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Propellers

 
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propellerdesign(at)tele2.
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PostPosted: Tue Oct 24, 2006 8:44 am    Post subject: Propellers Reply with quote

Hi

This is my first post on this list.
It seems that we have a good amount of knowledge collected in this group.
I want to share my opinions on prop's

Propeller.

I often hear that longer blades is always better, well I say that optimal blade length is best, and it is better if we can keep them that long as long as it is possible.
Why does some say longer is better then? mostly hearsay I think, and some testing where they tie a scale between the rear bumper of the truck and the plane and test the STATIC THRUST, here we come into the optimal diameter, the higher speed the smaller the optimal diameter will be! so at zero forward speed a bigger diameter is better, until the sonic wave drag eats up the power that is.

The big diameter force us to use less pitch to get the RPM, High pitch can lead to that the blade stall or cavitate at zero forward speed, and the propeller turn faster when it cavitate just as the vacuum cleaner motor do when we block the hose, the impeller/propeller is working in an vacuum! when we start to move forward the angle between blade and relative air is reduced making the propeller gradually unstalled, increasing the power needed to turn it. (ask Guy about this) If you are on the back side of the torque curve then ...
This effect the result we got from static test, big diameter low pitch = not stalled = high static thrust, small diameter high pitch = more or less stalled = low static thrust.

Except for on the top of an stall turn, we don't fly at zero forward speed.
Thrust is = Power times efficiency divided with speed. So the faster we fly we will see less thrust, this reducing thrust is coming from air going throe the prop disk at a higher speed, The prop wash have to be faster then the forward speed, so the faster we go a smaller prop disk is necessary to "squeeze" out the "jet" of air from the power we have. The efficiency increase with speed but not to that amount that it increase thrust with increased forward speed.

If we want the shortest take off distance, we optimize the diameter for the lift off speed! it is just before lift off we cover the most runway per second, not at zero speed.
If we want best climb rate we optimize it for that speed. (Vy)
If we want best cruise or top speed we .. Ok.

What effect the optimal diameter?
Except for speed, it is effected by the RPM and the HP.
Also the rest of the propeller, like blade thickness, blade width and Nr of blades.

The pitch and pitch distribution is also important.
The center of thrust and peak of thrust is said to be at 70-75% of the radii, and this "center" is also used as reference for pitch when we buy or compeer propellers.

Talking about Warp prop's if the 75% station is where we have the peak in thrust, to get aerodynamic balance it must be better to use that station as set point when it looks like the blades have some differences at the outer inches, Why are they not uniform?

Pitch.
What pitch is best? (=same question as what airplane is best)
The pitch will vary some with the diameter used. Or compensate for diameter.

On a standard purpose propeller (not Climb or Cruise but in-between) I have found if the diameter is correct the back side of blade (at 75%) is in line with the relative air.
On a Climb prop the angle of the blade will be little less then the incoming air and a Cruise prop little more angle, best efficiency is where the airfoil have it's best L/D (about 2-3 deg on most common prop airfoils) but that prop will not always be the most practical.

An example.

if we cruise at 88 mph and the prop is turning at 1935 RPM the relative air is 88 * 1055.77 = 92908 inch/min / 1935 RPM = 48" pitch
if the diameter is 68" the angle at 75% radii is = 68 * pi * 0.75 = 160,22,
48 / 160,22 = 0.29958 tan = 16.68 deg
At the tip it will be 12.66 deg
and at 50% radii it will be 24.2 deg

When we buy a prop the blade twist is (hopefully) optimized for an specific speed / RPM / diameter. Called VnD

Jan Carlsson
www.jcpropellerdesign.com

Ps. on the propeller tests made by NACA using on the ground adjustable blade, they are often made so that they have a uniform pitch distribution from tip to about 40% when the 75% station is at 15 deg. so when turned to greater angle the pitch will be higher at tip then the 40% station.

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dosmythe(at)cox.net
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PostPosted: Tue Oct 24, 2006 10:02 am    Post subject: Propellers Reply with quote

Jan,
From looking at your web site I'd say you know a bit about props. We welcome the post. You ask, "why are they not uniform". First off, we ("I") might be looking at this situation with too big a magnifying glass. Maybe the pitch difference of 1-2 degrees at the tips is acceptable. I just know that mine ran true until that last little bit. Warp seems to imply that they use the tips as a reference point so that Warp users would all be united when discussing pitch angle.

Don Smythe
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dave



Joined: 22 Sep 2006
Posts: 1382

PostPosted: Tue Oct 24, 2006 11:18 am    Post subject: Propellers Reply with quote

Ok on my Warp I did a quick peek before lunch and he is what I got............

