How strong of servos should I be looking for? It is of 40 inch span not a fast A/C either. Give me a number in Oz. inches.

The area to each flap is 39 S.I
The area to each aileron is 9 S.I.

Wm.

standard servos are fine for this, about 3 kg

The force on the flaps when down 90 deg will be about 3 lbs at 60 mph, only about 1/2 lb at 30 mph. If the flap chord is 3" the torque produced by the flap is about 60 oz-in at 60 mph and 15 at 30 mph. Torque is proportional to the chord assuming the area is fixed.
The ailerons won't delect nearly as much as the flaps and are smaller in area and chord. They probably require less than 5 oz-in.
Are the flaps really 40 sqin on each wing of a 40" span wing? That is a lot of flap for a wing half that is only 20".

Jack,

could you give me the formula that you are using for calculating air loads? I need it for a trim tab project I am working on for my expierimental aircraft.

Schmleff

For a quick check look at where the TOC champs put the most servos. They gang the bloody things together auch as four for the rudder two for each aileron (at least) Elevators get a bunch two. The point is the number of servos these guys use is as good indicator of where the most push and pull are required. F'rinstance a throttle servo needs very little power, elevators and rudder need more and ailerons can get by with a bit less. Most standard servos are 48 oz/in. this will fly most ARF and trainer types.

thats a very generic statement, theres a lot of bigger artf's on the market now.

for this guys application, standard servos are fine

I'm curious too if there is a calc/formula for determining the torque needed. I'm building a jet with all moving horizontal stab, a rather large surface area. Don't have a clue what's needed.

Yep, checked the area figures once again. The reason the flap area is large is that the model was intended to be a small S.T.O.L. type. I doubt it will go faster than 20MPH when completed.

Your numbers are aiding quite a bit as to choice and then position of servos.

Wm.

Staying away from the giants and jets the 48 inch oz servos are fine for most anything.

I have used them in a Robin Hood (99inch span, Zenoah 38 powered), a Senior Telemaster and a Ryan STAM ARF powered by an OS 4 stroke. All have worked fine with no gear stripping etc.

Years ago Bill Northrup flew something like a Gypsy Moth that must have been around 8 ft. span or so with a system that used pulse servos. They were a small geared coreless motor with a spring return. The output was less than our modern micros.

Years ago Don Brown flew in the old internats (and placed high) with a pulse system of his own design that used modified double geared mighty midget motors (if memory serves correctly). I have used them myself and again the modern micro servo has more output power.

I just saw a demo on the video Modelsport magazine where a brick was hooked to the output arm of a JR micro servo and it lifted the weight. Awfully impressive.

All of that is to indicate that the average servo is useable in most any airplane up to and including a 1.20 powered machine. Of course better precision and a safety factor is achieved when a larger servo is used.

I estimated the force on the flap as the dynamic pressure times projected area.
press,d = 1/2 x dens x velocity^2 in psf
dens = .07/32.2 slug/ft^3
vel in fps
For 90deg flaps projected area is simply the flap area. Use flap area in ft^2 . get force in lbs.
Torque is force times half the flap chord times sine of the flap angle. Sine 90deg = 1. If you use the chord in inches and convert the force to ounces you wind up with torque in inch-ounces.
At 30 mph, v = 44 fps.
Press,d = 1/2 x .07/32.2 x 44^2 = 2.1 psf
Force = 2.1 x 40/144 = .58 lbs 16 x .58 = 9.4 oz
I guessed the flap chord is 3".
Torque = 3/2 x 9.4 = 14 oz-in
If the flaps are deflected 30 deg I multiply the 90deg torque by the sine of 30 deg. That is rough but provides a method for sizing servos.

Jack:

The flap chord is actually 2 1/3", but your figures leave me with a big set of types of available servos now on the market.


That will do just fine. Will beep at you at I pass through Red Bluff next week en route to Oregon.



Wm.