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Torque Meter for very light helicopters


I like the "Low Fuel Level" warning device v. much. The differential pressure principle is so simple even I understand. Don't know why I missed the original post in 2011 as I check the forum everyday. Will incorporate in my continuing build and will post pics when the time comes. Thanks for your continuing contribution.


I like how simple it is Bryan. I am afraid it is not an accurate instrument. It will be close only in a hover, but not 100%. It will be way off in forward flight.
In a hover, Air passes through the rotor system (mass air + velocity (over time) = Induced Flow). We all know that induced flow is slower relative to close proximity to the ground. A tail rotor is affected by the induced flow on half of the rotor (tail) system. The direction of the tail rotor direction determines its efficiency relative to the induced flow. In the case of the mosquito, the blade closest to the rotor system is going down. Kind of like following the air as it goes through the main rotor system. This blade will experience far lower velocity (velocity squared) than one going up at to the main rotor. This in my opinion is better for a low disk loaded rotor system, which is what the mosquito is. This I feel will enter LTE a lot smoother, or, not as sudden.
The bottom of the tail rotor will see the same thing. The induced flow coming out of the rotor disk travels out in all 360 degrees. The rear sector is where the tail rotor lies. The bottom of the tail rotor blade will be sweeping aft on its bottom quarter. The air this section now experiences is far less (V^2).
What does this mean with regard to a tail rotor pedal indicator?
The weight will determine the induced flow through the main rotor system for a given height above the ground (say 3ft). The main rotor will experience the same velocity (rotor RPM remains constant) while the tail rotor will experience different velocities (tail rotor RPM minus Induced flow). This distorts the 1:1 ratio required for connecting over all torque with relation to tail rotor travel.
In addition, this will only work in a “no wind” condition. If you factor in a left quartering tail wind on the mosquito, it would require a higher pedal input to maintain heading. This torque increase will be only because of the tail rotor. The torque required from the main rotor does not change. This again distorts the 1:1 ratio needed to tie overall torque to tail rotor input.
In forward flight, none of this could work without an airspeed indicator to measure the change to the pedal input ratio to overall torque. If there is any kind of vertical stabilizing device on the helicopter, it will off load the tail rotor (by some %). Because of this, the ratio is changed from 1:1 to some other ratio.
What could work?
In my opinion, it must be something that is measured before inputs are made. If the helicopter were able to measure flexing in a belt, then it can quantify (in Newton Meters) belt flex relative to Newton meters.
How does that work?
As an engine pulls more torque against a main rotor and tail rotor combined via a belt, its belt will tighten up proportionately. If a spring loaded belt tensioner was placed on the tensed belts side, you could measure movement of the tensioner relative to Newton meters required to move it. This would need to be measure off of the engine, not the main rotor. If it were only the main rotor, it would not take into account the tail rotor inputs. This would give you an overall torque gauge that will be accurate.
bryancobb - 10/7/2012 5:05 PM

Everyone, including me, has been trying to come-up with a super-simple TORQUE METER for Helicycle, Mosquito, Mini-500, etc.

Some of the logic we have discussed is measuring twist in the mast...stretch in the belts...deflection of the transmission case...and even miniaturizing the HUEY "2-washers with steel balls" torque meter.

Another thread got me thinking about it this morning.

WELL...I'VE GOT I!!! Cost? Less than $20. Weight? Less than 1/2 pound.

Background: I learned to fly helicopters in Army Primary, in the TH-55 (269).
The manifold pressure gauge was color coded and it had nothing to do with limitations. The green / yellow / red arcs corresponded with a 1" wide color strip on 1 antitorque pedal, and a pointer on the other pedal. As pedal position changed (torque varied) the pointer gave an indication on the color strip.

This was for students that didn't have a good feel. They could look at the manifold pressure color arcs and PUT THE PEDALS in the same color zone.
This was a "Forest Gump" way of teaching students to apply the right amount of pedal for the amount of torque being used.

Now see the next post for a P E R F E C T torque gauge for little helicopters.
Gimme about 30 minutes to do an AutoCAD sketcQUOTE]

Bryan, I love the simplicity of this idea.
I have had a MP gauge sitting in my panel, unused, ever since a friend of mine pointed out that two-strokes "spit back" rendering the MP gauge useless.
But would a similar linkage attached to the collective not achieve the same end as the connection to left pedal?

