Propellor construction

[William Robison]

Without a prop it doesn't go. Almost all the micro threads have more or less info on making the props, why don't we try to get it all in one thread for easier reference? If anyone is interested, I wrote a spreadsheet model to figure helical pitch angles, as input it accepts diameter and pitch, returns the pitch angle at whatever diameter interval you want. Wrote it for Lotus, also runs in Excell.

So how about a propellor thread?

Win with a twin

WLR (Bill)

[Mike Taylor]

That sound intereting. How about posting a link?

[GHMBO]

Hi William
It looks like you are trying to get a lot of information organized. Have you also included in your spreadsheet the RPM, thrust and power input (watts) for typical altitudes and temperatures? It seems to me that if that is the case and we can get the motor manufactures to specify their products for load, RPM and power in, we can do some real equipment matching rather than quessing. I have mostly stayed with GWS because they have done some testing of their products and I generally know what to expect from their motor/prop performance tables. I would like to be able to mix manufactures with less experimenting. Keep us informed.
Jim

[William Robison]

Mike. I don't have the S/sheet model on a shared directory, tonight I'll set it up and post it here. I did not want to go to the trouble if there was no interest. It was originally written to make three bladed props for wet planes.

With two postings less than 30 minutes after I started this I think there just might be the interest.

GHMBO: No, I don't have anything in it other than the pitch angle calculator. BUT. I go back to the early days of computers (1956) and we posted (actually put paper tapes in drawers) our software welcoming others to modify/improve/make additions. Add what you wish, but try to keep it working on as many different version spreadsheets as you can, if you can't, IMHO Excell is the one to stay with due to its popularity. Doesn't everyone have Mocrosoft Office?

For motor current, battery life, and power calculations "MotorCalc" is almost free, but I don't know of any that work on propellor efficiency, so forth. Add/modify and be welcome.

Have a fling, a twin's the thing!

Bill

[William Robison]

OK, folks, here is a print of the spreadsheet for a 6x4 prop.

propellor:
diameter: 6
pitch : 4
rpm : 33614
eff % : 70%
circum : 18.85
tip angle : 11.98
speeds:
fwd mph: 89.13
static tip: 600.00
true tip: 606.58

Diam angle start: 1
Diam increment: 0.25

blade angle at angle
diam: diam: step:
1.00 51.85 6.33
1.25 45.53 5.20
1.50 40.33 4.29
1.75 36.04 3.56
2.00 32.48 2.98
2.25 29.50 2.52
2.50 26.99 2.15
2.75 24.84 1.85
3.00 23.00 1.60
3.25 21.39 1.40
3.50 19.99 1.24
3.75 18.75 1.10
4.00 17.66 0.98
4.25 16.68 0.88
4.50 15.80 0.79
4.75 15.01 0.72
5.00 14.29 0.65
5.25 13.63 0.60
5.50 13.03 0.55
5.75 12.49 0.50

From the top, left side: Propellor diameter and pitch, rpm is set to calculate rpm when the tip is at 600mph. This can be overwritten with any rpm number you want to check. Efficiency is a guess, (yours) enter as a decimal - 70% entered as 0.7. Next is the distance traveled by the tip in one revolution without any forward motion. (We'll come back to that) Finally the angle of the blade at the tip.

The "Speeds" entries are forward speed at whatever rpm and efficiency numbers you have, the tip speed with no forward motion, and finally the true tip speed including the forward speed indicated.

Next we enter the distance from the center of rotation to start the angle calculations, and below that the step distance between calculations.

You can start the angle calculations at any point in the length of the blade, and have the steps as close as you wish. For me, 20 steps (as written) is more accurate than my carving, but you can modify and have as many as you want. If you want fewer, either ignore the steps between or enter a larger interval.

Lower columns:

"Blade diam:" is distance where the angle equals the degree entry in the second column, "Angle at Diam:." "Angle Step:" is the difference in angle from one station to the next, makes it easier to see where the greater twist is.

I was going to attach the "wk1" file to put right into your spreadsheet, but while another host has the ability built in, I don't find it here.

DAVE ROBELEN! HELP! HOW DO I DO IT?

(I'll attach it when Ol' Dave tells me how, sorry)

You can get an idea of its capabilities looking at the answer print, I'll get it up as quickly as I can. If Dave tells me "No Way" I'll do a formula print, you'll have to manually key it.

