elevator or rudder, for 3D of course

Based on the idea that the 60-90 size 3Ders use a servo per elevator half, and only one for the rudder (which is usually bigger) I would say the elevator.

Most of the time, you use 2 elevator servos to keep from having to use a joiner between the halves. It tends to flex, allowing different movent from each side. You really need a honking servo to hold the rudder over in knife edge.

Good point, Ed. On my UCD 3D 60 size, I went with a pull-pull on the rudder. Made a HUGE difference in hover and knife edge. It's just a standard, but the pull-pull really works wonders. I guess the question is still open. One could make the arguement that, for instance; in a hover, the elevator has to work harder because of the pendulum effect. Ie, it has to keep that heavy nose from going astray. But, the same could be said for the rudder, it's just doing the same thing side to side instead of fore and aft.

Beats the heck out of me...just go fly! [8D]

I have calculated the necessary servo torque needed for my new .90 sized CA Model Epsilon using the recommended throws for 3D-flying at 60 mph.

The result are as follows:

Aileron: 35 deg deflection requires 28 oz.in. torque (one aileron)
Elevator: 40 deg deflection requires 42 oz.in. torque
Rudder: 45 deg. deflection requires 54 oz.in. torque

/Red B.

With split elevators, the rudder always requires more torque. Remember you have to stear it on the ground as well.

Rudder

The rudder on most 3D planes has a larger chord and thus has more leverage against the servo, assuming equal throws and surface area of course. Also, many 3D planes have large counterbalances on the elevator and tiny ones on the rudder.

Nothing is absolute, it really depends on the plane.

3D flying at 60mph?!? Wow. That seems a bit fast to me. I don't intend to get my new Funtana nearly that fast. I usually fly my 66" profile fun-fly about 30mph or so I would guess.

Dude you might better check your calculations again. You are crazy if you think a standard servo is gonna stand a 60 mph Wall. No way. Not on a 90 size plane and probably not on a 60- size. And dead eye there is very very little force on a control serface in a hover. Heck, a micro servo would work on a 120 size if all you were gonna do is hover. The only pressure on the sufaces in a hover is from the prop wash. Now i know that if you get the nose down a little it may take a little more power to get it back up, but it is an insignificant amount. In a hover torque has little meaning. Now lets get to flying straight and adding a control movement. The rudder has the most control surface area and a longer chord on most planes so it will take more power per degree of throw than the elevator. Also the split elevator is mostly for setup. Now on some planes both elevators halves put together may be larger than the rudder so then they would need more power than the rudder. Basically it does depend on the plane but mostly like what was said about the longer chord, it and surface area determine how much torque is required.

I will use this logic(I'm sure it's wrong but what the heck). I feel like the ELEVs have a lighter load. They basicly change the angle of the wing. In a wall for example the wing is what takes the beating. The rudder on a knife edge is holding the plane at the AOA. The fuse is forced to make an airfoil, but the rudder is having to hold it there.
I may be proved wrong. Oh well. It's just a theory. Ask me about about a La-Z-Boy and it will be fact. Ask me about aerodynamics and it's a guess.

David

Not sure about that logic daveopam

The control surface does not "know" what axis it is on, it is applying a given force to the plane depending on speed, area, throw etc. If a particular axis requires less force to change direction the plane just changes direction faster, that's why a wall is possible.

Imagine identical V8's, one in a suburban and one in a corvette (the vehicle represents the inertia of the axis and the engine the control surface or servo). Both vehicles (axes) will be pushed by the same force (servo), but the corvette accelerates faster. The engine doesn't know which car it's in

That's how I see it and I'm stickin to it

geoharry,

The engine in the suburban is still under more of a load, and working harder.

I really don't know the answer to this, it would be a good ? to throw at someone in the aerodynamics forum.

3D ONdaEDGE wrote: You are misinterpreting the results.

First of all, I have never stated that a standard size servo should be enough for this kind of aircraft!

Second, the calculated values are the torque needed to deflect the surfaces to the given angle at the given speed. Nobody does 3D at 60 mph. but I had to choose some airspeed for my calculation and I know for sure that this aircraft will now and again be flying at that speed.

Third, servo torque figures are often given for a stalled servo. This means that if the servo is expected to operate at loads close to its maximum torque it will move VERY slowly. Also the gear train in the servo is not designed to operate at maximum torque, but for very brief periods of time.

From my experience if I choose servos that has a torque rating that is double that of the calculated values, the servos will be O.K. and perform with adequate speed. For pattern planes were I want a really quick and exact response I use servos that have torque values trice the calculated torque.

Sure, but in my example the elevator torque is for the left and right elevators together.

/Red B.

fiveoboy01


Actually, the engine would work just as hard in both cases, but only as long as it is wide open (like in a drag race). I should have been clear that I was assuming a wide open throttle.

