Reynolds number and Angle of attack - HeliFreak
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Old 06-06-2012, 05:07 AM   #1 (permalink)
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Default Reynolds number and Angle of attack

It's exciting to see that there is an aerodynamics and engineering forum in Helifreak so I guess I'm going to ask a few questions I've had in minds for months.

Firstly is the Reynolds number, I've asked a question on Yahoo! Answers (http://au.answers.yahoo.com/question...1041948AAbq54E) a few months ago but I didn't quite get the answer so it would be great if it can be elaborated. When a helicopter is ascending vertically, the blade would be treated as a wing span. Do I have to do a summation of all the reynolds number at all points along the blade to get the actual Reynolds number of the blade when it's spinning, since the velocity at points along the blade are different to each other?

Secondly, how do you find the angle of attack of a blade? If a model helicopter is flown in still air indoors, does this mean that the relative wind direction is flat and so the angle of attack would be the same as the geometric pitch angle? This confuses me because the Trex 450 even indoors require some 3-4 degrees of pitch before it can lift off while if I use XFoil and ask it to output a chunk of lift coefficients at various angles of attack for NACA0012, I would get the amount of lift required for the Trex 450 to hover at 0.5 to 1 degree pitch, and that is not practical.

Thirdly, I've read many helicopter aerodynamics book (not completely since my calculus knowledge still have a long way to go), and I find that to find the thrust of the rotor from the Momentum Theory, the static thrust above the rotor, and below the rotor is usually assumed to be given. Say I would like to do some experiments and measure the lift of the rotor, can I place 2 pressure sensors, 1 above the rotor, 1 below the rotor to find the static pressures then use the Bernoulli's equation to find the downwash velocity? This will certainly help me with the angle of attack problem.

Most of the things I read from the net or in books are theories and some practical experience would be greatly appreciated.

Thanks - Cong.
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Old 06-06-2012, 05:50 AM   #2 (permalink)
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I'm not up to answering most of your questions...
But to get the ball rolling: The AoA in hover will always be less than the pitch because the blades are meeting a downflow of air.

Also you might find this website interesting http://www.regenpress.com/ It's not about helicopters but he emphasises the Newtonian mass flow aspect rather than analysing pressures and velocities.
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Old 06-06-2012, 06:16 AM   #3 (permalink)
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Cong

Short on time, so the answer will be brief-

1- Re is based the fluid, the characteristic length/dimension of the body in question, and the mean velocity. Since the first two are generally assumed to be similar for the span of a rotor blade at a given moment, the one variable is velocity. You cannot simply average the values, as the point of the Re is to characterize the nature of the flow. You have to solve for each station/position on the span, since the velocity differs so greatly. Just as an example, the Re for a 450 at 3200RPM in hover varies from approx 62,000 inboard to 310,000 near the tip.

2- AOA is not trivial to measure in our context. A generally accepted approach is to calculate the inflow velocity (and swirl component), and use that in combination with the known AOI, for each station. The problem is very complicated because the blades tend to flap, which alters the local AOA, etc.

3- I don’t think you will find much use in measuring static pressure. When you consider that our flow velocity is trivial (something like 10mph in hover), the pressure delta is very low. You would be better off using a pitot or a simple hand airspeed indicator and taking (CAREFUL!) plots vs station if you want to get some idea about the velocity distribution. Much easier is to just track down some CFD plots of velocity across the rotor, etc.
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Old 06-06-2012, 11:23 AM   #4 (permalink)
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The aerodynamics explanation based on Newtonian is interesting. I'm will read it in the next few days.

So to find the thrust generated by the blade, theoretically, I would need to find the Reynolds numbers along the span of the blade, then find the lift coefficients corresponding to each station; then I can use a handheld airspeed indicator to find the induced velocity to calculate the the thrust along the stations and sum them up to find the thrust of the rotor. Right?

Where do you get a handheld airspeed indicator? By the way, how do engineers at model helicopter companies design their helicopters (the aerodynamics side)? Do they try out various rotors through a wind tunnel or do they have a template? On a related note, I saw in your profile, extrapilot, that you are an aerospace/electrical engineer. I'm doing electrical engineering at the moment but I also really want to get into aerospace, but I can't because finding a job as an aerospace engineer where I live is extremely difficult. So did you do a combined degree or did extra study from somewhere to become qualified in both degrees?
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Old 06-06-2012, 11:10 PM   #5 (permalink)
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Quote:
Originally Posted by freakofheli View Post
Where do you get a handheld airspeed indicator? By the way, how do engineers at model helicopter companies design their helicopters (the aerodynamics side)?
I doubt they do more than prototype their models (build one and see how it flies) from an aerodynamics perspective. They would do basic analysis of the drive components to make sure they handle the power but thats about it. RC helis have so much power, a few aero inefficiencies are not really a problem.

The rest would be using rules-of-thumb developed during design of previous helis (model and full scale).
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Old 06-07-2012, 07:18 AM   #6 (permalink)
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Ah ok.
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Old 06-09-2012, 09:37 PM   #7 (permalink)
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One simple explanation as to why your blades need pitch in a hover is simply that they are symmetrical, and thus need some angle of attack in order to produce lift. They do make non symmetrical blades for model helis, but I haven't ever run one and think they are more designed for low headspeed camera birds.

Lift is going to be a function of the blade airfoil and its velocity through the air. Essentially the design of a heli is as stated above, mainly using previous experience with similar models, and refining vertical CG, tail moments, and mechanical issues. I also do not believe there is a whole lot of aerodynamic theory going on with the actual model manufacturers, but there is a ton of it happening at the blade manufacturers.
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