Enough for now.

Get your twin OFF the shelf, or put yourself ON the shelf.

Bill

[Dave Robelen]

Hi Bill,
That work you have started is looking really good. I have been hacking out props for a very long time, and my only layout method has been the old triangle block system that started at least a generation ago. I quickly learned that if I can make a prop that matches the model/battery/motor system, Things will get better.
I do not have a ready answer to your question about the entry, but I will put it to the board administrator and see what can be done. It seems I know more about hacking up decent wood than messing with these computer systems.
Take care, Dave

[William Robison]

OK, I think I've found out how. If it works you can pull the attachment to this post and do your prop numbers. Refer to my last for useage. Play with it, modify it, improve it, add to it, but tell the rest of us so we can use it also. Thanks.

Forget it. Didn't work. I'll key it manually. Later.

Props designed with Excell, the twin never flew so well.

Bill

[Dave Robelen]

Hello Bill,
I just got word from our administrator Leo Ficks at Airage. He is unwilling to work with a spreadsheet because it will expose the system to all sorts of viruses, etc. Go figure-
Thanks for being willling to share with us. Too few modelers today are benefitting from making custom propellers, and the enhanced performance they make available (especially two of them)
Regards, Dave

[William Robison]

Thanks for the try, Dave. I'll do a formula text print and post that. It's just so much easier to click on the file and have it run, but I can understand Leo nit wanting to allow any binary files. Oh well.

I'll do it later.

Disallow binary, protect your finery.

Bill

[bmutlugil]

Hi Bill,

I am quite interested in your work, although I do not know much about the term "helical". As much as I understand, it might mean the pitch angles are adjusted along the prop blade such that every part of the prop would have the same contribution to the thrust or pitch - is this correct? What about props that are not helical - what other alternatives or definitions exist, if not helical pitch?

I once had written a program to calculate pitch angles - way back when we had punch cards as you said :). In this program, roughly thinking, for any point along the blade, the product of that point's radius and the blade angle was a constant - does this make it a helical pitch one?

Now I am only copying props using carving ****.. See the Sperry thread.. I would be interested in trying some of the calculated props, though - if I can find the time..

Best regards,

Bulent

[Dave Robelen]

Hello Bulent,
I will jump in here with my definition of "helical" as I use it. This term applies to a propeller that has the same advance ratio at any point along the blade. Or, the pitch in inches would be the same at any point. There are other pitch distributions that have been developed for modern Wakefield models, man powered airplanes and such, but for our micro models, the "helical" pitchseems to be the simplest way to lay out an efficient propeller. This pitch distibution also performed very well in some wind tunnel tests I performed comparing commercial Park Flyer props with custom cut ones. In all cases the helical props had a higher thrust for a given power input at any useful airspeed.
Regards, Dave

[William Robison]

Dave, Bulent, and all:

I used "Helical" because it's easier than erxplaining parabolic, and your eyeball will never see the difference anyway. But the air stream will.

To visualize a helical prop take a strip of paper 11" long and about 1/2" wide. Hold one end against a flat board with the edge vertical, this is the hub end. Twist the tip end down to a 10 degree angle to the board. If you twisted it clockwise with no kinks, looking from the tip end, you have made a helical propellor blade, one half of a 22x12.5 helically shaped propellor. Congratulations.

Why efficient propellors are not helical: When the diameter increases as a factor of two the distance traveled by the tip increases at a factor of 6.2832>>> (2Pi). Roughly 6.25" at a 2" diameter, and 25" at 4" diameter. This results in a greater angle change per unit distance near the hub than the change for the same unit distance near the tip. Look back at the answer print I posted for a 6x4 - the column "Angle Step." There is a >6 degree change from 1" to 1.25", but only one half degree change from 5.75" to the tip at 6". A helical blade does not include this variation in the advance rate, merely approximates it.

Helical: Constant angular change.

Parabolic: Angular change varies with diameter and nominal pitch.

The spreadsheet model gives the correct angles in degrees for parabolic blades, I have built in the conversion from radians to degrees in the arctangent functions. If you're a geometry whiz that tells you how to write it for yourself. No trig required.

