### Current (and Airspeed) for \$1.45

This is a Current (And Airspeed) Sensor for \$1.45.

I raised this question a while back, and was reminded again by the Post below which asks how to stuff all of this into a small box. (Beautiful Glider)

Small planes should remains a primal focus because they reduce the real and regulatory risks and allow broader participation in more fields/parks, etc...

So I'm offering a size reducing part - add a current sensor in exchange for the whole pitot/airpressure spaghetti.

The Airspeed theory is simple - at a given RPM, power consumption is proportional to airspeed, The faster the plane, the less the motor consumes to turn the prop. One can either detect the rpm, or determine that ESC's are essentially RPM = Throttle. Then a bit of data collecting will show the relationship.

http://www.goldmine-elec-products.com/prodinfo.asp?number=G16976

• Current transformers measure AC.

But you can use a hall effect sensor to measure DC current. They are about the same size and weight, but cost a little more.

• Developer

There are other problems with measuring airspeed by this method and I suspect it would be fairly poor for the purpose of maintaining a constant airspeed.

Another problem for example - most esc's do not hold a constant rpm for a constant throttle setting. The only ones that do are high end helicopter esc's with a governor mode.  Cheaper esc's that claim to have a governor mode usually have a governor mode that doesn't really work.and most esc's targeted for smaller planes don't have a governor mode at all.

• And it's not so easy to convert current to prop RPM, let alone air speed. 5A may mean the prop is running at a constant 2000 RPM, or accelerating from 100PRM to 200RPM. I suppose if you considered the commanded throttle too, you could design a complicated filter for it.
• yes, V(t)= -L x di(t)/dt

This behavior will be detrimental to the system if fast power peaks are requested to the battery, that's why shunt resistors for current measurement doesn't imply to be a coil.

On the other hand a coil (long wire) will present some resistance that may be useful for current measurement.

• Matthew is correct, this is an AC sensor. v(t) = L*di(t)/dt.
• @Scotty

I suggest only that there is an inverse relationship between airspeed and power for a given RPM - within the range of powered flight.

It has to do with how many particles are hitting the prop blade - the same rule as the pitot.

• @John,

So you're right, at zero airspeed, the motor has to work hardest to maintain RPM, at full dive, the opposite is true, the motor has to work least to maintain RPM.

I should say it's inversely proportional.

When the prop is moving through the air at the rate of its pitch, the only power is drag, when pulling the plane

at the same speed, the power includes prop lift, thus the airseep is "behind" the prop speed.

As for gliding, tis mostly true - once that prop folds, there is no signal.

If you have a gyro, you can slightly deflect the ailerons briefly, and measure the Roll rate, an imperceptible twitch ever 2 seconds or so.

So one reason I think airspeed sensor should have multiple modes of detection is because pitots fail, whereas the prop is pretty big. Control surface twitching can be redundant on every channel.

@Matt, you can place it on one Motor lead - where current is AC... (Allegro has DC which I've posted previously.

• Correct me if I am wrong, an inductive current sensor measures changes in current, not dc current.
• It would be useful for current sensing, but there is not a linear relationship between current and airspeed. Interesting part though... And cheap...
• What John said.

It is entirely possible for the aircraft to be in a thermal, climbing, accelerating, and have the throttle at idle.