Jan 01, 2010, 07:17 PM
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Joined Dec 2002
5,481 Posts
Quote:
 Originally Posted by vintage1 PITCH SPEED. The speed at which a prop spinning at a given RPM generates no thrust whatsoever, and no drag along the axis of the aircraft. Yes, I know there are other definitions, but this is the one in use here, today, to avoid endless discussions about what pitch speed is.
Quote:
 Originally Posted by vintage1 It is, for an aircraft operating near the pitch speed (where lift to drag is best as the blade meets the airflow at just a couple of degrees or so))
Your definition of PITCH SPEED and the second quote above are mutually exclusive.

By your definition PITCH SPEED has zero thrust. Planes have a difficult time flying with zero thrust, except in a dive.

PITCH SPEED (RPM X PITCH) is the speed at which the vector sum of the forward and rotational velocity create a constant angle of attack of zero degrees along the entire blade length.

For a flat-bottomed airfoil (propeller) this is the point of maximum efficiency.

The ZERO THRUST SPEED occurs at an ADVANCE RATIO of PITCH SPEED ADVANCE + 0.2. At speeds above PITCH SPEED, the effective angle of attack along the blade length has begun to go negative.

For a prop with a 1:1 P/D ratio PITCH SPEED occurs at an ADVANCE RATIO of 1.0 and ZERO THRUST SPEED occurs at an ADVANCE of 1.2 (20% higher speed).

The prop in the Advance Curve below has a P/D ratio of 0.8. If its PITCH SPEED is 60mph, then its ZERO THRUST SPEED is 75mph (25% higher).

Quote:
 Originally Posted by vintage1 Another factor also comes into play at high blade angles of (geometrical) incidence. The lift vector is no longer along the direction of aircraft travel. Its perpendicular to the blade, so for a given incident airspeed, blade area, the thrust decreases as cos(theta) where theta is the geometrical angle of attack. So at, let's say, 60 degrees, the thrust is NOT what we might expect, but half that (cos( 60)=0.5) After that it drops like a stone. so we can probably say that a helically cut prop with the angle at - say- mid blade of greater than 60 degrees is probably simply too coarse to be generating any thrust much at all. This is useful, because it sets an upper limit on the propellor pitch to diameter. It turns out that for the conditions above, its Pi. That is no more than PI times diameter will be a effective as a prop. so scrap that putative 10x32 prop. It's not going to work.
I don't know whether a 10X32 prop is going to work or not, but there is another factor to consider statically. Although the geometric pitch might be excessive, the effective angle of attack statically can be considerably less due to inflow. Even though the majority of the blade is above the stall angle, air being sucked into the propeller disk can significantly reduce the static effective angle of attack.

The helicopter blade below has an geometric pitch of 10 degrees, but its effective angle of attack along the blade length is significantly less due to inflow.

# Images

Last edited by Martyn McKinney; Jan 01, 2010 at 08:02 PM.
Jan 01, 2010, 08:01 PM
Registered User
East Anglia, UK
Joined Sep 2002
29,705 Posts
Quote:
 Originally Posted by Martyn McKinney Your definition of PITCH SPEED and the second quote above are mutually exclusive. .
Not if the concept (near != at) has any meaning.
Jan 01, 2010, 08:11 PM
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Joined Dec 2002
5,481 Posts
Quote:
 Originally Posted by vintage1 Not if the concept (near != at) has any meaning.
I guess you're correct.

25% is close enough. 40% for props with lower P/D ratios.
 Jan 02, 2010, 04:13 AM <- Balsa flies better -> Gustafs, Sweden Joined Feb 2006 1,968 Posts Great reading to the morning coffee, thanks Peter
Jan 02, 2010, 07:43 AM
Registered User
East Anglia, UK
Joined Sep 2002
29,705 Posts
Quote:
 Originally Posted by Martyn McKinney I guess you're correct. 25% is close enough. 40% for props with lower P/D ratios.
What I do with the wood props, is cut them helical plus 3 degrees just about.