So, Dave, reach out and touch someone (me) with your boot and get my --- in gear, post the numbers for the model. Thanks, (in advance) I needed that.

Prop Helical or parabolic, a twin will cure your colic.

Bill

PS: Dave, substitute "Parabolic" in your post and you're correct. wr

[Dave Robelen]

Hi Bill,
Thanks for clearing that up. Now I have the opportunity to test still more props someday:) Maybe this is the breakthrough that will let my KP-00 with direct drive pull a Litestick through a torque roll. Or then again, maybe not. The fun part is that the propeller manufacturers use none of these systems, and no two seem to agree on the pitch distribution. Maybe Bill is right, if I build a twin there will be so much surplus thrust that I won't really care.
Regards, Dave

[William Robison]

OK Folks, here we go. It will be uploaded in parts because I had it all typed in, and punched the wrong key. Wiped it ALL out. By paging if I do it again I'll be somewhat ahead.

"Pre" Notes: I can run this model in Excell, but it's written in Lotus. Excell recognizes the Lotus ".wk1" models, but some functions may be different when written in native Excell. If it doesn't run in whatever spread sheet you have try changing the file extension to ".wk1" to indicate Lotus, that may be all that is necessary to make it run. I will list the function signs at the end to help you translate if it wont run as entered.

The model is listed by column, cell number, cell contents

In every case if there is no cell number the cell is blank in that column. Cells for data entry will be indicated by a double slash // followed by what is expected in that cell. Labels will have a quote mark " preceding the text.

The model uses columns A through F, and rows 1) through 22).

This part will have columns A through C, the next part will have columns D, E, and F.

Column A:
2) "Diameter, inches
3) "Pitch, inches:
4) "RPM:
5) "Efficiency %:
6) "Circumference:
7) "Tip angle:
8) "Speeds
9) "Forward:
10) "Static tip:
11) "True tip:

Column B:
2) //diameter in inches
3) //pitch in inches
4) 633600/B6
5) //efficiency (in decimal)
6) B2*@PI
7) @ATAN(B3/B6)*(180/@PI)
9) B3*B4*B5*0.0009469697
10) B4*B6*0.0009469697
11) (B10^2+B9^2)^0.5

Column C:
4) "Starting
5) "Diameter:
6) //diameter point to start angular calculation
8) "Diameter
9) "Increment:
10) (B2-C6)/20

That's it for the first installment, second follows after a "Smoke Break."

Bill

[William Robison]

Here is the rest of the model. All notes in part 1 still apply.

Column D:
1) "Blade
2) "Diameter:
3) C6
4) D3+$C$10
5) D4+$C$10
6) D5+$C$10
7) D6+$C$10
8) D7+$C$10
9) D8+$C$10
10) D9+$C$10
11) D10+$C$10
12) D11+$C$10
13) D12+$C$10
14) D13+$C$10
15) D14+$C$10
16) D15+$C$10
17) D16+$C$10
18) D17+$C$10
19) D18+$C$10
20) D19+$C$10
21) D20+$C$10
22) D21+$C$10

Column E:
1) "Angle at
2) "Diameter:
3) @ATAN($B$3/(D3*@PI))*(180/@PI)
4) @ATAN($B$3/(D4*@PI))*(180/@PI)
5) @ATAN($B$3/(D5*@PI))*(180/@PI)
6) @ATAN($B$3/(D6*@PI))*(180/@PI)
7) @ATAN($B$3/(D7*@PI))*(180/@PI)
8) @ATAN($B$3/(D8*@PI))*(180/@PI)
9) @ATAN($B$3/(D9*@PI))*(180/@PI)
10) @ATAN($B$3/(D10*@PI))*(180/@PI)
11) @ATAN($B$3/(D11*@PI))*(180/@PI)
12) @ATAN($B$3/(D12*@PI))*(180/@PI)
13) @ATAN($B$3/(D13*@PI))*(180/@PI)
14) @ATAN($B$3/(D14*@PI))*(180/@PI)
15) @ATAN($B$3/(D15*@PI))*(180/@PI)
16) @ATAN($B$3/(D16*@PI))*(180/@PI)
17) @ATAN($B$3/(D17*@PI))*(180/@PI)
18) @ATAN($B$3/(D18*@PI))*(180/@PI)
19) @ATAN($B$3/(D19*@PI))*(180/@PI)
20) @ATAN($B$3/(D20*@PI))*(180/@PI)
21) @ATAN($B$3/(D21*@PI))*(180/@PI)
22) @ATAN($B$3/(D22*@PI))*(180/@PI)

