The 7 Ton Brute
The Hawker Typhoon is an aggressive looking warbird. As part of a scratch building contest I opted to build this plane for the very challenging paint scheme.
As the contest allowed I spent a good deal of time designing this model inside sketchup and learned quite a bit in the process. I developed a new accurate way of constructing a complex semi-symmetrical airfoil. I even took the time to build in some retracts. Too bad the retracts only lasted long enough for five flights, but it was a fun adventure. (I'll get back to retracts later). I also explored using a slicer script to make section parts for the complex shapes that I otherwise would have carved. Using the Phlatprinter I made the most accurate canopy I've made to date along with a positive mold for a cowl.
As this one was for a contest I wanted to pull all the stops and have a fully featured aircraft.
Split flaps, check
Retractable undercarriage, check
Wing running lights, check check!
It was all there.
Applying the lessons learned from the Corsair, I mounted the servos to control the tail under a hatch up front. This actually was for another reason as well. The Typhoon has a very short relative area in front of the Center of Gravity (CoG/CG). To many this is nothing new, but to completely understand why the servos were moved towards the CG you must understand the concept of a moment arm, or sometimes called a lever arm. A moment arm, or a moment is simply a rotational force on an object. The longer this distance, or longer the lever arm, the greater the moment. A moment is figured by simple multiplication, and as I've written in Principles of Design, all forces on the aircraft act about the center of gravity. For instance, the weight of the tail creates a moment illustrated as this:
Typically, if I have 20 grams of servo weight in the tail at a distance of 70 cm from the CG, that equates to a moment of 1400g-cm.
To balance the aircraft at the CG I will need to counter that moment. The distance from the area in the nose to the CG is about 20 cm.
This means to counter the weight of two micro servos in the tail I will need additional 70 grams of something in the nose pulling the opposite rotation.
That's over three times the tail weight, not including the actual tail!!
For the egg heads out there, this is setup as the following equation:
20g * 70cm = 70g * 20cm
Or Expressed as: 20g * 70cm - 70g * 20cm = 0
(Moment & Opposite Moment must be Zero!!)
As you should be able to see, the shorter the nose, the greater the required weight to balance the tail.
Likewise, weight in the tail will require a multiple of that weight in the nose to counter it. Illustrated as this:
Now many folks will simply slide their battery around as a ballast to get the aircraft to balance. ...But I have seen others that will add lead weight or heavy spinners to achieve the same effect. Sometimes you simply don't have a choice in this matter, as with many WWI era biplanes, but going that route should only be done as a last resort.
In reality these forces are not linear as shown above. There are dozens of things that create more density in a given section of the airframe. Short nosed airplanes need a lot of parts crammed in up front to properly balance the tail. Additionally the shorter nose means that the weight up front will not be as forgiving to find a sweet spot. When you have a longer distance to balance the craft you also have the luxury of spreading the weight out over that distance. This could easily give you several inches to play with while obtaining good results though the band of where you actually stick the battery. On the shorter nose, you have more of a spike of weight, which may only allow a few millimeters of a band to find a good balance on the CG. The fact of the matter is, no matter what, if the plane is to balance these moment forces must be equal!
The actual usable numbers for the distances on my Typhoon are approximately 15cm from the nose to CG (not including the spinner) and 75cm from the tip of the tail back to the CG. For those two 9 gram servos that far back I would have needed 90 extra grams upfront in the nose, in English that's an extra 3.2 Oz!
Incidentally, I have recently learned that the Typhoon required a massive amount of weight in the tail! The original H-block engine was indeed a beast! So in short, I believe I could have increased the size of the power plant by quite a bit on this model airplane.
No More Math! ME WANT PICS!
The Typhoon isn't all tough as nails to build, but at this point it is already easy to see why the factory workers went on strike due to the advanced building techniques required to build this powerful bird. On of the easier things to deal with is the thickness of the wing. This made it ideal for retracts as most wheels should be able to fit inside. Thick wings are also easy to form. But before one gets too excited, the wing also has a bad set of features going for it. The type of tip dihedral used also creates a problem for the builder as it is difficult to get two identical anythings in this world, not to mention mirror opposites. On a straight dihedral it is easy to keep the wing tips at an even distance up as the wing can just be rotated about the middle. With the wing tip style dihedral the problem is both wings must be made evenly or very funny things will happen during flight. To counter this problem I came up with a three piece sectioned wing.
Because of the thicker designed wing, the foam is much easier to shape into the correct shape. The thickness also adds to the inherent strength of the assembly.
To aid in assembly shapes were cut into the parts so that they can be matched. In the following pic I show matching two pentagons. Also used are triangles, and triangles with "T" inside to mark the tops of parts.
Each section of the wing, except for the tips, was designed in three parts. First is a leading edge part. This part gets rolled and shaped to the leading edge of the airfoil, it can then be glued into place using hot melt glue. The bottom skin is then rolled slightly as is the top. The trailing edge of the bottom is sanded thin and then glued on. The tops can now be fit, but it may be best to wait so that access is available as you build.
On the thinner sections clear packing tape can be used to keep the foam from splitting as it is aggressively rolled. Notice that the washout designed in CAD transfers into the actual build. One must be careful however, as the washout must be equal. A bit of precision in building is required, or perhaps a bit of muscle to keep the twist equal.
To be continued...
Hawker Typhoon 1B
DIFFICULTY: Moderate - Compound bends are required on nose section of fuselage, care is required for built up wings to achieve symmetry.
SCALE: Approx 50 inch wingspan.
PLANS BY: Erik