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Re: Trainer Jet WIP

Posted: 06 May 2013, 14:39
by Steve
Do it! You'll love it

So, moving swiftly onwards and upwards from that. A quick amount of research and testing suggested that the plane will be a 'mid-wing' configuration and not high or low because it carries many more aerodynamic advantages.

So we're now moving on to aerofoils.

So, the aerofoil is what gives the plane lift. It, like every other component is selected out of compromise based on certain factors, and a lot of the time it's not which shape generates the most lift. It can be the most benign stall conditions, the best lift-drag ratio, thickness for internal storage and so on. But all are selected because it generates the correct amount of lift at minimum drag when the aircraft is in cruise.
Here's the ones I have selected, known as NACA 6 digit aerofoils. The aerofoil needed a lift coefficient of 0.31, therefore I have selected NACA 641,412 as it generates a good amount of lift for minimum drag at the required coefficient. But, this may be too thick an aerofoil to carry over the entire length of the wing, so I have selected a slightly thinner aerofoil, 64A410 to be used at the wingtips, providing minimal drag for maximum lift across the largest speed range possible.

Wing Root. 641-412

Wingtip. 64A410

I've also needed to decide what type of high lift device is needed. For that I've picked out a leading edge slat, which delays stalling effects over the wing by providing fast moving air over the top surface through a slot. And a slotted flap, which provides a greater down wash, larger wing area and a higher angle of attack. The flap itself has its own leading edge slat to make sure it doesn't stall too soon.

The anatomy of the devices looks like this:
You can see the small 'slot' in front of the main flap, called the vane and how in theory it will re-accelerate air through the gap between it and the main flap

This has required a new program, called 'Ansys Fluent', to check the aerodynamics of the aerofoil. I include this because later on it ties in with 3D modelling. Here's some of the results.

Where this sequence of images show the stalling properties of the wing with the flaps down at increasing angle of attack, 10-14 degrees. The colour plots show the amount of turbulence indicating stalled flow.
These are unusual results; a stall develops over the rear section of the wing and is transitioned into strong wake turbulence at a higher angle of attack only to then re-develop. While there is a possibility of simulation error a likely cause could be the effectiveness of the flap vane at varying angles of attack, which is a significantly negative, delaying the effects of a stall. As a result with increasing wing angle of attack the vane will become more effective at providing clean air through the slot gap. This is shown in area A, where separation and turbulence originates from the main wing elements, but pushed away from the flaps where the slot gaps are positioned. While this may indicate that the maximum angle of attack possible is 13 degrees, it may still be firmly in a stalled condition. Pressure plots varying from 9 degrees upwards to 13 appear to confirm that in fact 11 degrees is the maximum usable angle of attack.

And some steamlines showing flow direction around the leading edge slat. If you look you can see a lot of circulation inside the slot gap until the aerofoil reaches a certain angle of attack. That's not good, requiring a redesign

Next up. The wing shape, and some...actually interesting looking simulations

Re: Trainer Jet WIP

Posted: 03 Jun 2013, 13:28
by Steve
One more slightly boring bit to go. What comes next is known as the 'reference planform'. Which is what the wing would be shaped like if it was a straight edged trapezoid extending all the way to the centre of the aircraft.

Now. The wing has the shape it does for a few reasons, but the primary reasons are maximum lift, minimum drag, good stability and controllability but also stall quality. The wing gets thinner (tapers) towards the tip to improve the roll rate as more of the mass closer to the centre, and it reduces weight as less strengthening is needed further out. But it also rakes backwards (known as wing sweep), this is a stability thing, where sweep increases stability both in pitch, yaw and to a slightly lesser extent roll. It also allows aircraft to cruise at a higher mach number before supersonic effects take over.

So the reference planform I chose is here, The flaps cover 60% of the span, with the leading edge slats outboard of that, shown at a suitably high angle of attack to see how it goes. The wing was modelled in 'Alias Automotive'...which was just so fun :roll:


Next up is actually properly integrating that with the fuselage. The IP rights are now mine again so I am free to show off the actual design now

Stay tuned

Training jet fuselage, now with 50% more plane

Posted: 11 Jun 2013, 21:23
by Steve
Okay then. So let's look beyond that now, I can work on the proper aircraft form.

