Wood Bar Dyno

[Tom H]

This is a spin-off from Jim's XL Built topic.

Answering Gil's question in the other topic, the instrument I am talking about is called a "wood bar dyno".  From what I remember, it was developed by some French guy back in the late 1800's, and I believe is considered valid.  Several Eaglers have used it to check their engine performance.

A spreadsheet used to be on the old Yahoo EaglersNests site, but I was not able to find it just now.  I found one version of it on my computer, and tried to attached it to this post, but it won't allow a xls file.

Basically, the technique uses a wood bar of a specific length with a square cross-section, with sides of that square 1/15th of the length.  In the middle is a wider section to allow the bar to bolt to the prop hub.

Here is a print-out of one spreadsheet that I used:

Wood Bar Dynamometer
test club is square cross section, width = length / 15
HP = 0.0532 x (dia/450)^5 x (rpm/1000)^3
pick length(in)	calc length(mm)	calc width(mm)	calc width(in)	@ RPM	calc HP
28.00	711.20	47.41	1.87	2000	4.20
				2250	5.98
				2500	8.20
				2750	10.91
				3000	14.16
				3100	15.63
				3200	17.19
				3300	18.85
				3400	20.62
				3500	22.49
				3600	24.47
30.00	762.00	50.80	2.00	2000	5.93
				2250	8.44
				2500	11.57
				2750	15.40
				3000	20.00
				3200	24.27
				3300	26.62
				3400	29.11
				3500	31.76
				3600	34.56
32.00	812.80	54.19	2.13	2000	8.18
				2250	11.65
				2500	15.98
				2750	21.27
				3000	27.61
				3100	30.47
				3200	33.51
				3300	36.75
				3400	40.20
				3500	43.85
				3600	47.72
34.00	863.60	57.57	2.27	2000	11.08
				2250	15.77
				2500	21.64
				2750	28.80
				3000	37.39
				3100	41.26
				3200	45.38
				3300	49.77
				3400	54.43
				3500	59.38
				3600	64.61
36.00	914.40	60.96	2.40	2000	14.74
				2250	20.99
				2500	28.80
				2750	38.33
				3000	49.76
				3100	54.91
				3200	60.39
				3300	66.23
				3400	72.44
				3500	79.02
				3600	85.99
38.00	965.20	64.35	2.53	2000	19.32
				2250	27.51
				2500	37.74
				2750	50.23
				3000	65.21
				3100	71.95
				3200	79.14
				3300	86.79
				3400	94.92
				3500	103.55
				3600	112.68

If the sheet shows above - -   - - we used a 30 inch bar, and our engine, at full throttle, spun it to 3400 rpm, resulting in about 29 hp.

This is only one point.  If you want to generate a dyno curve showing max hp at every rpm,  you will need a series of bars of different lengths.  Then, each bar is run on the engine at full throttle.  Each rpm/hp point can then be put on a chart, and the dots connected, which will give a crude dyno curve for the engine.  I am sure that air temp, humidity, pressure etc. will alter the results, but I don't know of any correction factors for that.  This technique will give some validity to hp claims, but won't produce laboratory quality results.

Hope that helps.

Tom H

[grdev]

Looks like I will be making one, sure hope it don't come apart!


Thanks Tom

[Steve]

Looks like I will be making one, sure hope it don't come apart!


Thanks Tom

The dimensions of the bar require wood working accuracy - the edges must be true and with very little rounding of the corners...

Bars for prop extensions with bolt patterns < 4" should be laminated to assure adequate hub strength...

English Excel Spreadsheet Tool located in Forum Toolbar "Downloads"