IVOPROP IFA Medium - Moment of Inertia Test

[Geowitz]

Ran the test described here on my Medium IVOPROP IFA - http://www.800-airwolf.com/pdffiles/ARTICLES/part31.pdf

At 30 oscillations I had an average of 165 to 170 seconds. Prop assembly weight is 17.5 pounds which is actually off the chart, but if we extrapolate where it would be on the chart the Medium IVOPROP IFA appears to be around 7500 kg/cm2.

Just food for thought. Sorry about the crappy picture.

[rwaltman]

... weight is 17.5 pounds which is actually off the chart

Will try to work out the formula.

... the Medium IVOPROP IFA appears to be around 7500 kg/cm2.

Oops! The max for the 700E is 3000 Kg/cm^2 for the 2.58:1 reduction unit and 6000 Kg/cm^2 for the 3.47:1
I believe 6000 is the limit for the 700T
Need to measure the Powerfin, may have the same problem.
Thanks for the info.

Roberto.

PS: Found it - From the 700T Operations Manual v1.00,
"3.9.1. Propeller mass moment of inertia
Max. permissible propeller mass moment of inertia
6000 kg cm^2"

[Timberwolf]

inerpolated and got 8166 KG/cm^2....hmmm what does IVO say about using them on the 700T? There seems to be quite a few ppl running them on the Rotax with no problems

[rwaltman]

Will try to work out the formula.

Fortunately for my forgotten physics, many others did already.
The magic words to search for are "bifilar torsional pendulum"

A formula is derived in this article:
http://www.mathworks.com/tagteam/628...MMOI_final.pdf

And an excellent video lecture here (Purdue) shows the experiment itself. (equivalent formula, D = 2b, etc.):
http://www.youtube.com/watch?v=m9iHEanmNWc

Roberto.


PS: This is the video description:
In Dynamics classes, we learn how to calculate mass moment of inertia, but sometimes you will need to measure it instead. A common way to measure mass moment of inertia of an object is to swing it on a bifilar pendulum. The process is simple and the results are easy to interpret. I show you how to run the test using an aluminum bar as an example. Then I show how to calculate MoI from the results and compare the experimental results to analytical results.

PS-2: The page here mentions a "trifilar pendulum", which could save some work balancing the propeller, at the expense of the additional work making sure all three threads have the same length.
http://www.me.utexas.edu/~me244L/labs/filar/filaroverview.html

[rwaltman]

Using the formula from the video and plugging-in Geowitz numbers:

m = 17.5 lb = 7.95 Kg
b = 6" = 0.1524 m
L = 72" = 1.829 m
T = (165 + 170) / (2 * 30)
T = 5.58 sec

Pi = 3.1416
g = 9.81 m/sec^2

I = (m * g * T^2 * b^2) / (4 * Pi^2 * L)
I = (7.95 Kg * 9.81 m/sec^2 * 5.58^2 sec^2 * 0.1524^2 m^2 ) /
(4 * 3.1416^2 * 1.829 m)
I = ((7.95 * 9.81 * 5.58^2 * 0.1524^2) /
(4 * 3.1416^2 * 1.829) ) Kg m^2
I = ( 56.40 / 72.21 ) Kg m^2
I = 0.7810 Kg m^2
I = 7810 Kg cm^2

Note that the video defines L not as the string length, but as the distance from the upper attach point to the plane of the center of gravity of the body being measured. Also I believe the 20 degree oscillation angle described in the Rotax paper is too much, I would aim for no more than 10 degrees.

Roberto.

[szicree]

A small nit to pick: I believe the units would be kg*cm^2, not kg/cm^2.

[rwaltman]

the units would be kg*cm^2

Yes, thank you for the correction. [ Previous post fixed ]

Roberto.

[Geowitz]

Using the formula from the video and plugging-in Geowitz numbers:

m = 17.5 lb = 7.95 Kg
b = 6" = 0.1524 m
L = 72" = 1.829 m
T = (165 + 170) / (2 * 30)
T = 5.58 sec

Pi = 3.1416
g = 9.81 m/sec^2

I = (m * g * T^2 * b^2) / (4 * Pi^2 * L)
I = (7.95 Kg * 9.81 m/sec^2 * 5.58^2 sec^2 * 0.1524^2 m^2 ) /
(4 * 3.1416^2 * 1.829 m)
I = ((7.95 * 9.81 * 5.58^2 * 0.1524^2) /
(4 * 3.1416^2 * 1.829) ) Kg m^2
I = ( 56.40 / 72.21 ) Kg m^2
I = 0.7810 Kg m^2
I = 7810 Kg cm^2

Note that the video defines L not as the string length, but as the distance from the upper attach point to the plane of the center of gravity of the body being measured. Also I believe the 20 degree oscillation angle described in the Rotax paper is too much, I would aim for no more than 10 degrees.

Roberto.

So the crude number I interpolated as an average is pretty close if not close enough. Both my number and Roberto's calculated number are both quite believable in my opinion.

Yes, this was the blade recommended by IVO for the 700T, however, it was recommended based upon power and RPM requirements. They do not discuss MOI. That is left up to you. I had anticipated the 3 blade MOI being over the 6000kg*cm2, but was optimistic the 700T would handle it as the Rotax 912 ULS's do. I'm going to set it up as a two blade and try the test again tonight.

[rwaltman]

Another idle-setup data point.

The video here [ ] shows an HKS-700e installed in a Quicksilver with a 3-blade Kiev Propeller.

Caption at 00:21: "Idle is set at 1950 rpm, this is where the engine will smooth out with the prop installed."

Caption at 01:11: "The Quicksilver engine mounts really help, as they absorb almost all the vibration."

Caption at 01:36: "The entire airframe is mostly vibration free, and during cruise it's extremely smooth."

Caption at 01:53: "The HKS Coyote in the background will not idle this smooth (mostly because of the engine mounts) but it seems to be as smooth at all power settings above 2500 rpm."

What is so special about the Quicksilver engine mounts?
The engine can be seen shaking a little, it is difficult to judge from the video how "smooth" it is.

Roberto.

[szicree]

I'm going to set it up as a two blade and try the test again tonight.

The moment of inertia with two blades will (should) be a hair more than 2/3 of the MOI with three, assuming the blades are all the same. The hair more part is cuz you've still got the entire hub.