The Stall

In the last post,  I mentioned briefly how a stall works but this time, I will explain it in a little more detail and also talk about some other concepts involving the stall.


AIRFOILS AND LIFT COEFFICIENT

Picture an RC airplane from above, the fuselage in the middle and the wings on the right and left side. Now, look at the plane from the side, ninety degrees to the right or left from the angle you just looked at it from. What you see now is called an airfoil and they come in many shapes and sizes.

The one on the top, flat bottomed airfoils, have a lift coefficient of about 1.0 at a normal angle of attack.  The next one down, semi-symetrical airfoils, have a slightly lower lift coefficient of 0.8 to 0.9 at that same angle of attack. The bottom one, symmetrical airfoils, have a lift coefficient of 0.5 to 0.6. What's important is that higher lift coefficients decrease the stall speed of an RC plane and lower lift coefficients do the opposite to the stall speed. For example, an RC plane with a wingspan of 1 meter, weight of 986 grams and a wing area of 17.5 square decimetres with a lift coefficient of 1 will have a stall speed of 34 km/h. With a 0.8 lift coefficient, its stall speed will be 38 km/h. So, flat bottomed airfoils are generally better for lift and although symmetrical airfoils have a lower lift coefficient, they are much more suited to aerobatic planes that will need to fly inverted because there is an equal airfoil shape on both sides.  Faster RC planes often have a straight wing with no airfoil. There are lots of tables on the NACA airfoil website that will show exactly what the lift and drag coefficients are of different airfoils at different angles of attack. When the trend of lift coefficient starts decreasing rapidly, that is the point that the airfoil has stalled. Angle of attack or A.O.A is the angle of a plane's airfoil to the ground.



WINGS STALL ON THE INSIDE FIRST

Wings for full-sized airplanes are always designed so that the inside of the wing (section closest to the fuselage) stalls before the tip of the wing (section farthest from the fuselage). If not, there would be no way of controlling the roll of the plane. RC planes are sometimes designed the same way.

WASHOUT

One of the ways of making sure that the tips stall after the inner part of the wing is called washout.

One form of washout is when the tips of the wings have a lower angle of incidence or angle of attack A.O.A than the inner part of the wings closest to the fuselage.

Aspect Ratio, Wing Loading and Different Wing Shapes

As I mentioned in the previous post, the wing is the most critical part of the plane because it is what makes them fly. Different wing shapes correspond to different flight characteristics. Below are a few concepts involving wings that will definitely help with the design of an RC plane. This may be a good time to decide what flight characteristics you want your radio control model to have, or how you want it to fly e.g maneuverable, aerobatic etc. because a plane (radio control or full size) simply can't be designed for everything.



ASPECT RATIO


The aspect ratio is the length of a wing divided by it's width (chord). For example a wing that as a length of 98cm and a width of 15cm ... 98/15= . . . . . will have an aspect ratio of about 6.3. Why is this important? Here are the differences between high aspect ratio and low aspect ratio wings.



MODERATE -  HIGH ASPECT RATIO 8.0 - 15.0  (Gliders or small-midsize commercial jets)

Characteristics

• Less induced drag due to smaller wingtips and/or winglets. I will explain drag in more detail in a later post but basically drag is one of the four forces involved with flight and it is usually air resistance hitting the plane and slowing it down while in motion. In this case, it is different. Check out this link that explains induced drag. Sorry, it is a weird video. https://www.youtube.com/watch?v=xNEx7rEv4gs. Basically, he is saying that air travelling from the high pressure zone underneath the wing to the low pressure zone on the upper surface at the very tips of the wing creates vortices which is induced drag. Winglets are basically aerodynamic fences that reduce induced drag by lowering the amount of air that travels to the upper surface. 

•    Longer glide
•    Looks more majestic
•    High aspect ratio wings make RC aircraft less maneuverable 

LOW ASPECT RATIO (fighter jets, stunt planes) 

Characteristics

• Mostly the opposite of high aspect ratio wings. Higher induced drag, lower efficiency (for an RC plane, won't glide as well), more maneuverability

The main thing to notice is that the characteristics that these airplanes needed matched the types of wings that they had. For example, 787 airliner had the high aspect ratio wing for greater efficiency and the stunt plane had the lower aspect ratio wing to be more maneuverable. The same can go for your RC plane. If you want a glider, go with a high aspect ratio wing. 

WING LOADING

Wing loading is the weight (grams) divided by the wing area (square decimetres) e.g
986 grams / 12 square decimetres = 82 grams per square decimetre of wing loading.

Higher wing loadings correspond to higher stall speeds. The stall speed is the speed that the plane no longer has enough lift to fly. The wing loading will increase if the weight is increased or the wing area is decreased and the RC plane will have to land and fly at a higher speed to avoid stalling. Reverse of this if the wing loading is decreased assuming that no changes are made to the airfoil (shape of the wing from a side view.)

If the wingspan is the usual 1 to 1.5 metres, and a weight of 1-3kg, the wing loading of an RC ducted fan jet should be around 100-130 grams per square decimetre. This will mean that stall speeds will be somewhere around 60 - 65 km/h.

A glider of the same size would have a much lower wing loading closer to 30 grams per square decimetre and may stall at 15 - 20 km/h.

A good strategy is to look at websites such as E-flite, Parkzone etc. and find a plane much like the one you are designing. Looking at some of these will be a good idea so that you know what the wing loading for your model should be.



DIFFERENT WING SHAPES

Rectangular wings: These are wings with a low-moderate aspect ratio usually. Good for carrying lots of weight because it distributes the weight of the plane equally along the wing, and a balance between efficiency (high aspect ratio) and maneuverability (low aspect ratio) . The best wing for an all-round type RC plane but has no real specialized purpose.

Swept wings: These wings are effective at high speeds, because they reduce drag allowing the plane to fly faster, however, they have less lift so they need more speed in order for an RC plane to takeoff. This is why they are the ideal wing for RC jet aircraft, due to the higher speed that they can fly at.

Delta wings: These wings take the swept wing concept even further. Delta wings have even less drag than swept wings but require even more speed to fly. That is why they were used on supersonic airliners (full size) such as the Concorde and Tu-144 but have no real use today for airliners but they do exist on some fighter jets. 

This should help you make a good start on designing the wing for your RC plane. Remember to determine the characteristics you want and then find a wing loading, aspect ratio and wing shape that will give you those characteristics.  

How Does A Wing Work?

Really, the wing is the airplane, so it is a good place to start when talking about designing an RC plane. This is also probably the most critical part of the design. First though, it is important to know how a wing actually works to generate lift.

There are several ideas out there for how a wing actually works but one of the most popular is the Bernoulli principle.

The Bernoulli principle states that air moving over the top of the wing has a further distance to travel than the air underneath. So, it must travel faster to meet the air that went underneath the wing. The faster-moving air that travels above the wing has less pressure than the slower air beneath. This creates the lifting force. 

However, the misconception with this theory was that both paths of air have to meet up together at the other side of the wing. Some experiments were done recently that suggest that the air that travels over the top of the wing actually reaches the trailing edge (back) of the wing BEFORE the air underneath. 

Another theory for lift is that when the wing is at an angle of attack (angle relative to the ground) that a plane would normally take off at,

the air hitting the underside of the wing is being directed downwards. As this happens, this air  pushes the bottom of the wing upwards in a way that creates lift at a good angle of attack. 

The bottom line is that both of these theories work for how a wing generates lift, if they are explained correctly. They are just two ways of looking at it. Personally, I agree more with the first one. I think that the second one would work more for paper planes.