Welcome back! Whilst under lockdown I’m going to continue posting a blog post each week day, documenting the the RC model aircraft build journey, from beginning to (hopefully!) successful first flight.
It would be great if you could share the blog and we could get people building alongside.
Today, I’ve been modelling the tail. As I have said before we only need to model one side as we will use a mirror later on. This has the benefit of saving time, but also ensuring our plane is symmetrical.
I haven’t used any new Fusion 360 functions for this part today. The boom is made using the loft function and the fin using the sweep function. I think my trial and error approach is clear for all to see in the creation of the boom and that is probably my biggest tip for those new to any CAD program. Just keep tinkering… and try not to get to frustrated.
The fin is created using a sweep of an aerofoil. We discussed aerofoils last time, the only difference here is that I have made it a symmetrical aerofoil as it need not create ‘lift’ either way unless it is displaced. This is different for the horizontal stabiliser.
On a conventional aeroplane, that is an aircraft with a main wing forward and a small wing aft, the aft ‘horizontal stabiliser’ provides a downforce.
This is because the centre of gravity (the point at which the aircraft’s mass acts) is usually forward of the centre of lift (the point at which the upward force from the wing acts). If you think of the centre of lift as the pivot on a see-saw you can see that the tail needs to press down to stop the nose from going down.
On an airliner this is why we load the aircraft to favour an aft CoG by putting more luggage in the back or pumping fuel around to aft tanks. An aft CoG requires a smaller balancing downforce than a forward one. Since downforce adds to the effective weight of the aircraft, a smaller downforce means less lift required, resulting in less drag, resulting in the captain and first officer having bonuses and wealth rained down upon them by the CEO, board and shareholders. All of that apart from that last bit is true.
This is the reason that you will often see an inverted aerofoil on the tail. NB. It is perfectly possible to use a symmetrical tail, Piper’s stabilators are symmetrical, and even a normal ‘upright’ aerofoil will work, the angle of attack will need be negative and in the case of the normal aerofoil it won’t be hugely efficient…
There are aircraft which have designs to avoid a down pushing tail such as Piaggio Avanti (another model I’m working on) but for the Skymaster I have used an inverted stab.
P.S SORRY! The horizontal stab didn't make it into this screencast - will put in on the front of tomorrow's.