I used a GSC protractor and it slides on the warp about 7 inches in from tip
all 3 blades are same degrees on tip ( about 10,5 degrees) but the measurements at this station is about .82 the length of prop. Not ideal but I used what I had....
blade 1 15.25 deg
blade 2 15.50+ deg
blade 3 15.75 to 16 approx.
--note the prop length I measuring is from the centre of hub to outside of prop, But the actual blades are not 34 inches but shorter because of the large hub diameter. Do we measure from centre of hub ? Actual inner end of blade ? or in from outer tip of blade ?

So what is odd is that I think my GSC numbers are pretty close to these figures in degrees but not sure how far on GSC prop it goes.

Jan, Don and anyone -- Lowell you too since you got 2 degree variation but not at same station......
I think it worth a shot to set all blades the same? But if so what is the exact point to do it at ?
and should I first chart out the degrees are different lengths on prop? e.g. every 2 inches in from tip ?

Dave

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PostPosted: Tue Oct 24, 2006 12:11 pm    Post subject: Propellers Reply with quote

Dave,
See below
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Guy Buchanan



Joined: 16 Jul 2006
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Location: Ramona, CA

PostPosted: Tue Oct 24, 2006 1:37 pm    Post subject: Propellers Reply with quote

At 12:17 PM 10/24/2006, you wrote:
Quote:
--note the prop length I measuring is from the centre of hub to outside of
prop, But the actual blades are not 34 inches but shorter because of the
large hub diameter. Do we measure from centre of hub ? Actual inner end
of blade ? or in from outer tip of blade ?

That's the other problem. I had a 3/32" variation in blade length on my
Warp so I had to measure out from the hub to get a consistent place to
equalize the angle.
Guy Buchanan
K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.


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Guy Buchanan



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PostPosted: Tue Oct 24, 2006 1:37 pm    Post subject: Propellers Reply with quote

At 09:44 AM 10/24/2006, you wrote:
Quote:
This is my first post on this list.
It seems that we have a good amount of knowledge collected in this group.
I want to share my opinions on prop's

Jan's been helping me figure out my seeming lack of power / excess drag in
my 582 K-IV. He has some software that is, (from my perspective,) primarily
useful for propeller design, but can also be used for evaluation of
existing designs with some head scratching. He has been extremely helpful,
even digging up his own comparison data.
Guy Buchanan
K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.


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dave



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PostPosted: Tue Oct 24, 2006 1:57 pm    Post subject: Propellers Reply with quote

Guy, well have you tried the measurements like Don has suggested ?

I got he numbers now and it looks like I will change it .......very soon to
test and will report back my findings. Should only take 5 mins for a
flight, land and repitch and re fly and see how it compares.
Dave
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Float Flyr



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PostPosted: Tue Oct 24, 2006 4:09 pm    Post subject: Propellers Reply with quote

Prop stations should be measured from the centre of the prop hub. Your prop only shows a deviation of + - .25 degree at about .82 the prop length and spot on at the tips. Warp specifies setting the prop by the tips so it sounds to me that your set is good.

All blades should be set as close to exactly the same as you can get. The only place I know of where a blade is intentionally set out of pitch is on old car engines. They used to intentionally set one blade more coarse to avoid having the fan whistle at high rpm.



Noel [quote]
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PostPosted: Tue Oct 24, 2006 7:18 pm    Post subject: Propellers Reply with quote

At 02:56 PM 10/24/2006, you wrote:
Quote:
Guy, well have you tried the measurements like Don has suggested ?

Yes. I posted previously that I have about 1/2 bubble variation between
blades at the tip. I have yet to do a comprehensive survey or measure what
1/2 bubble is. I have been matching my blades at about 75% when I set
pitch, trying to hit a mean tip angle for comparison purposes. I'll get
some numbers when I get some time, (next week,) and report.

Quote:
I got he numbers now and it looks like I will change it .......very soon to
test and will report back my findings. Should only take 5 mins for a
flight, land and repitch and re fly and see how it compares.

Dang, I envy you. With the Warp HPL hub it takes me about an hour to re-pitch.
Guy Buchanan
K-IV 1200 / 582-C / Warp / 100% done, thanks mostly to Bob Ducar.


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PostPosted: Wed Oct 25, 2006 12:21 pm    Post subject: Propellers Reply with quote

Ok, if we take the average

25" 25,5" 34 1/8
15,58 deg 16,33 deg 10 deg
43,8" pitch 46,9" pitch 37,8" pitch hmmmmmmm

what is the relative air at full speed ahead?

Dave had 103 mph at 6900?

