Prop -v- RPM Mystery

[jesse]

At 1100 lbs the 1000 fpm increase would only require about 35 more HP, still too much to expect from a different prop when you're talking about a 100-125 HP engine.

Lance--could you clue me in on how you determine that number?

 

[Capt. Geoffrey Thorpe]

Okay, Here are the station readings we got after the re-pitch. Can anyone out there in POA land extrapilate these readings and give a pitch? Blade 1 & 2 were pretty close.

The formula Sterba uses is;

Pitch/Circ. = the arc. tang = angle (Degrees)

Blade #1 Station Readings

15" = 35.5 degrees
18" = 29.4
21" = 25.25
24" = 22.5
27" = 20.25
30" = 18.75
33" = 16.20

Any calculations you can do to determine pitch?

I could, except something doesn't add up in your original formula

Blade Angle
Pitch/Circ. = the arc. tang. = angle (Degrees)
To find the angle needed at any blade station for a desired pitch.
Example: 60 pitch, 68 diameter (214" Circ.)
60/214 = .2804, then arc. tang of .2804 = 15.70 Degrees

I dont see where the '214" Circ.' comes from and how that relates to the diameter of 68 inches or the stations in your data.

 

[Dan Thomas]

200-300 fpm does sound really low even at that 108 hp. I'll be interested to hear how the new prop works out.

Yeah. There's something desperately wrong with the Sterba prop if it's only getting 200 or 300 fpm now. When I questioned the 1000 fpm increase I was thinking that it must be already 800-1000 now, and to double that would be unusual indeed.

Dan

 

[docmirror]

214 is close to the circumference of a prop which is 68" diameter. If I do the conversion at station 33 and pitch of 62(target) I come up with 16.66 degrees. 16.20 should produce just under 62" pitch, but the profile is important too.

<edit: I spreadsheeted it and 16.20 deg is going to produce a pitch of 60.2".>

 

[brcase]

Here is what I came up with.

RadiusDeg Degrees Circumference Pitch
15 35.5 94.2 67.2
18 29.4 113.1 63.7
21 25.25 131.9 62.2
24 22.5 150.8 62.5
27 20.25 169.6 62.6
30 18.75 188.5 64.0
33 16.2 207.3 60.2

circum=2*pi*rad
Pitch=Circum*TAN(degrees)

Feel free to check it for errors.
Brian

 

[Capt. Geoffrey Thorpe]

Here is what I came up with.

RadiusDeg Degrees Circumference Pitch
15 35.5 94.2 67.2
18 29.4 113.1 63.7
21 25.25 131.9 62.2
24 22.5 150.8 62.5
27 20.25 169.6 62.6
30 18.75 188.5 64.0
33 16.2 207.3 60.2

circum=2*pi*rad
Pitch=Circum*TAN(degrees)

Feel free to check it for errors.
Brian

Ok, missed the circ = circumference. I put in pi and get the same answers as above.

 

[jason]

this thread makes my head hurt

 

[Geico266]

this thread makes my head hurt

Try thinking of this all day long!

Whoa. Now I wonder why this prop is not producing the RPM I need. Maybe when he rounded the "backside" it cancels some of the "lift from the front side?

 

[Dan Thomas]

Here is what I came up with.

RadiusDeg Degrees Circumference Pitch
15 35.5 94.2 67.2
18 29.4 113.1 63.7
21 25.25 131.9 62.2
24 22.5 150.8 62.5
27 20.25 169.6 62.6
30 18.75 188.5 64.0
33 16.2 207.3 60.2

circum=2*pi*rad
Pitch=Circum*TAN(degrees)

Feel free to check it for errors.
Brian

I see a problem. Not with your figures; I assume they close enough. I see a huge pitch difference in adjacent stations, between 15 and 18 and 30 and 33. The stations with higher pitch are going to be overloaded and those with lower pitch are either loafing or are actually contributing drag. The effective AOA of a blade in cruise is between 2 and 4 degrees, so we can see that the tip is doing nothing and maybe even has a negative AOA. It sure is contributing little or nothing in climb, too.

I once had a 14.5-foot outboard that I converted to a 13-foot cracker-box racer with a Chev 283. The prop I used was a second-hand thing that had been repitched. The boat didn't go quite like it should and I finally took the prop off and measured the angles at various stations, finding that the repitching had applied further pitch only in the outboard areas of the blades and so the centre was dragging while the tips were cavitating much of the time. I bent it back where it had been when new, and that boat jumped out of the water and went like a scared cat. Got 400 more RPM, too. More HP.

And then I sold it.

Boat props are a little fussier than aero props, but such big differences shouldn't be there.

Dan

 

[gismo]

Lance--could you clue me in on how you determine that number?

One HP = 550 ftLbs/sec or 33,000 ft lbs / minute.

So to get an additional 1000 FPM in a 2000 lb airplane you need 1000*2000/33000= 60.6060... HP

 

[Geico266]

The boat didn't go quite like it should and I finally took the prop off and measured the angles at various stations, finding that the repitching had applied further pitch only in the outboard areas of the blades and so the centre was dragging while the tips were cavitating much of the time.

Dan

I think you nailed Dan. When I talked to Ed he told me he only took wood the last 1/3 of the prop. The Catto prop will be here in 3 weeks.