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jb92563
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Posted Monday, November 16, 2009 @ 12:46 PM  

Just a few notes on the relationship between prop diameter, pitch and HP.

I just have some tips for those that are not up on the propellor science.

Really its hard to know exactly what prop is best with a specific aircraft/engine
combination because there are so many variables from plane to plane such as
the HP output of the engine at a specific rpm, the airframe drag and numerous other
subtle factors, including the specific mission (Best climb, top speed etc).

The way to find the best prop ends up being experimentation in the end.

Here however are some relationships that will be helpfull in adjusting for your next
experiment.

Greater diameter props create more THRUST which is beneficial to the climb.

A limiting factor to Diameter is the tip speed as it is better if it is ~80% of the speed of sound or less.

Additional blades also contribute to adding more thrust so when tip speed becomes critical, you add another blade, which
slows the rpm, reduces diameter and tip speed is reduced.

Increasing PITCH will improve your top end speed and fewer blades also contribute to greater top end speed.

Of course the HP available to turn the prop at a specific RPM is also a big factor as is the drag of the airplane.
If you adjust Pitch or Dia the RPM will be affected acording the the available HP at that RPM....its always a trade off of one thing for another.

I have reviewed the various prop testing people have done in this forum and along with my prop programs have some observations.

It seems that 53x33 prop produces decent reults but interestingly Sam's 57x27 prop also gets him 118 mph level with a climb rate of 690 fpm.

He is getting great climb due to the extra diameter and the reduced pitch allows the engine to wind up to 3800+ rpm and generates a bit more HP.

VW car folks run the VWs at much higher rpms(6,000+) to get more HP out of them.

This leads me to believe that the RF4D needs the extra thrust to overcome the drag for a higher cruise, plus the extra diameter improves the climb as well.

My experiment will be to take a 62x30 prop that I have and cut the tips down until I arrive at what I think will be a 56x30 size to get a good balance of climb and cruise and see how that works out.

Also in reading Paul Lipps(ellipse Props) article about props I think that beveling the tips ("Slash Tips" instead of just a straight cut will also help efficiency.

I'll keep this thread updated as my experiments progress.

[Edit by jb92563 on Monday, November 16, 2009 @ 02:54 PM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

Sam M.
First Sergeant

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Posted Monday, November 16, 2009 @ 09:58 PM  

Actually our RF4D N1700F was timed at 770 fpm. with a 54x27 and fresh 1400cc engine.

That is with only a 140 pound total pilot weight and aerobatic fuel.

[Edit by Sam M. on Monday, November 16, 2009 @ 10:00 PM]

Jorgen
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Posted Tuesday, November 17, 2009 @ 03:27 PM  

You beat me to it Sam, not only are you the youngest Fournieteer, I bet you are also by far the lightest!

Not that I doubt the Zipper is among the freshest 4´s in the gang...

May the 4´s be with you/ Jörgen

Bob Grimstead
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Posted Thursday, November 19, 2009 @ 10:25 AM  

Hi Guys,

I think I might have some interesting facts to add.

My Australian professional prop-carving buddy tells me that required horsepower increases with the cube power of prop diameter. I keep asking him for a longer prop, but he refuses, going coarser instead.

My new VW motor (a 1400, with standard 1200cc camshaft, inlet tract and exhausts) has just finished its dynomometer runs, and the horsepower curve turned out to be surprisingly flat, with its peak power at 3,600 rpm. Any increase in rpm led to a decrease in power. The drop off per 100 rpm over 3,600 was greater (steeper curve) than the rate of increase up to 3,600 rpm.

Max torque came at a surprisingly low 2,500 rpm, and again the curve is very flat, so there is still significant torque at 3,600 rpm.

All this has been borne out empirically in my flying with ever longer and coarser propellers (see my earlier posts).
Take-off rpm (1,400cc motor) are currently around 2,500-2,600, and yet take-off performance seems unchanged from with the finer standard Hoffmann prop and 2,900 rpm static.
Climb perfomance (new 1,400cc motor, aerobatic weight) is rarely better than 600fpm, and more often around 500fpm on a warm day, varying greatly with ambient temperature.