103 * 1056 = 108768
6900/3 = 2300
108768/2300 = 47,3"

so it have an negative Alfa (at the flat bottom) at 75% of 0,4 deg, that will be good for a CLIMB propeller, the high WOT RPM confirm that, and I don't think Dave say differently.

My program say 16 deg at 75% and 12,5 deg at tip, I don't know the thickness or cord length at tip on the standard warp but mine have an difference of 5,36 deg from the bottom to the zero lift line at tip ( airfoil is 10,5 % thick at tip)
10 deg + 5,36 is 58,9" so even if the angle is low at the tip it will make some thrust and not negative thrust. good but not perfect, but maybe necessary with the wide tip the standard blade have.

Jan
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PostPosted: Wed Oct 25, 2006 12:39 pm    Post subject: Propellers Reply with quote

I got in a few flights this am

one before and one after the reset .
I set all blades at 25.5 ": about 16+ degrees approx
tips were 10 10.5 and 11 degrees

might have been a little bit less vibration but performance about the same ........
did not have much time and full day ahead of me but will try later


Sorry I typed this at 9 am today and forgot to send was still sitting in desktop .

Dave

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PostPosted: Fri Oct 27, 2006 5:20 am    Post subject: Propellers Reply with quote

One propeller that have become popular in Europe is the French DUC and have proved its efficiency

se the info

http://www.duc-helices.com/anglais/catalogue.htm

Jan
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PostPosted: Fri Oct 27, 2006 7:07 am    Post subject: Propellers Reply with quote

Propeller

16 deg at 69% is 42,2" pitch.
16,5 is 43,66"
17 is 45,0"

16 deg at 75% is 45,9"
16,5 is 47,5"

What the pitch is at 75% on the GSC don't have to be, or more exact isn't the same as on 69% (depends on the blade twist and the angle)

Normally a propeller don't have true helix, meaning the pitch isn't the same along the whole blade, if the prop was sitting an a long shaft 10-20 feet ahead of the fuselage it might be ok, but at the nose the air is slowed down (pushed forward) by the fuselage, gear, wings ...) this make a propeller with less pitch near the hub better, on "normal" propellers the thicker airfoil near the hub also work better at smaller angles, and the opposite, the thinner blade near the tip work better at higher angles. (angles relative to the air)

Aspect Ratio.
Why is the warp doing so good? even with the wide tips on the standard blade.
Normally a propeller have a AR of 6 or 6,66 on an 2 bladed prop, the Warp is around AR 6 on 3 blades, meaning it would be AR 9 on 2 blade, a rather high aspect ratio.
We all know that a glider plane with AR 20-25 have better performance L/D then a plane with a AR of 6-7, same on propellers, the prop with AR 6 on 3 blade will have thinner blade in mm or fraction of inch, then a 2 blade with AR 6 if the airfoil(s) have the same percent thickness, and the losses at the narrower tip and blade is less with high AR. The 3 blade will have less diameter then the 2 blade with same blade, this also reduce the high drag from the higher Mach nr on the 2 blade.
Note that a 3 blade with AR 6 will have same diameter as an 2 blade with AR 6.

This may be the case with the Powerfin F model, it looks like it is wider then the Warp and GSC, this force it to have shorter blades to come to its optimum.
The pitch distribution maybe isn't optimal either. (I don't know)

Stuart at Powerfin say that it is not perfect to have just a few models and cut the length to perform, it might be so that the F model isn't designed for this speed/RPM/diameter VnD but better blade will come.

>The last few years I have been designing a manufacturing process that enables me to make drawings and toolpaths for molds very quickly with CAD/CAM. Just having a few blades and cutting them down to size is not a very good way to get efficiency when the main factors in efficieny seem to be proper lift distrubution and advance ratio to achieve the correct alpha angles. Since the optimal design for each application is unique, it is in my interest to figure out how to make molds and prototypes quickly. That’s been my main focus for a number of years.< Stuart.

Tests.

To make accurate tests, following test will be interesting, Take off distance (difficult to measure exact) time to take off might be better, but wind make a lot differences.
climb speed say at 60 MPH timing with the clock the climb from say 1000 to 2000 feet starting at 500 keeping the speed constant. feet AGL that is so we don't hit any Sitka spruce's.

cruise speed at say 5800 and 6000 but this mean the prop's have to be adjusted to exact the same WOT RPM others the cruise RPM will represent different HP's
and then WOT speed, all at same density alt. this have to be made same day on a calm day. (days are short this time of the year)

there is other aspects too, like vibrations and wear.

would be interesting to test the taper Warp and the standard on the same airplane/same day also.

Jan

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