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What difference does pitch make?

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Posted

Okay so I've been flying three years but have never understood why pitch on a prop can improve takeoff performance.  I am running a rotax 912 uls on an AVID C.  Have a Whirlwind 3 blade 75 STOL prop.  Currently have it set at 17.5 pitch.  I am not getting 5800 rpm at full throttle in flight or 5300 static.  My question is this.  If I backed off the pitch to 15 to achieve the 5300 static / 5800 in flight how would that improve takeoff performance since its not pulling as hard with the 17.5 pitch. 

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Posted (edited)

The power curve of that motor would show if you would get better TO perf with more static rpm.  Likely the engine will produce more HP (and thrust) at a higher static rpm.  Problem obviously is the revs might be higher  than you want in cruise when you set for optimal power during TO.

My plan is to do some static thrust pulls at different pitch settings and get an idea of where my prop-engine combo will work best (also considering that rpm "ram" rise during the TO roll and in cruise)

With a fixed pitch prop it obviously will end up being a comprise to a cetain degree

 

Edited by Yamma-Fox

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Posted

912_ePerf100.jpg

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Posted (edited)

Chart above is from CPS for the 912uls.

Pretty flat curve!   Looks like adding 400 static rpm would only add about 5 HP.

But enough paper speculation!!

Fly it and time a few runs from brake release to say 500 or 1000 agl.  Then pitch for 5300 and see what the difference is in time to climb and also see if you notice better off the ground performance.

In my O300 Maule (yeah I know, apples and oranges, but) I made a 300 rpm pitch increase and it was VERY noticeable!

Edited by Yamma-Fox

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Posted

Last night I set the pitch at 16.  Was set at 17.5.  Ill post the results on here as soon as I get a chance to fly.

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Posted

That's a pretty massive change for one adjustment. On my 582 one half of a degree is 3-400 rpm.

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Posted

Its a complex problem.. the faster you turn the blade, the more "air drag" each blade has. For a slow plane, a longer blade, with pitch flattened enough to get RPM in the max torque band would likely make best thrust for takeoff.... as shorter blade would need more pitch to keep the rpm in check but might yeild faster cruise at the expense of not as good take off perfrom....and each brand prop is a little different as well... seems most the big aircraft prop manufacturers re going to a semitar tapererd blade design and that makes sense, ,higher aspect ratio = more effecient wing(or blade). so ask around  what the other guys are doing , what prop do they have, how many blades, what engine, do they have big tires installed(these slow the plane down).

Ive talked to guys who flew thousands of hours in 503s and they say keep the rpm at 6000 cruise, egts around 1000-1100 and that seems to give best performance for fuel used.....I also know using 5500 rpm will use less fuel but  alot less power... so there are lots of variables.... fixed pitch prop is a compromise....

Leni has an in flight adjustable prop and these are ok but you better get a good understanding of them because I think you could get in trouble with too coarse a pitch or maybe too flat a pitch. IT would take a little work to set it all up, ask leni about it....aka..AK flyer

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Posted

Also blade angle, measured at some blade station with a  protractor or digital protractor just tells you the angle between the hub flat face and the  tip of the blade... 

IT IS NOT GEOMETRIC pitch.. .there are formulas to figure this out but it looks to me that most are 38 to 40 inch pitch in the end... this means if turned in a solid with no air slippage, you would travel that far in one revolution as if you were in a solid....of course there is air slippage but the faster you go generally the less it is...

Most of these 503-582 rotax engines with 68" diameter props run 11 degrees blade angle at the prop tip + or - a few degrees, but like C5 says, 1 degree can = 300 rpm change on some engines....

 

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Posted (edited)

....My question is this.  If I backed off the pitch to 15 to achieve the 5300 static / 5800 in flight how would that improve takeoff performance since its not pulling as hard with the 17.5 pitch. 

Let's look at the propeller as a wing, and recognize that the wing makes lift (thrust) by a combination of angle of attack (pitch) and airspeed (RPM). f the two, the airspeed is more influential, since the lift equation used the velocity squared.

At 5500 rpm the prop blade tip is moving at 740 feet per second (with a 2.43 reduction box), at 5800 rpm it is at 780 ft/sec. The difference i thrust is about 11%, assuming the engine was producing the same power. But with the engine producing 4 HP more power at 5800, that would make the prop thrust about 15% higher at 5800.

Edited by nlappos
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Posted

I adjusted the pitch on the whirlwind 3 blade 75" STOL from 17.5 to 16.  I can feel the added torque on take off and the climb angle is steeper.  Appears that setting puts the engine at around 5800 full throttle in flight.  I can go full throttle for a few seconds as the plane approaches VNE very quickly at 5800.

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Posted

Plane hitting vne at the same time engine hits max hp/rev limit..... that sounds like an ideal performance setting to me!

Maybe do 17.5 if you are going for a long cruise.

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Posted

Just picture less propeller pitch the same way as you do a lower gear in  car's transmission.  Less pitch will give you faster acceleration off the line than a higher pitch, but you run out of high end as the engine reaches its maximum speed.  Ideally, propeller pitch would vary with the load (lower pitch for takeoff and climb, higher pitch for cruise).  Aeromatic props did this automatically with air load and centrifugal force.  Some airplanes of years ago used a two-position prop (the Sensenich Skyblade on a Stinson comes to mind).

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Posted

I've been trying to get a better grasp of prop pitch too!  Avid90- That is a Great comparison /explanation to bring it into perspective.  Makes much more sense now! Thanks :BC:

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