Edited by Rob Hall 10/10/2012 12:40 PM
All your knowledge is kind of like the teacher on Charlie Brown. Smile
Fact is, accuracy is not as important as having SOMETHING. The TH-55's I flew in Primary, had a Manifold Pressure Gauge for accuracy, but the color coded pedal indicator
got you a good estimate, and the 1:1 relationship between pedal position to manifold pressure didn't vary much from hover to cruise to VNE.
Sorry friend, i would love to see it. Maybe the inaccuracy's are so tiny that it would not matter. I would love to see it work and for $10, you cant afford not to try it!
I agree with HAWKIP in that the ratio will change as airspeed tends to unload the tail rotor but the primary flight profile where this indicator would be used is in hovering flight and for this purpose the application is perfect. It is simple and fail proof and probably represents a near perfect 1:1 indication at speeds below ETL. It's basically 100% mechanical below ETL. Those of us who have flown the Mosquito know that even in the XE/XEL two cylinder versions you will reach VNE before you run out of power that results in rotor rpm droop (yes that is even with the floats installed). The tail rotor is so effective that when hovering in windy conditions one can easily droop the main rotor with the tail rotor input so Bryan is barking up the right tree by coming up with this super simple indicator. If we wanted a more exact indicator the one HAWKIP mentions could work but I think a sensor could be applied to one of the motor mounts rather than measuring the flex of a belt to get a more exact measurement. I prefer the simplicity of Bryan's idea and if you really need an exact indicator to successfully fly then you need to lose some weight, fly smoother or both. Great discussion!
Yes I do understand the error in forward speed. As you guys with experience in "EQUIPPED" helicopters know, ANY type POWER INDICATOR is used for several purposes, some of which are:
* Performance planing, to see if you have power for the mission required maneuvers
* Knowing when to stop pulling in collective BEFORE drooping RPM / LTE gets you
* Making PREDICTABLE configuration changes in cruise (I.E. descend at 500FPM or accelerate from 60mph to 70mph)
* Determine your fuel consumption for various settings

It doesn't matter if the slot cut in the panel is numbered from 1 to 10, 0 to 100, or accurate pound-feet of torque being crammed into the input quill. What matters is YOU HAVE SOMETHING IN FRONT OF YOUR FACE GIVING YOU A NUMBER THAT BEARS SOME CORRELATION WITH THE PERFORMANCE YOU MIGHT EXPECT.

When I was flying the Mini-500 regularly, 4 or 5 times per week, I did not have such an instrument. I just reached up in thin air and grabbed an arbitrary number from 1 to 10 that subjectively rated how much "juice" the little helicopter seemed to have during the flight I had just finished.

1 meant I had to do a running takeoff. 10 meant I successfully did a max performance takeoff out of the yard, going over the telephone-pole with power to spare. Every time I flew, I wrote this number and the density altitude in the log book. After a couple of dozen flights in all seasons and temps, I had a pretty reliable TABLE of EXPECTED PERFORMANCE, that I could use BEFORE takeoff, to predict how it was GOING TO PERFORM.

This little NyRod instrument makes my "1 to 10 out of thin air"... be a little more accurate...a little more OBJECTIVE...A little More safe!
Let me think some more...

If you had a PERFECT vertical fin, such that at 70mph, it created exactly the right amount of yaw force that the T/R thrust was ZERO. Then let's
say in that condition you could reach out and remove the un-needed T/R blades...

My little instrument could be 100% inaccurate in that condition if you move the pedals to the extreme. However, what actually happens is, as you gain speed and the fin does more of the work, you unknowingly remove an ever-increasing amount of left pedal pressure to keep in trim. At VNE the fin is doing all the work but you have not got the right pedal on the floor. You stopped removing left pedal (or applying right) much before it reached full right.

I agree, as airspeed increases, error grows, but it is still useful in many other phases of flight.
I agree Bryan! The level of accuracy that will be presented by your indicator will probably be more accurate than the airspeed indicator in the Mosquito. This is the best idea and I love it! Way to go man!
Thanks Mark. I'm gonna FISH for some more ego-building complements. Smile

Did you look at the equally-simple flashing low fuel warning system? What do you think? I stole it from my Brantly.

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