Make your own R/C Aeromodel
Flying Aeromodel is so fascinating....right??? :) Ofcourse it is...But then flying Aeromodel made by yourself is even more fascinating... or even just watching your creation flying fills you with the joy of lifetime. To be frank, I haven't flown my own creation till now. However, I have made some Aeromodels of different kinds. I am waiting for the day.. when I'll be able to fly ... what I designed and fabricated myself. Alright... lets begin with Aeromodel design techniques....
In this article, I am mainly going to focus on the design and not explaining the basics of Aerodynamics or not gonna introduce the basics of flight. I am presuming that you are aware of all the basics of Aeroplane and its stable and controlled flight.
First of all when you design an Aeromodel; you have some constraint around which you design your Aeromodel. Typically, constraint could be one or more of (i) Weight (ii) Size (iii)Powerplant (iv) type of aeromodel.
When Aeromodel is flying level, it's wing should generate the lift equal to total weight of the Aeromodel.
So, W = L = 0.5*rho*V^2*Sw*CLw
where, rho = sea level air density
V = flying speed of Aeromodel
Sw = Area of the wing
CL = Lift coefficient of wing
W/Sw = 0.5*rho*V^2*CLw (W/S is called wing loading)
There are some typical values for wing loading (in Newton/m^2 unit) according to the type of Aeromodel,
15-30 : Park flyers, basic trainers and powered sport sailplanes
30-45 : Fastest sport flyer, smaller size trainers
45-60 : Larger size trainers, sport scale models and sport aerobatic models
60-75 : Fast sport models usually with more than adequate power
75-105 : Scale models and larger multi-engine models
105 plus : thts a lot!!!!
Now based on the type of Aeromodel you want to design, choose a value of Wing loading. Suppose it is 25 for small trainer plane. Along with this you also need to assume a value of Aspect ratio (AR) of your wing. Typically, AR for powered R/C model is between 4-7. Lesser AR means faster and sporty plane while higher AR means slow and glider type of plane. Lets suppose AR is 5 for our Aeromodel.
So at take-off from wing loading formula,
0.5*rho*Vto^2*CLwto = 25
(where, Vto: speed at take-off
CLwto: wing lift coeffiecient at take-off)
Here, value of "rho" is known; the two unknowns are Vto and CLwto.
Now here comes the first iteration process you need to go through...... Assume some take-off speed Vto, starting from 4 m/s and calculate CLwto.
As in our case, for Vto = 4m/s
CLwto = 2.55
Which is way too high for typical Aeromodel wing. So you need to increase Vto some more and calculate CLwto once again and see if it there exists any airfoil with which wing can give this much lift coefficient.
There is one very good software DesignFoil for getting virtual wind tunnel data of various airfoils. Its demo version is available online for free. You can use it for generating airfoils and running virtual wind tunnel to get the lift and pitching moment coefficients. The data you get from this software may not be accurate enough for real Aircraft but surely good enough for designing an Aeromodel. One more thing to note here is that you need to convert airfoil CL to wing CLw by the formula given as follows:
CLw = CLalpha_wing * (alpha – alpha0)
(where, alpha: Angle of attack, alpha0: Angle of attack at zero lift)
CLalpha_wing = CLalpha_airfoil / (1 + CLalpha_airfoil / (2.51*Aspect Ratio))
(where, CLalpha_wing: slope of CL vs angle of attack plot for wing
CLalpha_airfoil: slope of CL vs angle of attack plot for airfoil)
So at the end of iterations, you will get your airfoil fixed and you would know at what speed would your plane take-off from the ground.
Now, when we calculate wing dimensions, the constraint on the design comes into picture. Suppose, you have constraint on weight… so W is known, from wing loading value, you can calculate wing area. If you constraint on power-plant or you know what’s the max thrust your powerplant can provide. So from typical value of thrust to weight ratio from 0.2 to 0.4, we can calculate, maximum weight of Aeromodel allowed. So again from wing loading, we can calculate wing area. Now suppose we have constraint on size or on wing span. Then, from aspect ratio we can calculate mean chord of the wing as aspect ratio is equal to span divided by mean chord. So in the end, we know airfoil, wing area, wing span and its mean chord which defines wing, very much for us.
There are some rules of thumb for deciding the horizontal and vertical tail dimensions…..
Horizontal tail: Its area is typically taken as 20-35% of wing area. It should be mounted to fuselage at 3-4 degree negative to wing. It should have Aspect ratio lesser than the Aspect ratio of wing but not less than half of it.
Vertical tail: It is designed with respect to the horizontal tail. It should have area equal to 40-60% of Horizontal tail area and almost half the aspect ratio.
Most of the time, horizontal and vertical tails are made from flat plate. And sweep back is given to both the tails.
Elevator: Elevator area is typically, 25-35% of horizontal tail area.
Rudder: Rudder area is typically, 30-45% of vertical tail area.
Note that, horizontal tail includes elevator and so vertical tail includes rudder while calculating are of both the tails.
As wing is made in one single unit… so it your Aeromodel should be high wing or low wing. High wing plane is more stable suitable for trainer and beginner aircrafts while low wing plane is suitable for sports and high maneuver flying. Generally, high wing Aeromodels are designed for the ease of wing assembly.
Typically, fuselage length should be 55-70% of Wing span for High Aspect ratio Wing, and 70-90% of Wing span for Low Aspect ratio Wing. In addition to this, pitching moment of Aeromodel should be balanced at cruise and pilot should not be required to apply the elevator control surface for that. Assume wing setting angle to be 1-2 degree and balance the pitching moment about center of gravity (CG) of the aircraft. The CG of the Aeromodel should be at 10-20% of the wing chord from wing’s leading edge. And that’s only what you need to know… Don’t worry you won’t need cruise speed to know while balancing the pitching moment.
The last thing left in this article is the selection of appropriate engine for your Aeromodel. Info for the same would be added soon. Check back soon... in the mean time you can start planning the design of your own Aeromodel or you can even do start calclulations.
Note: In this article, I have intentionally not written some equations and have not shown mathematics. It is to encourage the reader to learn the basics and explore some more about Aeroplanes and its design. I’ll write some more articles on basics of Airplanes and its flight in near future. Do check back if you find yourself curious about it. In case you could not understand anything on this article or have some suggestions for improvements, do write me at email@example.com