Wing Incidence Angle


Wing incidence angle

Wing incidence angle

The wing incidence angle on fixed wing aircraft is the angle between the chord line of the wing and the longitudinal axis of the fuselage. The angle of incidence is a fixed setting built in place as the airplane is constructed. There is no normal method to vary the angle of incidence while in flight.

The wing incidence angle should not be confused with the term angle of attack. Angle of attack, in regards to aviation, is the angle between the wing’s chord line and the relative wind on the aircraft. Angle of attack is under the complete control of the pilot. Exceeding the angle of attack results in a wing’s aerodynamic stall and the loss of control until sufficient airspeed is gained to recover the aircraft.

Setting the Incidence

Setting the angle of incidence is important when designing an aircraft. For a variety of reasons model aircraft usually fly best with 1-2 degrees of positive incidence. This incidence should be drawn into the aircraft plans so the builder installs the wing at the proper setting. Programs such as TurboCAD make adjusting a wing rib to the correct angle an easy task through the angle rotation feature.

There has been a tendency in recent years to create model aircraft designs with zero wing incidence. This approach works with certain types of aircraft such as lightweight 3D acrobatic models.

Bleriot nose detail

Bleriot’s high positive wing incidence

These smaller models tend to fly much of their profiles directly off of engine power, such as with hovering flight. These unique aircraft spend little time in what could be termed “normal” flight, where the aircraft’s lift is primarily supplied by the airflow over the wing and not via engine thrust.

For most sport RC models a slight amount of positive wing incidence will lead to a higher degree of neutral pitch stability. In other words the model, once trimmed, will tend to stay in the pitch attitude that it is placed in until changed by the pilot.

There are some cases where an RC model airplane design needs more than the normal 1-2 degrees of positive wing incidence angle. This situation happened to me while drawing up the Chickadee, Blackburn and Fokker Spin plans.

First, some background. Full scale powered aircraft during the first decade of flight had similar design challenges. The engines of the day were heavy and did not produce much thrust. The aircraft were typically designed with large wing areas to provide sufficient lift to accommodate this lack of power. These larger wings also added to the overall drag of the aircraft.

In addition, the wing airfoils often had a large curvature. This wing shape generated a lot of lift at low airspeeds, which was the design objective, but they also created a lot of drag.

The early aircraft designers set the wing with a high amount of positive incidence. This aided in the generation of lift at lower airspeeds.

There is a limitation with designing an airplane with a high angle of incidence. If the aircraft has a more powerful motor the increase in thrust combined with the wing’s large incidence setting can cause a rapid climb or ballooning as airspeed is increased during flight. The airplane is thus restricted to a narrow and lower band of airspeed.

As plane designs evolved and more efficient engines became prevalent the wing incidence setting was decreased to the more normal 1-2 degree range.

Chickadee experience

This is an appropriate time for a discussion of what happened to me during the Chickadee design.

Chickadee RC plane nose section

Chickadee’s front wing posts were increased in length to add positive wing incidence

As I laid out the Chickadee’s dimensions I used the Sig Demoiselle parameters as a template. You’ll note similar wing shapes, tail surface areas and incidence arrangements between the two designs.

On the initial test flights of the Chickadee, although there was plenty of power from the electric motor, the aircraft would simply not lift off of the ground during takeoff roll.

This surprised me. I went back to see if I had missed anything from my survey of the Demoiselle. I noticed the 7 degrees or so of positive wing incidence on the Demoiselle, but a lesser amount on the Chickadee. I added the appropriate amount of structure to the Chickadee’s front wing posts to incorporate 7 degrees of positive wing incidence. On the next test hop the model took off normally and flew great.

When designing the Blackburn I copied the Chickadee’s wing incidence setting exactly. Using the magic of TurboCAD, I simply copied the Chickadee’s wing rib shape and angle drawing directly overlaid onto the Blackburn fuselage. The size of the Blackburn’s front and aft wing mounting blocks was accurately drawn on the plan. On the Blackburn’s first test flight it flew exactly like the Chickadee.

Fokker Spin settings

I followed the same design approach to setting incidence when preparing the Fokker Spin plans. While the Spin is a smaller aircraft than the Blackburn, I copied the Blackburn’s settings and overlaid the smaller wing rib and angle onto the Spin’s fuselage plan view.

1911 Fokker Spin radio control model airplane in flight

1911 Fokker Spin in flight

During test flights the Spin did not respond well to liftoff during the takeoff run. Just like with the initial flights of the Chickadee, the Spin accelerated smoothly and had plenty of airspeed, but did not want to lift off.

Through trial and error I progressively lowered the Spin wing’s trailing edge. This had the effect of increasing the wing’s angle of incidence. The final result was around 13 degrees positive incidence. The Fokker Spin flies fine at this wing setting, almost twice that of the Blackburn.

The only thing I can think of regarding the need for a greater angle of incidence for the Spin as compared to the Blackburn or Chickadee is the Spin’s aft wing sweep. There are so few swept wing propeller driven aircraft flying today that there is not a lot of research in this area. However, the results of my test flights led to the higher incidence setting for my RC model of the Spin.

I must note that Werner purchased a set of Fokker Spin plans and pointed out the discrepancy in the incidence settings between the Blackburn (which Werner has completed) and the Spin. See Werner’s hand written notes on the plans. Werner is building his Spin now, and is planning on a 200% enlargement for his variant.

Blackburn plan wing incidence detail

Werner’s note showing 7 degrees of positive wing incidence on the Blackburn

As both the Blackburn and the Spin fly well it did not occur to me to have a detailed examination of the differing incidence arrangements. Werner’s note got me thinking, and thus this recap on how I went about drawing the wing set up.

In conclusion I recommend that you design in 1-2 degrees of positive wing incidence to a regular sport RC model aircraft design. I followed this path with the Electro Aviator. If creating a scale model of one of the antique flyers, typically aircraft designed before 1914, add around 5 degrees of positive incidence for the initial test flights.

Adjust the wing incidence as required during the model’s test flights. If the airplane is reluctant to takeoff add more incidence. If the model balloons off the ground during takeoff reduce the setting. Once you have a well flying model, retain these parameters, areas and moments for future designs.

Fokker Spin plan detail

Werner’s note showing around 13 degrees of wing incidence

Author: Gordon McKay