[hevansrv7a]

I just noticed in some air-2-ground photos that I could see my prop blade's blurred image covering an arc.

I put a piece of paper on the PC's screen and traced the angle, completed the angle to its apex while off-screen and measured it with a protractor. In this case it was about 50 degrees. To be more precise I could subtract for the width of the prop blade in degrees.

Then I viewed the EXIF information for the image (IrfanView, a free download, can do this if you don't already have a tool which does) and it showed me that the exposure took 1/200 second.

200 times 50 degrees / 360 degrees = 27.7 revolutions per second which is 1667 revolutions per minute.

I did not note the indicated rpm when I took the picture, but that's about right. I think this may be a nice way to quickly check the tach.

Does anyone with more technical knowledge of digital cameras have more information?

 

[Tomasz]

200 times 50 degrees / 360 degrees = 27.7 revolutions per second which is 1667 revolutions per minute.

I did not note the indicated rpm when I took the picture, but that's about right. I think this may be a nice way to quickly check the tach.

Does anyone with more technical knowledge of digital cameras have more information?

There is no need for more information - your calculation is pretty much accurate.

There is a question though: why would you want to go such long way to check your tach? I mean it either works OK, or you can clearly see something is wrong. Tach is simply an impulse counter so in case when:
a) impulses are intermittent (bad wire/sensor) it will 'shake' the needle and you will clearly see it or
b) it counts them wrong, but then it will show a number way off (like x2 or x0.5).
I can't see a situation where it would show just a little bit wrong (like 50rpm all the time) and even if so that probably wouldn't matter in your flying.

 

[n5lp]

...Tach is simply an impulse counter so in case when:
a) impulses are intermittent (bad wire/sensor) it will 'shake' the needle and you will clearly see it or
b) it counts them wrong, but then it will show a number way off (like x2 or x0.5).
I can't see a situation where it would show just a little bit wrong (like 50rpm all the time) and even if so that probably wouldn't matter in your flying.

This is true with the newfangled electronic stuff. There are huge numbers of mechanical tachs around and they can indeed be just a bit or sometimes a pretty significant bit off.

 

[Jamie]

Guys:

I think he was just posting this for the 'coolness' factor, not for the practicality of it all. I personally think it's a rather neat hack!

Jamie

 

[BrentHumphreys]

One thing to consider in your calculation is the blur is from the back edge of the blade when the exposure started to the leading edge of the blade when the shutter closed.

Another thing to consider is the "curtain" of the shutter. In order to get fast exposure times the mechanical shutter (if so equiped like in my D80) is more of a slit traveling across the sensor.

That may cause some distortion as well.

 

[az_gila]

Another way to do a check at night....

...is to use the strobe effect of airport lights...

Borrowed from an on-line newletter...

A simple method of checking the tachometer in your aircraft is to run it up at night with fluorescent (or mercury vapor?) light shining on the prop. Since 60 Hz current is universal in the US, and maintained very accurately at 60 Hz, you can check your tach at multiples of 60, such as 600, 1200, 1800, and 2400 RPM (if you can turn up that high on the ground). Just adjust the RPM until the strobe effect stops the prop and check your tachometer. Make sure you take appropriate safety precautions while doing this.

Very accurate, since clocks are run off the 60HZ...