Good luck with your propeller-matching Jorgen, and I look forward to hearing the results, as I continue to strive for maximum thrust from this valiant little motor.

Incidentally, the Aussie props I currently run have square chopped off tips, and I've often thought that a thinner section towards the tips with a tapering planform (like Hoffman's) would be more efficient.

Yours, Bob

[Edit by Bob Grimstead on Tuesday, February 2, 2010 @ 04:02 AM]

jb92563
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Posted Monday, November 30, 2009 @ 04:51 PM  

I have prepared a prop to test on the RF4D.

It was a Tennesee Prop 60x30 that I cut down to 57.5 x 30.

I also added the Paul Lipps "Slash Tip" which is nothing more than a bevel of the square tip,
starting 15% of the blade chord back from the tip on the bottom (Flat) side of the bade tapering to meet the top surface.

According to Mr Lipps the resulting sharp edge is supposed to reduce/change the tip vortices in such a way as to increase rpm and
acts like a prop of greater pitch, as compared to a squared off, straight cut tip.

Quote:
A slashed tip on a square-tip prop is created by first drawing a line across the blade on the bottom surface about 15 percent of the tip chord in from the tip. Shape the tip from this line straight radially outward up to the top surface of the tip, forming a very sharp edge. This sharp edge trips the vortex at the very outer edge, giving the most efficient tip. It will increase full-throttle rpm by 20 to 50 and give a 1- to 5-mph speed increase.......Paul Lipps

http://www.eaa.org/experimenter/articles/2009-02_elippse.asp

Here is the blade tip that is beveled over 0.9mm (3/8") which is 15% of the blade chord as Mr Lipps specifies.

http://picasaweb.google.com/lh/photo/4G8jX2T-rbBEFUWeg6aODQ?feat=directlink

When beveling the "Slash Tip" a square tip geometry changes to rounded corners due to the blade profile intercepting the bevel.

http://picasaweb.google.com/lh/photo/-dHFTBhpRXRy8kR9rEU-bw?feat=directlink

I will cut this test prop back gradually to see where optimum climb rate is produced.

I have been getting about 500 fpm climb with the current prop. (Not sure of the pitch but a 53" diameter)

Thanks Bob G for the 1400 Engine power curves, it was interesting to note that the power curve is so flat.

[Edit by jb92563 on Monday, January 11, 2010 @ 11:47 AM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

jb92563
Second Lieutenant

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Posted Monday, December 7, 2009 @ 11:51 AM  

I have test flown the 57.5" x 30 modified from a 60x30 Tennesee prop this weekend.

The Static Revs were 2600.

Flat out level was 3000 rpm @ 90 mph.

A shallow dive to 100 mph yeilded 3200 rpm.

The takeoff distance was about the same.

The climb did not seem any better and it was so windy I decided not to even bother with a timed climb.

I cut the prop down to 56.5"x30 and recut the "Slash Tips" to try again next weekend.

It was definately over propped.

[Edit by jb92563 on Monday, December 7, 2009 @ 11:52 AM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

jb92563
Second Lieutenant

Gender: Male
Location: Lake Elsinore, CA, USA
Registered: Mar 2007
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Posted Sunday, January 10, 2010 @ 05:42 AM  

The 56.5"x30 prop had 2600 static and 3000 rpm flat out level at around 95mph.

Seems like its the same as the 57.5x30.

I'll take the Diameter down to 55.5" for the next test and see how that goes.

After that I will try narrowing the blade chord, a bit at a time, from mid span to the tip like the eLipps props, but not as drastic.

[Edit by jb92563 on Monday, January 11, 2010 @ 11:52 AM]

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

jb92563
Second Lieutenant

Gender: Male
Location: Lake Elsinore, CA, USA
Registered: Mar 2007
Status: Offline
Posts: 582

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Posted Monday, February 1, 2010 @ 11:53 AM  

My latest revision of the prop to 54x30 seems to be improving the climb rate on my first test flight.

I did not collect accurate numbers but it has definately improved and looks like I am getting 500+ fpm
with a Statc runup to full throttle of 2700 and 3000 rpm at flat out level.

I do not think top end speed has improved.

I'll have to take another flight to record the real numbers and make a timed climb.

