V-Tail Torque requirment

[Lynford]

I am building a light (23 oz) 2 meter sailplane with V-Tail. About how much servo torque would be required assuming free linkages, moderate ruddervator size, etc. I am thinking about two Hitec HS-60s which are about 16 oz @ 4.8 volts. Ideas?

TIA

Lynford

[Ollie]

The load on the servo depends on the square of the airspeed, the area of the control surface, the width of the chord of the control surrface and the deflection of the control surface.

With the light wing loading, the 2-meter sailplane will not have a fast glide but, what if it is in a prolonged dive?

See:
http://www.multiplexrc.com/calcservo.htm
By playing with this on-line servo torque calculator you can find out how fast it is safe to go with 16 ounce inches of torque. Then you can deside if that speed could be part of your flying style.

A little over 30 years ago 16 ounce inches of torque was considered "normal" and some standard size servos had about that much torque. Much bigger floaters with more than 2-meter spans were flown with V-tails and about that much servo torque. In other words, 16 ounce inches of torque will be fine if you don't do wild and crazy things like hard zoom launches off a winch or prolonged vertical dives.

[davidleitch]

Also in my very limited experience, linkages are not 100% friction or weight free.

Many new V tail designs have the servos in the tail. Although this is prone to higher drag my belief is that smaller servos can be used than if they are mounted under the wing, and there is likely less slop as well.

[Lynford]

Ollie: Thanks for the link but unfortunately Multiplex Web Site cannot find the page (disconnected or something). I might end up having to add some nose weight. If so a couple HS-81s are on the sidelines. It will be a sqeeze to get them in but a little persuasion with a big hammer :-) will work.

[bjaffee]

FWIW, my Opus 2m sloper uses 2 JR 341's for the v-tail. I believe they have around the same torque as an HS81. This plane is regularly DS'ed at over 100mph, so I'd say for a light weight thermal plane, 81's would be more then enough.

[Steve Diebolt]

Lynford,

Here is another link for calculating servo load.

http://www.csd.net/~cgadd/eflight/calcs_servo.htm

Hope this helps,

Steve

[Ollie]

Sorry about the non-working calculator. The address gets you to the calculator page but when I put in a test calculation, the calculator didn't work. The Multiplex dealership in the USA has changed hands resently. Maybe that has something to do with it.

[Kestrel]

I can't get either of the calculators to work. The one that Steve posted will allow me to put in the values and the button "moves" when clicked but no answers appear.

does it work for anyone else?

[Ollie]

Kestrel,

Steve's servo torque calculator works for me. I put in 100 MPH, 1.5 inch chord, 12 inch length and 20 degrees deflection. It gave back a servo torque load of 4.682 ounce inches. I think it is safe to say that 16 ounce inches are more than enough for any 2-meter plane weighing only 23 ounces.

[davidleitch]

Its interesting that plane mass is not relevant to the torque requirement according to Steve's calculator

[Steve Diebolt]

Gentleman,

This isn't really "my" calculator. Looks like the credit goes to Chuck Gadd from Colorado. If you follow the links back you will find more calculators and his Home Page.

I wish I was that good with math...:(

Regards,

Steve

[Ollie]

David,

In a way, the plane's weight puts a minimum on the speed because the plane's stall speed depends on the weight, other things being equal.

[Lynford]

Thanks Steve for the alternate calculator. For kicks after looking at the formula, I found that it would take a near infinite torque to drop 90 degree flaps on an open ship. Oh well, I don't need them on the ship I am building. :-)

Lynford
Reno Flightline