Optimum Tail Boom Length

[SDA1]

I have a Blade Nano CPX and an MSRX. I am getting ready to redo
both tail booms using 7mm tail motors. I have 2mm carbon fiber rod
stock so I can make the tail booms whatever length I want. What I would
like to know is- What is the optimum relationship between the tail rotor
and the main rotor? Should the tail rotor be under the main rotor? Outside the downwash of the main rotor? What is best aerodynamically? Any
thoughts will be appreciated.

[extrapilot]

There is no optimum, unless you define a specific operating condition (i.e. hover, or heavy lift at some defined cruising speed, etc).

There are design guidelines full scale, where the assumption is that you are hovering or in FFF. In both cases, a longer boom can help (TR authority, TR power requirements, and weathervaning). But you have limitations on length for ground handling, for CG, etc. So, lots of compromises. You also need to prevent the main and tail rotors from intersecting- so that sets a minimum offset.

For us, I think many designs are just based on what people are familiar with. There are still some compromises, but some of the advantages full scale hurt us in acro (a long boom is not good for flips, can create a lot of drag in travelling piros, etc), where we have lots of TR authority, and don’t care much about efficiency.

So you still need to worry about blade intersections, but CG (and the distribution of mass- where a big mass at the end of a long boom hurts flip performance) matter also.

[SDA1]

Thank you for your response. Let me respond to your first statement
and define my goals for these birds. I am not a 3d pilot. I am much more interested in a machine that is slop and vibration free. In stock form both
of these helicopters are made primarily of plastic. This allows excessive
(in my opinion) slop in the drive train and control linkages. I have gone
to great lengths (and way too much money) to convert them into precision
flyers. Both are completely rebuilt using primarily Microheli running gear.
Their landing gear is from Airtime. The Nano uses the carbon reinforced
rotor blades from Xtreme. It also has the MMW motor running an 8 tooth
pinion-plenty of power. So far I'm very pleased with the way they fly. The
Nano especially is very smooth and precise in control response. My concern at the moment is tail control. The MSRX tends to overshoot
yaw commands and then bounce back. If I could adjust gyro gain I would turn it down-but that isn't possible- hence the tail rebuild. There must be
some interaction between the downwash of the main rotor and the thrust
of the tail rotor.My question is- What is the best position- aerodynamically- for the tail rotor in relation to the main rotor to minimize
this interaction? I could build several different length tails and use trial
and error but I would prefer to get it right the first time. Help me out guys- lets see some discussion.

[extrapilot]

Originally Posted by SDA1

Thank you for your response. Let me respond to your first statement
and define my goals for these birds. I am not a 3d pilot. I am much more interested in a machine that is slop and vibration free. In stock form both
of these helicopters are made primarily of plastic. This allows excessive
(in my opinion) slop in the drive train and control linkages. I have gone
to great lengths (and way too much money) to convert them into precision
flyers. Both are completely rebuilt using primarily Microheli running gear.
Their landing gear is from Airtime. The Nano uses the carbon reinforced
rotor blades from Xtreme. It also has the MMW motor running an 8 tooth
pinion-plenty of power. So far I'm very pleased with the way they fly. The
Nano especially is very smooth and precise in control response. My concern at the moment is tail control. The MSRX tends to overshoot
yaw commands and then bounce back. If I could adjust gyro gain I would turn it down-but that isn't possible- hence the tail rebuild. There must be
some interaction between the downwash of the main rotor and the thrust
of the tail rotor.My question is- What is the best position- aerodynamically- for the tail rotor in relation to the main rotor to minimize
this interaction? I could build several different length tails and use trial
and error but I would prefer to get it right the first time. Help me out guys- lets see some discussion.

There isnt an optimum, as was stated. If you are in still-wind hover, you can find the value that gets your TR least power per unit thrust, or max thrust per fixed RPM/pitch. But once you add any lateral airflow- the rotor wash moves, and it is no longer optimal- it may be worse than stock in important ways.

In hover, the average air flow velocity through the MSRX is only about 5mph. So that means, just using basic geometry, a small sidewind, or slow forward flight, dramatically changes the angle of the flow from the rotor. Even just a 2mph sidewind can take that outflow from barely clipping the TR to fully engulfing it… Once you start with backwards flight or travelling piros, you have a whole other series of variables on the outflow.

There is about a 5% increase in efficiency by flying the TR blade into the rotor wash- where half the TR is in the wash. So, best case you gain 5% on TR power/efficiency. In most cases, you gain nothing.

I think you would be better off posting in the appropriate manufacturer subforum- and talking with others who have tweaked their machines. Its not that you cannot enhance their performance, but the idea that there is an optimal setup is wrong- outside of a very limited flight context, and generally it is a tradeoff (i.e. hover efficiency for FF efficiency).

[MariaFan]

SDA1,

Go here:

https://www.helifreak.com/showthread.php?t=711850

The "Maria Stretch" thread.

[SDA1]

Thank you- I'll take your advice. Moderator- Please move this entire thread
to the Blade helicopter forum.