Designing and Flying Vintage Model Aircraft

Bleriot during a fly-by

Bleriot during a fly-by

I enjoy designing, building and flying vintage model aircraft from the early days of flight, especially those airplanes designed during the first decade of flight. Many of these early flyers exude character and offer a slow and enjoyable flight experience

One item to keep in mind as you learn to fly some of these designs is that they can share adverse flight characteristics that are not common with today’s airplanes.

It is easy to lose sight of the fact that during the first decade of powered manned flight (from 1903 to 1913) everything was new to that period’s airplane designer and pilot. Construction materials and methods, aircraft engines, flight control schemes and even a basic understanding of how to fly were all concepts in their infancy.

Curtiss Pusher aircraft had a lot of parasitic drag from struts and the landing gear

Curtiss Pusher aircraft had a lot of parasitic drag from struts and the landing gear

We know of these older aircraft flight characteristic shortfalls due to the number of full scale replicas currently flying.  Today’s pilots require a keen understanding of how antique aircraft behave as compared to aircraft designed to contemporary standards.  Items such as control harmony, stall characteristics and engine out performance can surprise the uninitiated modern day aviator.

The Shuttleworth Collection of historic aircraft in Bedfordshire, England flies a replica of the Bleriot. The chief test pilot reports that Bleriot is “only just capable of flight with essentially no excess power for climb.”  Similar to the Bleriot rebuilt by the Experimental Aircraft Association, both aircraft are typically flown in zero wind conditions with takeoff and landing straight ahead, without any turns.

Demoiselle replica in flight

Demoiselle replica in flight

Understanding these performance issues can lend awareness as to how these aircraft should be flown as RC vintage model aircraft. These insights can lead to relatively minor changes in model aircraft design as well as adaptations of pilot technique, resulting in an enjoyable flying experience.

Some issues that related to full scale flying during this early period do not apply to models of these aircraft. Finding a lightweight engine with sufficient power to actually take off and climb was a major challenge during the dawn of flight.  The Bleriot, for example, employed a 35 horsepower engine.

Sufficient power is the key to effective aircraft performance. The technology of the time led many early aircraft to use rotary engines.  A rotary engine had the entire cylinder assembly rotating around a fixed crankshaft.  This provided a lot of power for the weight of the engine as well as cooling for the rotating cylinders.  The gyroscopic forces of the rotating engine mass were severe and made safe turns a true challenge.  Rotary engines fell out from general use shortly after the end of World War I, supplanted by radial and more advanced in-line engine designs.

Avro 504 in flight - note small size of all-moving rudder with no fixed vertical fin

Avro 504 in flight – note small size of all-moving rudder with no fixed vertical fin

While a highly skilled modeler could create a functioning miniature rotary engine, I am not aware of anyone who has tackled this extraordinary challenge. The usual approach for the RC pilot to mimic a rotary engine is to mount a dummy set of cylinders over the model’s motor.  Some models allow the dummy cylinders to freely rotate making for a convincing rotary engine effect.  The net result is we are spared the harsh impact on the model’s flight characteristics from the rotating cylinders.

Many vintage model aircraft suffered from under-powered engines. For modelers this is not a major concern as we can easily add as much thrust as we need due to the efficiency of today’s electric and gas engines.  It is a good model design approach to ensure we have more than adequate power and plan on normal cruise flight at a reduced power setting.

Avro Triplane in flight - note the number of drag inducing struts and landing gear

Avro Triplane in flight – note the number of drag inducing struts and landing gear

A similar discussion applies to aircraft bank control. Early aviators used wing warping for lateral control.  Wing warping provided marginal bank control and depended on a specialized wing construction that allowed the trailing edges to be bent (warped) during flight.  Ailerons rapidly replace wing warping and became the accepted method to allow for positive aircraft bank control.  Ailerons are used on virtually all aircraft to this day.

Building a vintage aircraft model that used wing warping on the original, such as a Wright brothers biplane or perhaps the German Taube, the model designer is faced with the choice to engineer a wing warping system, make a modification to add non-scale ailerons or employ a three channel scheme and use only the rudder for turns.

Bank control is probably the one aspect of early aircraft flight control that cannot be easily accounted for in a model. Due to the design approach of these designs there is a great deal of inherent parasitic drag.  Struts, flying wires, non-retractable landing gear and generous wing area lead to flight at slower airspeeds.  With these conditions the possibility of adverse yaw is a prime consideration for any flight.

