is speed (m/s) * chord (m) * 70000

That gives min 101000 and max 832000 so your calculations are o.k.

Two problems....
1) propeller pitch speed is not a very good indicator of aircraft speed. In particular the minimum practical speed of the plane (stall speed) has very little to do with the tickover speed of the engine
2) it looks like you are quoting theoretical motor rpms. In the real world a .51 with a 10x6 prop will probably have a minimum speed of around 3000pm and maximum of no more than 15000, more likely around 13000

Steve

[JetPlaneFlyer]

Slowest flying speed as noted above is really nothing to do with pitch speed. It should be calculated like this:
V = √( 2 W g / ρ S Clmax )

where:
V = Stall speed M/s
ρ (rho) = air density KG/M^3
g = 9.81 m s^-2
S = wing area M^2
Cl_max = Max Coefficient of Lift
W = weight KG

Or; a simplified approximate calculation in ‘old units’ =
Stall speed = 3.7 x √wing loading

speed in mph
wing loading in oz/sqft

[B1Z4RR0]

Thanks for your help Steve and JetPlaneFlyer. I try to do it as you say. If you can tell me more about this I apprecite so much!!! (or links)

Thanks a lot!! :)

[BMatthews]

You're also going to be off on the top end. The drag of the model will cause a "slip" of the prop pitch. For clean models flying level I've read years ago that the effective speed will be around .8 time what the prop pitch and RPM suggests. So more likely your 160'ish kph will be more like 128 kph. And this assumes a fairly clean and slippery shape. A biplane or a scale model with a big flat radial cowl isn't going to even manage 0.8.

And all this falls flat on its nose when you go into a dive and the model speeds up enough that it actually over revs the engine and the engine and prop are now actually working like an airbrake to retard the speed of the model.

All of which goes to suggest that you're best bet is to stick a small GPS unit into the model and fly it for a while and then haul the GPS out and see what it tells you for best speeds at the various extremes. Get the model way out in one direction, fly it back to yourself and out the other way to as far out as you're comfy with at low and high level speeds to give the GPS enough of a steady basis for a good average reading. Do it both ways at both power settings to equalize any wind effects. Take the average of the two passes at each setting.

[vintage1]

What he said plus don't climb the model to turn at the end of passes..doing wide open turns so as not to bleed too much speed of.

And models often do fly above pitch (as printed on the prop) times rpm (as measured on the ground). For many reasons to do with static versus inflight loads, and the difficulty if actually working out what what's printed on the prop actually MEANS.

[B1Z4RR0]

Thanks to Bruce and Vintage for their answer. You are right, I am making calculations for an ideal system, not real. It is my error and I recognize it, but I am making the initial calculations before beginning to draw, and I thought that it could get a formula that helps me to get the speed for an specific motor, it could determine this way the number of Reynolds and to establish other parameters like the profile of the wing and the stall speed.

I like the idea of the GPS, but it is not favorable at this time to me. This airplane exists only on my "Microsoft Excel". Because each airplane behaves different for its physical characteristics it is that I look for an standard, approximate mensuration that allows me to complete my calculations.

Thank you for your interest and collaboration. :)

[slipstick]

Of course we might as well say that knowing Re accurately isn't going to help much anyway. There's so little accurate data available for airfoils at the sort of Re we fly at that for choosing an airfoil you might as well take a middling value like say 400,000 and use that.

Or just use a standard airfoil as used for lots of other models of similar type and size to the one you want to design. Though it seems to me that if you already know the motor, propeller and wing span and chord you have quite a lot of the design there already.

Steve

[BMatthews]

Also keep in mind as shown already that truly accurate Reynolds numbers mean very little to model flyers. A general RANGE of operating numbers can indicate how to design a model but we just do not fly at a steady speed for any appreciable time. Airliners care about this sort of thing because their flight profile is largely spent at a small variation in altitude and speed for much of the flight. But with our models climbing, diving, turning and only rarely flying straight and level it is enough to have an idea of the general value give or take 25% to 30%. Yes, that's right, 25% to 30%. Because when we are flying at full throttle that's at least the sort of variations well see during that part of the flight and when gliding again it'll be at least that much.

The only time this would change is if you're looking at a specialty model such as a racing plane or a sailplane where you're wanting to optimize one aspect of the flying. Even then the variation typical in the critical portion that you're specifically designing for would likely be around 10 to 15% or more.

So I'd have to say that it's good to have a general idea of the value but it sounds like you're trying to pin it down to a few % error at most. We just don't fly our models that way so I'd have to suggest, no offense intended, that other than a mental exercise it's really quite meaningless to try to pin it down that closely.

EDIT- Oops, I just realized that I parroted back what Slipstick wrote.

There is a fairly well tested and pretty decent tool for predicting airfoil behaviour at "our" reynolds numbers. It's Xfoil. It's available in command line format as a free download or if you download and install Profili2 and then pay the small pittance for the unlock code one of the extras is a really nice GUI interface to the embedded Xfoil. With this you can generate a few different charts for any Reynolds number you want. Although with the difficulty in predicting airflow at Re's under 60K to 80K the curves get pretty crude and again can only be used as an indicator rather than as true fact.

Keep in mind too that any curves at any Re is only good for a fully sheeted, vacuum bagged or otherwise consistent shape holding construction method that follows the true airfoil shape to within a very small error. Selig in his original study noted that errors of as little as 0.5% in the wrong spot on an airfoil can totally distort the behaviour. And trailing edge errors of around 1 to 1.5% did the same thing. It's a tough world out there......

[slipstick]

EDIT- Oops, I just realized that I parroted back what Slipstick wrote.
I thought I recognised some of that, but you put it so much more elegantly ;).

Steve