Being a fast aircraft retractable undercarriage was a necessity on the de Havilland Mosquito. Since this model is going to be a replica that is as close as is physically possible to the original it is going to require retractable undercarriage.

There are manufactured retractable undercarriage systems available for model aircraft but they are highly simplified systems that do not operate or look like the undercarriage on the real aircraft. They are also phenomenally expensive and chew up a large percentage of your budget. The solution is to construct the undercarriage from scratch utilizing commonly available materials like music steel, aluminium, brass, copper and glass & carbon fiber reinforced polymers in various formats like rods, tubes, channels and I beams etcetera.

Brian Taylor's plans for the Mosquito do have the details for constructing the undercarriage however they utilize materials like piano wire which can easily carry the loads required but needs additional cosmetic materials like balsa wood and plastics to make it look like the undercarriage on the real aircraft. The design was also done some time ago and does not use many of the modern materials like carbon fiber reinforced polymers and kevlar that are now readily available.

Background

Before we go into the reengineering of the undercarriage there is a concept that mainly relates to aircraft and airframe engineering that is worth discussing. Unlike many other disciplines of engineering over-engineering components and assemblies on aircraft can often have undesirable and even dangerous results.

• Increased Mass: Mass and its associated weight are always critical factors when designing aircraft. Every gram that can be removed from the airframe translates into an additional gram of payload or improved performance. Steel has been a favorite of model makers for some time but there are readily available new materials that can reduce the weight considerably.
• Transferred Damage: Airframes are designed to take a certain maximum load and it is important that you engineer items and systems that are attached to and can apply forces to the airframe that can not overload the airframe. This can be very difficult to achieve and in some cases can not be achieved. For example if an aircraft is flying above the maximum maneuvering speed it is important that you never apply full control surface deflection as the resultant force will overstress the airframe and result in damage. If the undercarriage was over‑engineered a heavy landing that resulted in no damage to the undercarriage could have easily transferred loads to the airframe that were beyond its structural capacity damaging and weakening the airframe. Damage to the airframe is always difficult to fix and is often difficult and sometimes impossible to detect prior to a catastrophic failure. However, if the undercarriage is engineered so that it will progressively fail in a predictable manner before the airframe is over stressed it will protect the airframe from damage. Detecting and repairing damage to the undercarriage is always preferable to airframe damage.

With a full scale aircraft the acceleration it can sustain is normally limited by what the people flying in it can tolerate. Factors like age, health, sex, position, weight and height all affect the acceleration a person can tolerate and consequently cause it to vary from person to person. In general most people will find it difficult to tolerate much more than +5 g or -4 g. Interestingly women perform slightly better than men and while nobody is totally certain why it is hypothesized that the reduced distance between their hearts and brains makes it easier for the heart to keep blood flowing to and through the brain. You can however utilize things like G‑suits and reclined seats to increase a person's tolerance to acceleration but with nearly all aircraft the airframe is considerably stronger than the people inside it.

The acceleration an airframe can tolerate without distorting beyond its elastic limit varies from aircraft to aircraft and the intended use of the aircraft. Civilian passenger aircraft will normally never need to carry out the sort of radical maneuvers that military aircraft like fighters perform and the passengers certainly do not have G-suits or greatly reclined seats. In general a good place to start is an airframe that can tolerate +10 g and -9g.

In the previous thread The Search for Plans we discussed how the Square-Cube Law can have a dramatic effect on the ultimate strength of a structure. In this situation it means that the cross sectional areas of any elements have been changed by a factor of 8^-2 (squared factor) while the volume and mass have changed by a factor of 8^-3 (cubed factor). This ultimately means that the load any structural member is subjected to has changed by a factor of 8^-1. The dramatic reduction in loading would normally allow the model designer to replace materials like aluminium with balsa wood, which is capable of tolerating the reduced loads and reduces the mass and final weight of the aircraft.

The de Havilland Mosquito is somewhat different as the real aircraft was constructed from balsa and ply wood in a way that is similar to the way the model is constructed. The result of this is a model that is very close to the original but is 8 times stronger than the original.

So how does this relate to the design of our model? Since the Square‑Cubed Law will mean the model is relatively 8 times stronger we can apply this to the loading of the real aircraft to get some idea of the sort of accelerations the model will be able to tolerate. We can then apply the loading and factor from the Square‑Cube Law to the 6.2 kg final mass of the model aircraft to give us the following figures:

Re‑engineering the undercarriage

There are three basic ways to approach the re‑engineering of the undercarriage:

1. Reverse Engineering: Basically this involves analyzing the existing design and then working backwards to calculate the ultimate loading it can cope with. You can then use these calculated figures to re‑engineer the new design so that the ultimate strength and loads the new design can tolerate are the same as the original design.
2. Engineering from Scratch: This involves starting from scratch and carrying out all the load calculations from the beginning, as you would do if designing a new model.
3. Scaled Engineering: This technique involves taking the full scale components and just scaling them down to the scale of the model. Normally you can't do this as many of the materials in the original full scale aircraft have been replaced with materials like balsa wood and as a consequence the loading is not the same. The Mosquito was made out of balsa and ply wood so since the materials have for the most part remained constant the Square‑Cube Law will guarantee the new component is correctly engineered. However, it is often difficult to manufacture some of the smaller and more complex components so there will always be a certain amount of alteration and reengineering..

Ultimately I will be using a combination of Scratch and Scaled Engineering, but I will leave it here for this week. In the next installment we will look at the actual design and reengineering of the undercarriage.

As usual you can read more on the concepts discussed here by following these links.

• G Suit: Wikipedia
• Square-Cube Law: Wikipedia
• The Search for Plans: Masu on Mozzies
• g‑force: Wikipedia