In the servo example the load drops off very quickly for the elevator (like in a wall) but stays on the servo longer for the rudder during KE flight.

Even if the throttle were wide open the engine in the suburban will still be working harder than that in the corvette due to the weight of the car.

Therefore if a standard size control surface is used, a 4 pound plane would need less servo torque to move the suface than a 12 pound plane. Due to aircraft inertia, and the larger force required to move a larger object.

So the size and torque of the servo is proportional to the size of the aircraft and the size of the surface you wish to move. So if the plane is large, (with a small elevator for example) you only need a medium torque servo. However if the plane is large and has a large elevator, you need a high torque servo.
On the other hand if you have a small plane with a large elevator you can use a lower torque servo.

However this is where we runn into the issue of servo speed, and how long the servo takes to move the control surface.

All very interesting theorys.

I determine servos by the size of the control surface regardless of how fast I intend to fly. The Funana S 90 states a 50 oz as a minimum. This is too low IMO, but it will work. The book states 80oz as optimum, but I just figure, if the minimum required is 50oz, then go with atleast a 100oz. I have HS-5645MG digitals which produce 168oz torque. Just have to be mindful of weight of the servo if it is a 3D plane.

You can never have too much torque. A servo working at it's max will burn out quicker due to the servo having to work harder all the time, instead of working with much less effort, but the strength is there if it is needed.

In my Fat Free Taco (.25 size fun-fly) which has big control surfaces for it's size, I use a HS-85MG (49oz torque), but all that is required is about 25-30oz. I'd always choose to have more, than barely enough.

On a plane that has small surfaces such as my Super Stick 40, I use standard servos, HS-425 (47 oz torque) because the surfaces are small and will not require a lot of power to be moved even at a fast pace of speed.

Determine the servo more by the control surface size and less from the intended use of the plane.
Big Surfaces with speed = Hi servo torque
Big Surfaces with low speed = Semi Hi torque servo
Small surfaces with speed = Medium servo torque
Small surfaces with low speed = Low Servo torque

Big ailerons doing semi fast walls will put a ton of force on the aileron servo. Blenders, parachutes, flatspins, etc... will all require stronger servos on the tail section.

So we get more horsepower and torque out of the same engine by putting it in a suburban?



I was using the suburban vs corvette to illustrate the inertias of different axes on the same plane. The TLAR method seems to work fine for most people when picking servos.

But it's fun pulling theories out of our .......

What's the TLAR method?

This engine in the car theory is a very good example, You have torque and resistance. At first it seemed wierd to me but after I thought about it, it is almost the same. Now What you are saying is that the same engine is in both cars. Yes the Suburban will be working harder. If it wasnt then the two vehicles would get the same gas mileage (lets forget about gearingcause we are pretending it is the same in both vehicles) but as it is at wide open throttle the engine requires more fuel to move the same distance than it would in a lighter car. Oh, and Red. I know about the gear trains and all that crap. And you are right, it may only take that much to get the surface to 45 degrees, but it will take quite a bit more to keep it there at that speed. I didnt metion that and I should have.

TLAR = That looks about right

Works every time

Geoharry,

What I mean buy the engine working harder in the suburban is the load on that engine would be greater than that of the one in the corvette, due to the weight of the suburban and the larger force required to accelerate a greater mass!

So if you put a standard servo on a 12 pound 3d plane elevator, it would be working alot harder than the same servo in a 5 pound trainer.

So servo chose really comes down to what you want to get out of the plane (eg. 3d , IMAC, Sport), and the size of the surface(s) you wish to move.

For the engine part, forget about the Suburban and the Corvette. Too many variables. Let's use something simpler and easier to imagine.
Get a 5lbs plane with a .50 engine turning a 12x4 prop at 12500 RPM and a 10 lb with the same engine, prop and RPM. The power trasmited from the engine to the prop is the same in both cases, and so is the thrust. But which will accelerate faster?? The load on the engine is the same, but the results are very different.
Put that in a control surface. One that needs 30oz to be deflected will deflect with 30oz torque, but much slower than if you have a servo applying 60 oz on it. I guess this is obvious.

Now, about which surface needs more torque. We need to see which has the largest chord, area and deflection rate. Usually it's the rudder, but in some cases it can be the ailerons (see the 40% planes with 4 servos on the rudder but 6 on the ailerons). The rudder, when on KE flight, will require a lot of "holding torque", due to the high deflection coupled with a considerate speed to make the fuselage produce the desired lift. Why a plane, when slighty banked (i.e. 80 degrees) in KE flight can fly slower than at fully 90 degrees?? At 90 degrees the wing is not helping at all, so all the lift must be produced by the fuselage, induced by a large moment-arm created by the rudder. Decrease the lift needed, and you decrease the needed moment-arm. Less moment-arm, means less force on the rudder, which in turn means less deflection and less load on the servo.