So, here's what it looked like based on the results of the wing test


So it's taken a drastic leap. Explanation below, copy pasted from the coursework because I'm quite lazy :roll::

The aircraft will sit on the ground with a 2 degree nose-down attitude, this is to minimise unwanted lift from the wings as a result of gusting causing difficulty to the pilot during taxiing manoeuvres as well as improved forward visibility from both seats.

There are multiple changes to the initial fuselage design, notably the elongated and sharper nose cone and wider, blunter tail. Also because of limited fuselage space and the need for good rear seat visibility, the canopy has been made taller to accommodate a raised rear seat.

There is insufficient space to accommodate the retracted nose gear underneath the cockpit; as such it must retract forwards into the nose cowling.

A crease from the nose to over the leading edge strake will provide assistance in generating vortex flow.

The leading edge strakes are longer than the reference area and form an ā€˜Sā€™ curve to improve pilot visibility from both seats, raise the sweep angle and thus leading edge vortex strength whilst retaining the required exposed area. The strake also blends with the fuselage to maximise lifting potential and minimise interference drag.

Engine intake shape has been altered in lieu of leading edge strake placement; as such they are now fitted underneath the strakes to allow airflow to be delivered to the compressor face cleanly at high angles of attack and are rounded and enlarged.

The body has been widened to accommodate the main landing gear, which must also retract forwards. In a further attempt to shorten the wheelbase, increase longitudinal taxi stability and to ensure that on landing the forces through the landing gear leg are entirely axial, a forward rake angle of approximately 11 degrees has been included.

The wing trailing edge Yehudi will serve to blend the wing with the fuselage at its trailing edge.

The engine exhausts are further forwards and entirely shrouded from the top by the fuselage and the horizontal tails have been moved inboard. The vertical tails have also been canted outwards by 25 degrees to provide further longitudinal stability.

A bit more to come later

Re: Trainer Jet WIP

Posted: 11 Jun 2013, 22:19
by jamesmc
Fascinating details, enjoyed reading the technical aspects and the results.

Quite the lovely design on the model as well.

The image below is where I worked for awhile about thirty years ago. (human centrifuge)

The Human factor is often the limiting factor in aircraft design (if a human is in the craft). People tend to lose their eye level blood pressure on high turns and twists causing loss of consciousness.

In the early times of 1960s it was used to test G(y) forces on astronauts. They were laying down as they would at take-off. Was an award called "Order of the Elephant" (15 G(y) forces across the torso).

Since fighter pilots don't like to lay on their back to fight, they have upright seats, so we tested the G(z) forces that pilots took on tight turns and anti-combat maneuvers. Most common profile was 3 to 5 to 9 G(z) up and down for about a minute.

Saw a lot of folks come through there, NASA, Air Force, Navy, Foreign military, etc. That old centrifuge had 4 - 250 HP DC electric motors on it. It could roll your socks down in a hurry.:)

Humans would ride in that enclosed gondola. There other end was for instruments and animals tested.

Re: Trainer Jet WIP

Posted: 12 Jun 2013, 01:16
by Steve
Seen a test done in a similar sort of gondola on something called the Giraffe suit, featurers a whole heap of water filled capillaries running along a full body suit to increase pilot g tolerance. In the test this guy put it up to 9 G and sustained it for 30 seconds, then the test engineer took over and put it up to 11...whilst the pilot then took out a rubiks cube and solved it!. I've pulled 5 in a plane and it's certainly not a pleasant experience so I can't imagine 11!

This aircraft is nominally designed to operate with current G-suit equipment, though I know that'll only increase G-tolerance for a bit, maybe by 1 or 2 if you're lucky. Though I see the modern bang seats cant the pilot back by about 15 degrees, which probably helps quite a bit.

In reality though, it's doubtful whether this aircraft will go up to the 8g limit load required normally, unless the pilots are being utter hooligans.

Re: Trainer Jet WIP

Posted: 13 Jun 2013, 22:34
I saw one of these in 1972 at Nasa space center in Houston...... :bananacool2:
that was in the golden days of spaceflight...