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

Bob Grimstead
Captain

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Posted Tuesday, February 2, 2010 @ 04:24 AM  

Hi Guys,

If I've learned anything about propellers in my 40-plus years of flying, it is that one man's 54x36 prop will not perform anything like another's so, to compare props, you either have to compare props made by the same guy, or else simply borrow a succession of props, try them, and see what you get.

I think I've flown either six or seven props on my blue Aussie RF4, and the best is the one currently fitted, a Heliptera 52 x 36.

For a couple of years, I have been flying my 1400cc Fourniers with these 52 x 36 propellers that were originally designed for the 'new' 1750cc motor that has been sitting in my hangar for three/four years.

In the knowledge that these props worked well with the 1400cc motors, giving good climb and cruise performance with low revs, so they keep the noise down for spectators when I'm flying aerobatics, I asked for a prop that would give me comprable performance with the 1750cc motor when I fit it later this month (well, that's the plan).

He said:
With 1750cc I estimate power will be at least 60hp@3600rpm (compared with 46hp for 14000cc).
(We actually got 52.2hp at 30C from the 1750, 40.9hp from the 1400)

Power required varies with the propeller-diameter to the FIFTH-power.
ie 60/46 = (D/52)^5

(oops, I got that wrong)

Thus the new diameter (D), all else being the same except power (ie same max revs), is around 55-inches.

I could go to 56-inches diameter.

Airspeed increment varies with the cube-root of the power-ratio:
ie 60/46^0.333 = 1.0926

Thus, going from 1400cc to 1750cc, max speed should increase by around 9% ie from 103 to 112 knots

My estimates for the new propeller for your 1750cc VW engine, are a D56xP36 propeller.

This will keep the revs about the same at the same flight conditions, but should absorb the additional power.
Static revs should be about 2800rpm.

That's all just for info.
Ill let you know how things turn out in practice.

Yours, Bob

Mike-RM
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Posted Friday, February 12, 2010 @ 06:44 PM  

We tried Bob's 52 x 36 inch Heliptera on our RF-3 last year and were so impressed by the performance that we got Alan Newton to make a 52 x 36 inch prop for us. We have the 1400cc mod and the old 52 x 28 prop was just too fine pitch. Now she cruises at over 110mph at max continuous (careful! that's well over RF-3 Va of 93mph!) More usefully the 90mph cruise is now a very relaxed and quiet 2700rpm. As others have found, takeoff performance is little different. The rate of climb is somewhat reduced but still gives over 600 ft/min - rather better than our standard RF-5 so 52 x 36 seems a good combination.

Newton Propellor seems to be the only bespoke prop maker in the UK at the moment. I got a very good service from him and the price is reasonable. He does polyeurethane injection leading edge protection, which seems very good so far. The Heliptera was cheaper, but carriage from Oz was prohibitively expensive.

[Edit by Mike-RM on Friday, February 12, 2010 @ 06:45 PM]

jb92563
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Posted Wednesday, February 17, 2010 @ 12:08 PM  

The results are in on the tennesse prop 54 x 30 with "Lipps" slash tips.

I am getting a solid 500 fpm at 2850 rpm at 65 mph airspeed.

This is a noticable improvement in the climb rate even before clocking it to confirm.

The top level speed is about 105 mph at 3000 rpm.

The next modification to the prop will be to taper the chord from its 2/3rd diameter to the tips
in an effort to increase the rpm by 100-200 and see what that does for the climb.

I'd like to keep the diameter at 54" since it is a props increased diameter that contributes more to the thrust,
where as increasing pitch contributes more to the top speed.

My goal for this prop is to improve the climb rate.

That 52x36 does sound very good though if you are getting good speed AND 600 fpm climb.

--------------------
Ray
RF4D #4057 N-1771 Rectimo 1400cc
http://picasaweb.google.com/jb92563/FournierRF4D
http://www.touringmotorgliders.org

Sam M.
First Sergeant

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Posted Wednesday, February 17, 2010 @ 11:09 PM  

Im getting 3350 on climb and over 120mph flat out

[Edit by Sam M. on Wednesday, February 17, 2010 @ 11:10 PM]

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