So at a given speed all the prop parts are at + 3 degrees incidence.
Anyway the point was to emphasis that props should be run close to pitch speed for efficiency. So the model that cant do more than 50 mph running at 80mph pitch speed is definitely capable of a better prop.
 Jan 02, 2010, 08:57 AM An itch?. Scratch build. South Wales U.K. Joined Mar 2003 13,773 Posts Sorry, as soon as I see formula and graphs I usually skip past those posts, (I blame it all on my maths teacher. Well how could it possibly be my fault ). Anyway back to reality. My good old B-25 has had a range of motors, (all brushed), driving props through a 3.6:1 belt drive. Initially the props were 12x8's Puntilio wood, (IC engine props). They worked, the plane took-off, flew round on about 3/4 throttle. After my last 're-think', (another motor change, brushed again), I fitted a pair of 14x12's, (by accident, I thought I had bought 14x10's). She took-off about the same, climbed about the same, and cruised on about 1/3 throttle. To me a winner, as she still drew about the same old current at WOT. So 12x8 to 14x12, (different motors), and she flies quite similarly. But......? I don't understand the maths behind it. But I do wish some one would recommend, (in VERY simple terms), what is probably the better route to go regarding pitch. Most regular readers/fliers probably know the 'big diameter, finer pitch for torque, (grunt) and acceleration', and 'small diameter, coarser pitch for speed'. But there is more to it than just that. I think somewhere in the above posts was probably explained what I'm referring to, but as I said, maths just turns me off reading them. So can anyone explain the above without any formula or graphs, please.
 Jan 02, 2010, 10:12 AM Registered User East Anglia, UK Joined Sep 2002 29,705 Posts I thought I had mate. The conclusions section. Basically unless you have special requirements don't go much below 1:1 pitch to diameter. Since no one makes a prop coarser than that, pick the coarsest pitch you can find. Then gear your motor to about 2-4000 RPM and pick the biggest that will not overload the motor. Or torque roll the plane. One third of wingspan is a practical maximum. One quarter is a nice balance. If you are stuck with an ungeared higher revving motor, its not a huge sacrifice to go to 1.5:1 diameter to pitch. With scale diameters, you have to balance the model power requirements against diameter to get the RPM, and then calculate the pitch needed from the likley pitch speed. As you discovered, a very high pitch speed on a large prop will cruise on less than half throttle. I personally like that. YMMV. What I was trying to establish as a first base, was the sorts of pitch to diameter ratios that are best, and shoot at a typical RPM range. Its not surprising that I found that the pitch to diameter that is optimal is about twice that of the sort of props that are easy to obtain, and the RPM is about half of what an outrunner likes. In short, the props you buy are geared (sic!) for use with ungeared outrunners. They are sub optimal, but for many, so are gearboxes..
 Jan 02, 2010, 11:35 AM Registered User Chattanooga, Tennessee, United States Joined May 2003 27,206 Posts What I was trying to establish as a first base, was the sorts of pitch to diameter ratios that are best, and shoot at a typical RPM range. Its not surprising that I found that the pitch to diameter that is optimal is about twice that of the sort of props that are easy to obtain, and the RPM is about half of what an outrunner likes. You have it in a nutshell! Basically it tells us that the typical outrunner RC power system, though convenient, is very much sub-optimal/wasteful, and that without a gearbox and especially crafted props it will always be so. Aaagh!
 Jan 02, 2010, 11:43 AM Motors beat engines! Milwaukee Wisconsin, United States Joined Feb 2001 4,564 Posts Very nice post Vintage1, I have bookmarked it! DeaninMilwaukee
Jan 02, 2010, 06:30 PM
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Edina, MN, USA
Joined Oct 1999
11,445 Posts
Quote:
 Originally Posted by vintage1 A prop much less than 0.5 pitch to diameter is a waste of space and power
Or, to quote AstroBob -- "best used to stir paint"!

Quote:
 Originally Posted by vintage1 Over 1:1 you are beginning to sacrifice low end thrust for a more efficient cruise, or a higher top speed. RPM may well come down a LOT for low powered models. I think a rubber model typically flies with oversquare props and sub 1000 RPM. 1:2.5 represents the extreme limit of what you probably want to consider, for extremely high top speed, or on a one speed model where RPM is severely limited. Probably the sort of prop you would use to try and reach 300mph, or conversely, for absolute best performance on an indoor rubber model at 5mph! In short it is the ultimate sort of efficiency, but at the cost of hugely unsatisfactory off- pitch speed performance.
I've always thought operatinf rpm was a key factor -- tend to stay around 6krpm; usta be w/GB, now using lower Kv motor (they're normally a big larger/heavier - won't burn thme up!).