Propellor Design

[robbert]

Hello guys
I am a 737 capt and in my spare time I fly model airplanes. At the moment I am working on new propellors for the new planes for this years world championchips.
The tips of the props reach mach 1 at this moment so I'am looking for profiles suitable for this kind of mach numbers (Re 150000)
I also like to know the RPM airspeed and prop diameter from turbo props like saab 2000
Thanx robbert



[This message has been edited by robbert (edited 10 January 2001).]

[Mark 1]

I'm not a propellor designer, but did some work on propellor acoustics in the 80s.

As you've probably found out, the aerodynamics get very problematical as you approach transonic relative Mach no.s.
The work on prop-fans in the eighties led to swept airfoils (largely for noise aswell as aerodynamics) and composite shell with titanium spar construction to help combat the twisting moments.

Hamilton Standard, as I recall, did a lot of research in this area, also Dowty, Boeing et al.

A lot of reports were published, and the RAeS library in London is a good source of information.

Ducted fans offer a way of increasing Mach number and disc loading with fewer problems.

[avoman]

Ref RPM prop diameter and airspeed of a Saab 2000: dunno.
An ATR is a sizeable turboprop. Standard prop RPM is 1200, diameter 12 feet-ish, so a tip speed of 514 mph. Aircraft IAS up to 250 kts.

[Cuban_8]

Rob,

Not that I am an authority on the suject, but I do fly model aircraft and am doing an aero eng degree. From what I understand in the past, the aim is to prevent the blade tips from touchin supersonic speeds. At these speed, the blade section becomes very inefficient as the normal shockwaves developing over the blade promote seperation of the flow - essentially, when the blades hit sonic speeds, they stall.

A lot of work was carried out, as mentioned by Mark 1, in the late eoghties/early nineties on large scale propfans. These utalised blades that were swept rearwards to delay the onset of shock development on the section - the same reason as civil jets have swept wings. The aim was to allow propeller engined aircraft to travel at higher speed, as propellers are very efficient! Noithing really came of this as far as i am aware (if anyone knows anythinf, please correct me!).

In terms of your model (what class are you - F3A?), then the idea is usually to prevent the blade from touching sonic speeds. As you will know setting a higher pitch will allow you to go faster at the detrement of static thrust - a fixed pitch propeller can only be designed to be most efficient at one single pitch value, hence vaiiable pitch props on most full scale A/C.

In my experince, gearing has often been used to allow the size of the propeller to increase whilst still lowering the blade tip speed. Fixed pitch propellers are a bit of a black art, but what I would personally suggest is avioding sonic tip speeds - it doesn't work!!

Hope this is of some help,

Cuban

[Lu Zuckerman]

To: Robbert

Try this:

