THE SMASH DRONE - designed by CStence.
Here’s my version of the Smash Drone – it’s a little stubby because I was using what was to hand, but the wing and tail separation are still roughly the same as the Old Fogey.
Here's the battery pod - my biggest battery is only 1000mah, so I had to add some weights. During the test flights I added even more weight - 50g in total. Depending on how the plane performs with some recent modifications, I might try doubling my batteries with a Y-harness and removing the weights. We'll see how it goes....
Here's the servos and servo tray.
The main difference between this Smash Drone and the original is the boom. Mine is made from a triangle of 6mm dowel rods I had sitting around. They seem to be performing well enough so far, but they're probably heavier than the specified balsa or hardwood boom. We've been discussing booms in the Smash Drone article thread - arrow shafts etc. but I just came across another possible source - carbon fibre umbrella shafts! I'm keeping my eyes open for a suitable donor umbrella. :)
Here's a bit of tidying detail around the tail.
Here's the beast in flight.
Please Remember - these flight observations and comments are on my own build!
After its first flight (before the video) I felt the 8x4 prop was a bit anaemic with the 1250KV motor, so I shifted to a 9x3.8 prop which ‘just’ fits. If you've got a higher revving motor it won't be a problem. After the second flight I noticed a few tiny nicks on the boom and worked out the fuselage was springing open during landing (and possibly during some manoeuvres) and this was allowing the boom to move towards the prop. I put in the cross brace you can see in this picture to hold this part together and stop that movement. I think that could be a useful addition to the existing design.
Regarding flight characteristics - I was a teeny bit disappointed as my plane seemed a little ‘leaden’ in the air. I'm not familiar with pushers, but it seemed to be flying like a much heavier plane. It also took a bit of effort getting it to climb-out at take-off. I concluded it was a combination of motor thrust angle and high drag from the Old Fogey wing that was causing these issues. Once the plane was up it was pretty steady, but the motor always seemed to be working quite hard for the amount of forward movement I could see. It was a fairly windy day, so that could have been confusing things. On the plus side, with zero power, it glided beautifully in a steady very predictable descent. On a calm day it's probably much easier to fly.
I wanted to see if I could improve the handling so I measured up my angles and concluded the Angle of Incidence* for the wing is about 3.5 - 4 degrees, which is quite steep (relatively speaking) - plus the Old Fogey wing has significant undercamber, making it even more ‘draggy’.
I decided to reduce the Angle of Incidence by lifting the back of the wing. First I reinforced the wing to replace the stiffening the wing gets from the top of the fuselage. These stiffners straddle the fuselage and are a fairly loose fit. Then I placed a foamboard pad at the back of the wing to sit on top of the swappable pod mount - I had to glue the pad onto the wing to prevent the wing sliding backward in flight, as it could now slide over the top of the swappable pod and disconnect the rear elastics. I'm glad I spotted that! I doubt the fuselage would fly very well without the wing! This pad raises the wing by 6mm.
Changing the thrust angle was much simpler - I just punched through extra skewer holes at the forward end of the power pod. I tried a couple of different angles before taking it through the plane's centre of mass. The pod still sits between the cheeks of the mount, which stops sideways movement, and fore-and-aft movement is taken care of by the rear mount and motor thrust, so all the skewer has to do is prevent up and down movement.
Here's a closer view of the back of the wing - you can just see the pad lifting the wing above the fuselage. This is about half the displacement needed to bring the wing to a zero Angle of Incidence, so the new A of I is about 1.75 degrees. Again, that's supposed to be plenty... but can you believe everything you read on the interweb? If this works out I'll rebuild the top edge of the fuselage to give me the new Angle of Incidence.
Here's the next round of flights - I was able to remove that extra 50g of balancing weights and now I find the plane much more responsive and generally freer flying.
As you can see the plane now tracks a lot flatter and smoother, but that seems to be at the loss of some of the slower speed flying performance - it will fly slower but starts to sink as the speed drops. However it still glides and descends extremely well. I was on the lookout for a wrecked golf umbrella to get a lightweight carbon fibre boom, but then I spotted a carbon shaft golf club - which I just bought for a pound ($1.60 ?) from a local charity (thrift) shop. When I build this plane again I'll go for a longer 'original length' boom, but I'll also include some of what I've learned from this build.
Again, thanks to the designer for a great swappable!
* 29/08/2014 - I reviewed this article and realised I was using the wrong term when discussing the modification to the wing. I was using Angle of Attack instead of Angle of Incidence. Angle of Attack refers to the angle that the wind is striking the wing and is influenced by the attitude of the whole aircraft in flight. Angle of Incidence is the difference between the straight line (baseline), which the horizontal stabiliser 'projects' along the fuselage, and the angle of the wing to this baseline. Some planes have zero Angle of Incidence, ranging through to quite high (10 degrees or more) for extremely slow flyers. For faster moving planes the Angle of Incidence is generally only a few degrees.
UPDATE - 31/08/2016 - I've explored this topic of Angle of Incidence more recently, both from research and from experimenting with builds. The conclusion I've come to is that for most model planes, no more than 2 degrees Angle of Incidence is required for the vast majority of more modern designs. This actually led me to reduce the 'real life' Angle of Incidence on model versions of biplanes I designed - why? Because with the 'real life' Angle of Incidence, the model plane wings produced too much lift, which needed to be countered by negative inputs like excessive down elevator, or a steep downward thrust angle on the motor. Basically the plane was fighting itself - one part trying to lift the plane, the other part trying to keep it level - it was a waste of battery power that could be used to keep the plane flying longer. Another 'historical' problem with this plane is that I was flying it with a very nose-heavy CG. With wings capable of producing a lot of lift, like the Old Fogey wing, with a steeper A of I the front wing becomes progressively more effective than the horizontal stabiliser as the speed increases. This 'nosing up' tendency is often mistaken for the plane being tail heavy and the intuitive solution is to add more nose weight. This just loses the plane's ability to fly slower, the real answer is to reduce the Angle of Incidence, OR fly slower. I discuss this in my recent article - 'Grouchy Old Fogey getting you down?