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I have known of the de Havilland Mosquito since I was a boy but my interest was perked up after seeing a short documentary on the aircraft and I decided to do some further research. The method of constructing the DH98 has made preservation or restoration fairly problematic and it is believed that there are currently no airworthy Mosquitoes left in existence. I was pleasantly surprised when my research revealed that a group of volunteers in conjunction with current and ex Royal Australian Air Force RAAF personnel had undertaken restoring an ex RAAF Mosquito PR Mk XVI s/n A5-600 to flying condition.
The restoration of A52-600 is not the only Mosquito restoration project and a New Zealand team has built a mould for the fuselage and is planning to construct a new airframe while utilizing as many original parts as possible to construct a Mosquito. There is also a Canadian venture that purchased the first fuselage produced by the NZ team and is building a Mosquito from scratch.
As I wandered through the corridors, passages and dungeons of the internet I stumbled across a site that were selling plans for a 1:12 scale control line flying model of the Mosquito. The first glimmer of an idea started to form in the dark and rarely visited back reaches of the mind and since the cost of the plans was fairly inconsequential I decided to order a set. I didn't like the idea of trying to control a model of the Mosquito from control lines but since the site intimated the plans would be in DWG and DXF format I thought modifying them to suit my needs would be fairly simple. Unfortunately the person that ran the site was moving house which delayed the delivery of the plans considerably.
By now I had broached the idea with several family members and a few CR4 members and the glimmer had now grown into a raging fire. I decided to return to those dark and mysterious corridors of the internet and search for an alternate source of plans for a flying model of the de Havilland Mosquito.
The name Brian Taylor soon appeared in an inordinately high proportion of search results and I soon learnt that he had two sets of plans for the Mosquito. Brian's plans are renowned for being amongst the best in the world and he only publishes plans of models he has built and flown successfully. He has also won several awards for his models for things like being the most accurate and detailed reproductions. At this stage and as the other plans had still not arrived I decided to bite the bullet and go for the plans to his 2.057 m (81" 6' 9") wing span model which comes in at scale of around 1:8. This is considerably larger than the original 1:12 scale plans I originally ordered, but the plans are specifically of the PR Mk XVI which coincides exactly with the ex RAAF Mosquito A5-600 that is currently being restored.
My enthusiasm soon turned to despair as when the first set of plans eventually arrived at the end of August they were in PDF format rather than DWG of DXF format as had been intimated by the web site. As a result I could not easily import them into the CAD package I use and while it is possible to import images to perform any modifications pretty much means redrawing the entire set of plans. The plans came with screen captures of the drawings utilizing AutoCAD so I concluded that there was obviously a copy of the plans in a format that could readily be converted to DWG or DXF format. I sent off an enquiry to the now back in operation supplier asking if it was possible to get the plans in an appropriate CAD format and must give the supplier credit for their quick response. However, I was informed that the plans had been drawn up by a third party whom I was informed my request had been relayed to. I am still waiting for a response.
I still had my backstop though and as I had decided to go with the larger scale plans from Brian Taylor I wasn't too worried. Despair then turned into outright depression as I learnt Brian Taylor has decided to no longer sell or distribute copies of his plans or kits as he wishes to concentrate on building models and the distribution was consuming too much of his time. Oh well, back to the dark and dingy corridors of the internet and further searching.
Eventually my persistence paid off and my searches led me to a UK based supplier and what appears to be the last set of Brian Taylor plans for the Mosquito. Martin, the UK supplier, has been a real trooper, putting in the little bit extra and the plans arrived on my doorstep in Sydney, half way round the planet, in just under a week.
Unfortunately the plans are hard copies which makes alterations and preparing data for things like CNC LASER cutting machines somewhat tedious. I have not made a final decision yet but considering the difficulty I had obtaining the plans I will more than likely redraw them utilizing a CAD package. This will make the construction process somewhat simpler as well as ensuring the plans are preserved for the future. I am not sure about the legalities and copyright laws but considering the person that originally drew the plans is no longer interested in selling copies and the CAD versions would be drawn by me from scratch there is a chance I would be able to distribute copies to people interested in constructing models of the Mosquito.