Design of a Propeller
The computer program is based on the formulas presented in [10] (comparison Adkins vs. Larrabee). Based on the theory of the optimum propeller (as developed by Betz, Prandtl, Glauert), only a small number of design parameters must be specified. These are
 the number of blades B,
 the axial velocity v of the flow (flight speed or boat speed),
 the diameter D of the propeller,
 the selected distribution of airfoil lift and drag coefficients Cl and Cd along the radius,
 the desired thrust T or the available shaft power P,
 the density rho of the medium (air: ~1.22 kg/m³, water: ~1000 kg/m³).
The design procedure then creates the blade geometry in terms of the chord distribution along the radius as well as the dirstribution of the blade angle. When you have a look at the resulting geometry, you might understand why highly efficient propellers for man powered aircraft, indoor free flight models or FAI-F1B rubber powered models look as they look like. The local chord length c depends mainly on the prescribed lift coefficient Cl - if you would like to have wider blades, you have to chose a smaller design lift coefficient (resp. angle of attack) and vice versa. If you receive nonsense values for the blade chord, the power loading of the propeller is probably too high. Check if the power coefficient Pc is less that 1.5, otherwise the theory is not fully applicable and may lead to errors.
The Number of Blades
The number of blades has a small effect on the efficiency only. Usually a proeller with more blades will preform slightly better, as it distributes its power and thrust more evenly in its wake. But for a given power or thrust, more blades also mean more narrow blades with reduced chord length, so practical limits have to be considered here. The chord length can be increased by decreasing the diameter, but this is usually a bad idea in terms of efficiency, as long as the tip mach number or tip cavitation is not an issue.
The Velocity
The velocity of the incoming fluid together with the velocity of rotation (r.p.m.) determines the pitch distribution of the propeller. Large pitch propellers may have a good efficiency in their design point, but may run into trouble when the have to operate at axial velocity. In this case, the blades tend to stall. usually the best overall propellers will have a pitch to diameter ratio in the order of 1.
The Diameter
The proeller diameter has a big impact on performance. Usually a larger propeller will have a higher efficiency, as it catches more incoming fluid and distributes its power and thrust on a larger fluid volume. The same effect can be shown for lifting surfaces, which results in sailplanes having large span but slender wings.
Lift and Drag Distributions
Instead of the lift and drag coefficients, it usually convenient to specify an airfoil with a prescribed polar and the design angle of attack at each radius. In JavaProp, four definition sections are spread along the radius where airfoil and the design angle of attack can be prescribed. The distribution of Cl and Cd along the radius can be examined later by performing an analysis for the design point. For maximum performance, the airfoils must operate at maximum L/D. But if the propeller should also work reasonable well under off-design conditions, it is usually necessary to use a lower angle of attack for the design. Again, you can check the Cl and Cd distributions for off-design cases by performing several single off-design analysis for different settings of the flow velocity v. Stall should occur gently when the velocity is reduced. The analysis code will probably give unreliable results for very small velocities.
The Fluid Density
The density of the fluid has no influence on the efficiency of a propeller, but strongly affects its size and shape. As the forces and the power are directly proportional to the fluid density, a hydropropeller will have much smaller dimensions than a propeller working in air. Also, lifting surface under water tend to develop caviation when the local pressure of the flowfield falls below the vapor pressure. Therefore it is not possible to use high lift coefficnect in hydroprops, usually they have to stay below Cl = 0.5. The same is true for high speed tips of aircraft propellers, where no cavitation, but supersonic regions may occur if the pressure gets too low. Therefore the tip sections of propellers opearting at Mach numbers above 0.7 should be designed to operate at small lift coefficients below 0.5 too. The analysis module of JavaProp uses the fluid density from the Design page to calculate thrust and power during the multipoint analysis. The dimensionless coefficients Ct and Cp are not affected by a variation of density, but the thrust and power are. Thus a propeller engine combination will find different operating points depending on the fluid density. This makes a difference for aircraft propellers, where the performance of propeller and engine depends on the altitude.
last modification of this page: 01.01.01

I got this off the internet under Model Propeller Design. There are a lot of computer design programs on this site.

Good luck in the contest.

.

------------------
The Cat

[This message has been edited by Lu Zuckerman (edited 11 January 2001).]

[Mark 1]

As a follow on: I agree with Cuban8, supersonic or transonic Mach no.s should be avoided.
If your problem is not being able to absorb enough power within the constraints of a propeller diameter, the answer would probably be more blades.
If the overspeed is because you have optimized the pitch for level flight and powered dives are causing the overspeed, then some kind of pitch control may help.

This need not be as complicated as you think, some time ago I met a chap who sold his own design of wooden propeller at Avra Valley airport in Arizona, he called it the 'almost constant speed' propeller. I believe the trick was to use the aerodynamic twisting moment to increase the pitch as the loading increased by having an airfoil section where the centre of pressure is forward of the torsional neutral axis.

Sadly the designer of this died a few years ago and I don't know of any technical reports, but food for thought anyway.

[robbert]

Thanx guys for taking time to answer

Some remarks
The rules do not alow use of constant speed props
At the moment we are doing 33000 rpm with air speeds up to 330 kph (telemetry) on 180 mm props so mach 1 is reached.
I think I will go for reduction of the diameter with wider blades and reduce the alpha at the tips and hope for better results
I hope the reduced efficiency due to smaler diameter will be made up for by the increased performance of the tip profiles.

Will see what happens.

Robbert

[This message has been edited by robbert (edited 11 January 2001).]

[robbert]

Forgot,
Does anyone know where I can find coordinates of the profiles used in modern commuters????

robbert