Hello, I am new here and have a question about an optical tach… I have and use a small handleld digital tack on my model aircraft…this is usually held within a foot of the spinning prop and the rpm is displayed on the LED panel. The photocell for this tach is in the top of the insturment and is pointed toward the prop.
I want to make a Tach for a full sized homebuilt aircraft and was wondering if it is possible to get an accurate reading if I physically move the photo cell to a point on the cowling of the aircraft (within about a foot of the spinning prop) by using a long lead from the photo cell to the circuit board? I am hoping to mount the model airplane tack on the insturment panel by velcro and have the photocell wire equipted with a plug to make the tack removable without much trouble… the tach is powerd by 2 AA batteries.
I plan on putting the photocell in a small tube to shield it from surounding light and have it ‘focus’ on the spinning prop…
Will this work?
Thanks.
It should, but please remember that there are very specific rules about what you can and cannot do on an aircraft - even a home-built experimental craft.
And, I wouldn’t trust my life to an instrument such as that. For testing and a general indication, maybe, but for flight? Nope, I’m worth too much.
Give it a try, maybe with some duct tape first, and let us know.
Wouldn’t you want an engine tach rather than something measuring prop RPM?
Is there a practical difference?
Alan,
I have seen on some home builts using non-aircraft engines that there is some transmission like connection. They are not all direct drive.
From a technical stand point it should work. You might need to put some reflective tape on your propeller. As others have said the FAA may not approve & I would be hesitant to depend on it in a life critical way.
I think the point that Alan was trying to make that even with reduction, the engine speed is directly related to the propeller speed. I have never heard of anybody using a CVT on a plane.
Frank,
Lot’s of airplanes use speed reduction on the prop. The Cessna 400 series and Beech Bonanza have them. Many experimentals that use automotive engines use them to keep the engine RPMs up in the power range. Lycoming engines with the “O” prefix are all reduction engines.
My point was that even with speed reduction, it is a constant. So if you know one RPM you know the other.
Fixed reductions are pretty common. CVTs (constantly variable transmission) is the red herring since I do think they get used on planes. I should watch putting too many thoughts in one paragraph.
Lycoming IO series engines are direct direct drive though
Only on CD can you learn about aircraft engines in a discussion probably started by a spam bot 
I love this place!
Thanks guys for the responses…
To help clarify the issue… I have an experimental aircraft that uses a full VW engine in direct drive… I have a shielded ignition system using a Magneto…so the ‘wire to distributer’ wont work, nor will my Tiny Tach that has the wire that wraps around a spark plug wire…Since it is direct drive, I thought that using the ‘model airplane tac’ with extended photocell would be the best way to monitor rpm…and as usual…that would only be a reference…Im sure that I could tell if there was a problem with the engine by listening to the noise…and since I dont plan on flying this is any kind of bad weather, nor is there room for two people…it is all relative!!
Thanks again for the replys
If you are using a VW engine, couldn’t you just use the tachometer that was sold with the vehicle for which the engine is designed?
I do not have one available…actually, I was mistaken…it is a 1/2 VW…and most of the unwanted holes have been welded shut!! So to use the mechinal tach wont work…besides…too heavy and cable too long!! and no connection for an electrical tach.
How about a hall effect sensor instead? You could set it up to sense the passing of gear teeth, or bolt heads…
Aye, but weather has a way of sneaking up on you.
I agree, your ears will give you a good indication, using this as a secondary reference may be acceptable. Again: Tape it to the cowling and see what happens!