jakevegas

Hello People,

I am a new member of the forum. I have been reading the forum for a couple months and I have finally deciced to join and start contributing.

I am designing a large RC airplane from scratch and I have been able to identify most of the parameters I need to start construction. These include sizing, configuration, stability, structural design and other things. However, I would like to know if there is a way to calculate how much the Cl on the wing changes when the control surfaces are deflected.

Any ideas would help. Thank you.

BMatthews

A control surface deflecting alters both the camber of the airfoil and the angle of incidence both at the same time. How that will alter the Cl would be easy if it were only an angle of attack change. But throwing in the camber change at the same time confuses the issue.

Then there's also what happens as the wing begins to accellerate into the rolling rate. The angle of attack of the wings when the control surfaces are first deflected is one thing and easy to figure out. But then this immediately begins to change as the wing begins to gain rolling velocity. What the final value of a constant roll rate would be has to be calculated based on the rolling rate and how far out from the fuselage each position of the wing is located at since the feet per revolution covered is different for the root than the tips.

All of which is to say "I don't know". I only know enough to know that it's a messy situation.... which is likely why few folks this side of Boeing and Airbus and other such places would bother.

jakevegas

If I understood correctly it would be really difficult to solve for the changing Cl on the based when the control surfaces deflect since there are too many variables. Probably, a "close enough" estimation can be made by putting a piece of my wing in a wind tunnel and record how the Cl changes as the control surface deflects. However, it would not be 100% since all the variables would not be present when testing.

Do you know any literature that I can read that talks more about the deflection of the control surfaces?

Thank you again.

Shoe

Not trying to be a smart a&&, but why do you care about the connection between cl and control surface deflection? In other words, how are you going to use that information in your design? There are some very experienced people here who that can probably provide useful information if you tell them what you are trying to achieve with your design. The challenge is often knowing the right question to ask.

jakevegas

It think is fair enough for me to give more information. Basically I will use this information to optimize the internal structure of the wing. As it is I have a three section wing and the control surfaces are in the outer sections. I want to know the moments or forces that these produce so I can design a proper joint between them. Maybe obtaining the change in Cl might not be right approach but instead in finding a way of how to obtain that force and moment about my joint when the control surface deflect.

I have calculated wign lift distribution, the tail loads, and the load factors. I was thinking just to use moment equation on the wing and find my stress at the joint. However, I don't know how accurate this might be. Any suggestions is greatly appreciate it.

Thank you.

BMatthews

The closest you'll find to such information is some work based on flap angle deflections for a few airfoils.

As for using this information to calculate how strong a wing joint or spar or wing torsional stiffness needs to be you're REALLY barking up the wrong tree. The loads you need to be dealing with are the span loading from the aircraft weight and expected G loading in turns and pulling out of dives and the wing's torsional stiffness for dealing with the pitching moments produced both by the wing airfoils camber induced pitching moments and changes in that pitching moment produced by the camber changes that come with control surface deflections. But generally by the time you make a wing strong enough span wise to carry the loading and stiff enough not to twist in torsion the control surface effects are again minor and "lost in the noise" of all the rest.

I take it from all this that you're new to model designing and this is another one of those school projects. If so you're right to consider all these aspects but you're assigning too much value to their effect. Your primary needs for the wing are to produce a wing that will carry the expected loading at the maximum G rating and that will be torsionaly stiff enough and place the spars in the correct locations to produce a wing that does not twist while flexing under that maximum loading. If you do that the wing and joints will soak up any other loading that any control surface can possibly generate with ease.

Shoe

Not sure what it costs, but this program should allow you to accurately estimate changes in Cl due to control surface deflection...

http://www.desktop.aero/linair.php

rmh

wow -many questions -
I must say however, that trying to solve the problem with numbers and formulae is a very chancy approach if any real degree of success is anticipated -

jakevegas

Thank you all for your time and your replies. I will take into account your suggestions and I will come back with some results, hopefully.

rmh

I hope the project goes well.
The thing I noticed about the requests, was that apparantly the concept portion of a design became - using math to determine the design parameters.
Math is useful- of course- once the desired flight parameters desired are nailed down.
These, can be easily sorted out -in most cases - by researching past results for similar designs.

I am a great believer in experience and the direct hands on approach to all this stuff.
Those who know of Paul McCready-who designed the first workable man powered aircraft-may be interested in his approach to that design:
Apparantly - he was on a trip -driving a car full of shool kids -and the noise level was a real problem
he "shut out" the noise by thinking about how he could best approach the design for the Kramer Award.
The result was he decided the best approach was to work with materials which lent themself to easy and quick repair and modification.
A thoroughly practical approach.
Others labored over design tables an kept coming up with the same thing-a craft too heavy for the power available.
On a side note - I am doing a camera plane - which has to be light simple and a safe vehicle for carrying a 5 ounce video camera -A powered glider (I am using a RADIAN as the basis) converted to using two small wing mounted motors and inboard extended area flaps seem to be the best approach at the moment.

augustine

About controls.

it has 'Force', 'Deflection' and 'Harmony'. what we are interested is in the force/unit deflection. The force that servo will feel will be force exerted on the control surface. this can be calculated by applying simple formula 1/2 x 1.225 (Air density at sea level) x square of max speed that you will envisage that your model will travel (Keep the units same) x the surface area. this will give the max force that your servo will feel from an unbalanced surface (??!! ) add 1/2 kg for friction and bends, your servo should have twice the the torgue needed. why twice ? reason 1. in its life time the servo will continue to loose its torgue. reason 2. after 75% of its rated torgue, servo can experience something akin to 'Jack Stall'.

Theoretically it has nothing to do with weight. in fact in a non-powered controlled aircraft, there is a protrusion ahead of the hinge line (Refer image) this is called the horn balance (Not the control horn we know of), purpose of this horn balance is to reduce the control force. in an RC aircraft if this horn is long enough even a micro servo would do (In an aircraft called Devon, sometimes the balancing force was more than the opposing force and once you moved the stick the control surface used to travel to it max deflection of his own accord, this is called overbalance). if you are designing an aeroplane and want to knock weight and money, carefully design long enough horn. (on the lighter side, Make it more Horney).

Mathematically.

Oops ! i was dreading this, let me try and keep it as non mathematical as i can.  for example if you have a model which has a surface area of say 1/2 m²(S)  and it travels at say 60 kmph and if the aerofoil's efficiency at max deflection is say 0.2 (Most symmetrical aerofoil has this value), now let us see how do we arrive at a number. 1/2 ρ v²  is the dynamic pressure. Ct 1/2 ρ v² S total force it can generate. This is what we need to see, and is the total force on the control surface. Let us see the number in SI (Système international ). (Thats what i meant by keep the units same).

0.2 ( 1, being a flat plate at 90 degs to the air flow [Aerofoil efficiency]) x 1/2 x 1.225 (ρ air density at Sea level) x 16.66² (60 kmph converted to mtrs/sec) x 1/2(Surface Area) = 17 kgs.

this is for an unbalanced (Without the aerodynamic horn balance) controls, if you aerodynamically balance it, this can be reduced to 3-7 kgs.




Did I make matters worse ?

charlie111

I can tell you that your servos will have less torqe on them by moving in towards the servo with your linkage.This should allow a longer through at the control surface.This should help if your servos aren't moving properly at High Speed?