The Blackburn Monoplane has ideal nose and tail moments, as well as wing and control surface areas for a successful RC model

The Blackburn Monoplane has ideal nose and tail moments, as well as wing and control surface areas for a successful RC model

What is adverse yaw and how can we deal with it? When an airplane enters a bank preparing for a turn, aileron and rudder is applied.  Let’s use a left turn as an example.  The pilot moves the control stick to the left.  The right aileron goes down, raising the right wing.  At the same time the left aileron goes up causing the left wing to dip down.  Rudder is applied to keep the turn in coordinated flight, i.e. not slipping into the turn.

Adverse yaw is always a concern flying a high drag aircraft with minimal extra power. Following our example above with a left turn, the downward moving aileron on the right wing causes an increase in induced drag, as the aileron moving down caused the necessary increase in lift to raise the wing to commence the turn.

1911 Fokker Spin RC model airplane top view showing aft wing sweep

1911 Fokker Spin top view showing aft wing sweep

This increase in drag on just the right side of the airplane from the downward moving aileron can actually cause the aircraft to veer right even though the right wing is raised and the pilot intends to turn left. Adverse yaw was so severe in some early biplanes, such as the Avro 504, that a pilot could actually stagger around in a right turn with application of left bank controls if no left rudder was applied.

Modern day aircraft designs have largely eliminated adverse yaw with methods such as differential ailerons. A modeler flying a replica of an early flyer should always be on the alert for adverse yaw.  The fix is simple: apply as much rudder as required to keep the model’s turn headed in the proper direction of the turn.

Virtually every aircraft flown during the first decade of flight employed wood and fabric construction that included a large number of drag inducing items such as interplane struts, turnbuckles, flying wires and oversize landing gear.

BlackburnTopView-1000This built in drag results in slower fight speeds and adverse yaw as discussed earlier. Another impact from drag is a greatly reduced glide capability during landing approach or engine failure.  This needs to be understood by the modeler.  The fix is to anticipate how your aircraft behaves at slow speeds or engine out and fly accordingly.

A great example of an extremely high-drag aircraft is the 1908 Sig Demoiselle. This is perhaps the most “draggy” model I have ever flown.  Sig did a great job outlining these flight characteristics in the building manual.  The drag of the Demoiselle is so pronounced that you need half power to complete a normal landing flare.  The model will simply stop flying and drop to the ground if power is reduced to idle while at approach airspeed.

Sig offered a useful technique to learn how to deal with the drag challenge. For your first flights plan on taking off straight ahead and climbing a foot of two from the ground, and then very gradually reduce power, lower the nose and land straight ahead.  Once this maneuver is mastered you can proceed with shallow turns left and right. This was sound advice as you could see instantly the Demoiselle’s abrupt drop in altitude with the most modest reduction of power.

The model plane designer can use several techniques to counter any adverse flight characteristics of early aircraft. As we discussed the challenge of a heavy and underpowered engine is easily countered with today’s lightweight power sources.

Early aircraft often used airfoils with large amounts of camber to generate sufficient lift at low airspeeds. Models can often fly well with far less wing camber, to include such common airfoil shapes as the Clark Y.

Many full scale designs of this era used smaller control surfaces than required in an effort to reduce drag. With a radio controlled model one can make the elevator and rudder control surfaces slightly larger than scale without noticeable visual effect.  An optional approach is to increase the surface control throw.

Sig Demoiselle rear view

Top rear view of the Demoiselle – note large tail surfaces

In the case of my Blackburn Monoplane design, the overall layout of the aircraft in terms of wing area, nose and tail moment and control surface areas looked very close to present day aircraft, thus making the Blackburn a good candidate for a radio control model. Many of these early airplanes had quite short nose moments that made locating the correct center of gravity on a model a challenge.  The Blackburn literally flew off the drawing board on its first flight without any need for CG or control throw adjustment.

In summary the range of aircraft designed and flown prior to World War I make for a fascinating collection of interesting modeling projects. The wood and fabric construction is easy to replicate with a balsa wood model.  Ensure you have sufficient power and control surface throw for your model and the center of gravity is in the right location.

An understanding of some of the unique flight characteristics shared by these early designs, in particular anticipating adverse yaw and engine out profiles is helpful to the RC modeler who has not encountered these performance issues in more modern designs.

Author: Gordon McKay