The final footnote to the search for the plans was good old Murphy who just had to make sure his law was never overlooked or unapplied. About a week after the plans arrived from the UK and while I was doing something totally unrelated I stumbled across an article in Airborne Magazine about constructing a 1 to 7.5 scale model of the Mosquito. It turns out there is a local Australian supplier that could have sent me a copy of the plans all along and my global search was completely unnecessary. Oh well, at least I have a copy of the plans in my possession.
While we are on the subject of plans for scale models I would like to introduce a little bit of the engineering that goes with scaling things up or down. Galileo in his final book "Two New Sciences" introduced the concept of the Square-Cube Law which deals with the mathematics of scaling.
The Square-Cube Law relates how the volume, mass and surface area of an object changes as you scale its size up or down. It states that the volume of an object will change by the cube of the scale while the surface area will change by the square of the scale. For example, if we were to halve the size of an object then its volume would be ½3 or 1/8 of the original size while the surface area would be ½2 or ¼ the size of the original.
Before we get into the mathematics, variables with sub-script that start with S refer to the scale model and those that have subscripts that begin with M refer to the real aircraft.
So, how does the Square-Cube Law affect a model of the Mosquito? Equation-1a & 1b shows how the empty mass MMin of 6,490 kg would reduce to slightly over 6.5 kg and the Maximum Take Off Weight MMTOW of 11,000 kg scales to slightly less than 21.5 kg.
Next off let's look at what happens to the surface area of the wing. Equation-2 shows how the area of the wing A scales from 42.18 m2 to 0.658 m2.
An important factor with an aircraft is what is referred to as the wing loading. Basically the wing loading is the pressure differential between the upper and lower surfaces of the wing that is required to counteract the force of gravity. Equations-3a & 3b show how the wing loading of the scale model varies from 190 – 200 pa depending on the all up weight.
Equation-4a & 4b look at the wing loading WL of the real aircraft
As you can see the wing loading of the real aircraft varies between 1.5 – 2.6 kpa while the wing loading of the model is considerably lower at 180 – 300 pa.
Let's have a look at what happens when we scale the wing down and how it creates the lift needed to get the aircraft off the ground. Equation-5 shows the standard equation we use to calculate the lift produced by and aerofoil.
Where:
- Cl is the coefficient of lift for the aerofoil in question
- ρ is the density of the air
- V is the velocity of air passing over the aerofoil
- A is the area of the aerofoil.
If we now rearrange Equation-5 to solve for Cl we get Equation-6
If we now use the stall speeds Vs for the Mosquito at MTOW with flaps at 50.1 ms-1 (112 mph, 97 knots, 180km/h) and without flaps 58.3 ms-1 (130 mph, 113 knots, 209 km/h) and Equation-6 we can calculate the coefficient of lift of the Mosquito with and without flaps as shown in Equation-6a & 6b
By rearranging equation 8 to solve for V we get:
Now our model is a scaled replica of the full sized wing so the angle of attack, and ratios of length, thickness etcetera will all remain constant and for all intents and purposes we can use the coefficient of lift Cl calculated for the full scale wing on the scaled down wing. We can now use Equation-7 and the figures for the model to calculate the stall speed Vs for our model as shown below:
If you now compare the stall speeds of the model and real aeroplane you will start to see where the problems start to appear: The model's dimensions have reduced by a factor of 8, the areas by 64, the mass and volume by 512 yet the stall speeds have only gone from 50.1 to 17.3 ms-1 and 58.3 to 20.2 ms-1 or a factor of 2.886.
This has some serious implications because it means that the model would need to fly at a relative speed nearly three times greater than the real aircraft.
We could reduce the overall mass by more than the scaling factor cubed to keep the speed proportional to the scale factor. If we modify Equation-5 to solve for Lift L and then substituting mass M multiplied by g, the acceleration due to gravity we get Equation-8
If we now use a scaled speed and area we can get a mass that will give us a scaled speed as follows in Equation-8a & 8b.
Clearly it would be impossible to build a powered model of this size and keep its mass below 3 kg, however, controlling a model that has a relative speed close to three times the scaled speed would be equally as difficult.
It all gets complicated and I have really only touched on the problems that stem from scaling and the implications of the Cubed-Square Law but it will give you a feel for what is going on. You can also clearly see from Equations-7 & 8 that keeping the mass of the model as low as possible is incredibly important and will have a dramatic effect on the end product.
As usual you can read more by following the links below.
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