Mike's Flying

Mark5

Originally uploaded by Mikebert4

Now we’re getting somwhere.

This fellow did fly, and fly well. We managed to coax glide ratios of 4:1 out of it (just) and it coped with quite serious changes in wind speed/direction almost effortlessly.

If you look closely, you might well notice that it’s pretty similar to Mark4 in it’s fuselage and tail. Indeed, tis the same aircraft, only with the insane-dihedral cut down to a more acceptable 5cm or so. and longer wings added.

I apologise for the sorry state of the craft in the photos, I made the mistake of flying Mark5 before I photographed it. The nature of the construction means that after a few launches (maybe 20 or so) the joints and bends become very weak and the performance suffers. It’s somthing I’m trying to address in Mark6.

Flight of the Mark5

In fact, I’ve now brought you up to date on these here model gliders, and Mark6 is still under development, with the initial test-flights looking very promising.

The Techy Bit:

The only thing I will point out on Mark5 is somthing called gurney flaps. The trailing edge of the wing is folded down at 90º to the airflow and extending about 5mm below the wing. This increases drag (which is bad, but on a model this size, going this slow the additional drag is actually trivial), but it also increases lift. Simply put, it traps air under the wing a little bit, causing a higher pressure and hence more lift.

Mark4

Originally uploaded by Mikebert4

Ahh, Mark4.

Good idea, bad implementation. Glided exactly as one would expect from a brick suffering vertigo. The wings suceeded in making the glider (pah!) wallow on it’s little-slowed descent.

Mark 4 during construction

Seriously though, the idea of including an insane-dihedral to secure stability, and then straight wings to produce the lift was a good one. I just over-did the dihedralto the point where the whole middle section is producing no lift at all, and a lot of drag. Plus the little straight wings had a habit of folding up and aiding the inevitable plumet earthwards.

On the plus side, it was incredibly stable.

I’m not even going to go into the technical side of this fellows failings.

Mark3

Originally uploaded by Mikebert4

Ok, this here is Mark3 with the wing’s properly bent up. She flew alright, though had a tendancy to flip over or dive suddenly.

The wings are the big breakthrough with this design – as I mentioned in the techy bit for Mark2 thin wings are the way forward.

The Techy bit:

The failing of Mark3 was simply it’s stability, or lack thereof. The progressive dihedral of the wings added a lot of lateral stability, but longitudinally it was lacking.

The center of gravity was a good 2cm behind the center of pressure which was the root cause.

By this time it became evident that stability is the big thing for these free-flight gliders. Being so light and being made of such limiting material one has to pay careful note to how the wings will bend in flight, and the loading they undergo.

The phugoid (porpussing) mode of flight seems to be the easiest to maintain, a steady glide being near impossible when the rough air is combined with the light weight of these gliders.

Interestingly, and in support of the arguments I made for Mark2, roughening the leading edge aided the glide substantially.

Mark3

Originally uploaded by Mikebert4

Well, Time for you to hear about Mark3, this is a picture of the basic construction – before some fairly major tweaking to improve it’s stability.

Mark3 was a fantastic flyer indoors, with glide performance that outshone 1 and 2 by a massive amount. The main points of note are the thin (extremely thin) wings. I came up with a whole new way of building the wings by building a composite from layers of paper and glue. The upshot is that she flew wonderfully.

I’ll supply the techy bit with the second photo of Mark3.

Mark2

Originally uploaded by Mikebert4

Right. Given the failings of Mark1, I tried a new tact with Mark2. You’ll notice that it looks considerably different from Mark1 with massively swept wings. It flew very well, though couldn’t cope without still-air conditions and really didn’t suceed outside.

The Techy Bit (read at your own peril):

The swept wings add a lot of lateral (roll) stability whilst also moving the centre of pressure rearwards (which is effectively the same as moving the CofG forwards). So, 2 is markedly more stable. Drag is reduced too.

The problems arise from the large leading-edge radius and the method of mounting the wings onto the glider. To explain this I’ll have to give you a tiny bit of theory.

When considering a full-sized aerofoil (such as on a large transport jet) we attempt to keep the flow over the wing as smooth as possible (‘laminar’ flow) this reduces friction and helps the aerofoil move through the air. This is why you see a very smooth surface on a wing.

Contary to some teachings, having the air flow over the wing in a turbulant manner doesn’t reduce the amount of lift the wing generates. Indeed turbulent flow brings a lot more energy into the layer immeadiately next to the wing surface (‘boundary layer’), which helps to delay the stall. Though it achieves this at the cost of increased skin-friction drag, which affects fuel burn and top speeds.

On a model aircraft we have a different battle to fight. The air flowing over the wings is too smooth, meaning that we don’t get as much of a difference in pressure between the top and bottom of the wing as we would like. Another affect of the reduced scale of models is that only a small proportion of the total lift is gained in the conventional manner (faster air over the top surface droppping the pressure, causing th higher pressure underneeth to ‘push’ the wing up).

On a model we rely much more on the effect of the difference in angle between the chord of the aerofoil and the free-stream airflow. The process is simply that the air hits the underside of the wing and this increases the pressure under the wing and gives us lift (as well as deflecting the air downwards and giving us a small amount of newtonian lift). This is known as reaction lift.

Last bit of theory, this should bring most of it together for us.

Because of the reliance on this reaction lift, the aerofoil tends to sit at a high angle of attack – close to the stall. So, to delay the stall and hence allow us to gain more lift, we need to create turbulant airflow over the wing.

Thats it, turbulant airflow over a small-scale wing is prefered. You can test this by building a paper aeroplane and flying it, then taking a pair of scissors and cutting lots of small nicks (~2mm long) all down the leading edges, and flying it again. You’ll notice a marked improvement in it’s glide. It really does work.

Anyway, now we can get back to why Mark2 doesn’t fly so well. Because of the large leading-edge radius, the wing favours laminar flow. This means that when the aircraft settles down into it’s normal flight attitude, it’s close to the stall. Indeed, it does stall and then the nose drops (good ol’ longitudinal stability) and it picks up speed. and the nose starts to rise again. Now, at a faster speed the nose rises and the wings generate their reaction lift. Lots of it. The wings bend up. Suddenly all that lift isn’t holding the glider in the air, and it (all the while accelerating) spirals down to the grond.

There are two ways of thinking to solve this:

1. Delay the stall, strengthen wings

Later stall means longer time flying, but it also measn the glider will have to be accelerating for longer to bring the nose up – this means it’ll be going faster and it’ll more likely than not over-pitch up and stall again – given that the strengthened wings won’t bend up.

2. Promote earlier stall, recover quicker.

This means the wing stalls quickly, and the glider has to accelerate less to bring the nose up to normal flight again. Because we’re not accelerating as much, the wings don’t tend to bend up and we remain nicely airborne. Incidentally, the motion of diving, accelerating, climing slightly, stalling, diving, accelerating (etc…) in effect the apparant porpussing of the glider is known as a ‘Phugoid’ motion.

Deep breaths all, that was a long one