Column F:
1) "Angle
2) "Step:
3) E3-E4
4) E4-E5
5) E5-E6
6)E6-E7
7) E7-E8
8) E8-E9
9) E9-E10
10) E10-E11
11) E11-E12
12) E12-E13
13) E13-E14
14) E14-E15
15) E15-E16
16) E16-E17
17) E17-E18
18) E18-E519
19) E19-E20
20) E20-E21
21) E21-E22
22) E22-B7

In Lotus functions are started with the @ sign. Two functions are used in this model. @PI returns the value of "Pi," @ATAN is the geometric function arcTangent. The dollar sign ($) indicates an absolute cell reference, if you type all formulae by hand they can be omitted, if you use a range copy to duplicate formulae, as in column D, they will be needed to maintain the reference to the step increment, and so forth.

That's all of it. Usage notes in part three.

Bill

[William Robison]

Using the propellor pitch angle calculator:

Data entry is done mostly in column B. Enter the diameter and pitch in cells 1) and 2), (Use 6d and 4p to test) go to 4) and enter efficiency. Start with 0.7, (70%) this indicates the actual travel of the propellor as opposed to the numeric travel, taking no note of slippage. For example, a pitch of 22" would travel forward at 150 mph at 7200 rpm if there were no slippage. But at 70% efficiency, or 30% slip, the true speed is only 105 mph. Now go to C6 abd enter the number 1. This is the diameter where we will start the angle calculations.

If you entered the model correctly you will see "RPM: 33614" in row 4, "Forward 89.13" in row 9, "True tip 606.58" in row 11. D6 will say "1", and E6 will say 51.85. Congratulations.

Let's give cell B4, RPM answer, its own separate coverage. As written it indicates the rpm at which the diameter you entered will be running at 600 mph tip speed with no forward travel. The speeds answers use this rpm for their calculations, so the indicated speeds will be meaningless until you enter a practical rpm.

Column D will give the DIAMETER where you should find the corresponding angle in column E. Diameter is emphasized because, for simplicity, I have not used radius. The diameters indicated must be divided by two to get the distance from the propellor hub.

You can enter any point in the starting diameter cell C6, and any increment you wish in C10. If left as is the increment will self calculate 20 even steps from your entered starting diameter out to the tip. I see no reason to change it, but you might want to see just a small section of the blade, Example: Start 2", increment 0.1, would give the angle stepping from 2" diameter every 0.05" for a total of 20 steps. 0.05? Radius, remember?

I think the rest will explain itself.

Enough for now - it's 1:30 AM.

If you have further questions I will be happy to answer them. Post your questions, use PM or Email. If it is something I have missed or covered poorly, or most emphatically if I have made an error I'll post the answer here.

WLR

[Dave Robelen]

Hi Bill,
Boy, that's quite a job you have done. On one run through, it appears to work fine for me. We go about this business of prop design in seperate ways. Way back in my pylon racing days, I purchased a Prather pitch gage. More than likely, you know what I am talking about. A heavy aluminum base with slots cut every .4" parallel to each other. There is an adaptor block for holding the prop, and a swinging arm that contacts the back face of the blade. This arm sweeps across a series of scales related to the distance out from the hub. Each scale is marked in steps of .5" pitch up to some high value. A mighty handy tool for propeller rework, truing up the hub, or checking a blank as a new prop is being fabricated.
Thanks for taking the time to share with the rest of us your analytical method.
Regards, Stone-age Dave

[William Robison]

Thanks, Dave, for the kind word.

If the model ran once, it should continue. Let me know positive/negative results, please.

"Stone Age" implies refusal of modernization.

For this you do not qualify, therefore I will not accept the application of the sobriquet "Stone-Age" to you. Even if you are the party attempting the self-application.

Is that sufficiently erudite in manner?

Called a fellow "Illiterate" one time, he insisted his parents were married.

Enough.

If your twin knows the word, it will always be heard.

Bill

PS: I have Scotch in my blood - and just raised the percentage. Sorry. wr