Aerodymamics...

General Dynamics F-111

Wikipedia - General Dynamics F-111

That is an F-111, FIN. You don't get the same effect with every VG plane though, the Tu-22M Backfire, for example, loses no area because of its pivots being mounted well outboard on stub wings so none of the wing goes into the fuselage.

Indeed it is an F-111, and how did I not put the model together with that plane. Now I get all sentimental again, I even remember when I wated to put Swiss decals on the plane when I had ruined my American ones by simple trying to glue them on the plane.

You all don't seem to be grasping the concept of a wing through air very well.

Take a knife - stick it in a block of cheese straight up and down. Try to pull through it.

Now angle the knife (logically) and cut through the block of cheese. It got much easier because the distribution of force is different. More of it is applied to pushing the wing 'inwards' (or the knife up and out of the block of cheese).

For an understanding of the lift difference - take a styrofoam wing from one of those gliders at walmart - or something halfway equivelant (but large enough so that you can feel the difference in force). Stick the wing straight out and spin around in a circle (primitive wind tunnel testing - tie it to a string and spin it around!!!) - note the amount of force that is trying to lift the wing upwards.

Now do the same thing, only with the wing angled back - note the difference in force.

This, obviously has nothing to do with wing area - and more to do with the angle of the wing. The angle causes a net decrease in airspeed over the wing because you are taking the same length of wing and exposing the leading edge to less air per unit of measure.

Another way to look at it is to take a stick much like you did the wing - only do it through water. This might be an easier way to observe the dynamics of a wing surface.......... you could even use your arms while swimming to physically feel the effect.

That's the same principle.

Now vortex generators can't be demonstrated with a block of cheeze.... not exactly sure what you can use in everyday life to get the handle of those...... I can't describe them mathematically - but I know how they work...... oh - okay.

You could kind of compare them to a very high velocity fan. Have you ever seen how a piece of paper will fly into the 'exhaust' of the fan, even though it was not directly in the exhaust or behind the fan to begin with? This is because the movement of air causes a sort of a suction effect - pulling ambient objects towards it. By creating a vortex, you're compounding this force along the surface of an object - like taking the exhaust of the fan and wrapping it in a 'coil' that will make the net 'suction' stronger.

Similar effects are also used to pre-feed jet engines so that their turbines will perform more efficiently. There's also some stuff to do with supersonic flight... and I'm not too keen on that - never had the opportunity to get to play around with supersonic stuff.

And don't expect me to explain any math on that - as I do not know the math - I just know how it works and how to use it.

EDIT: added further examples relating to variably geometry.

[edit on 14-7-2006 by Aim64C]

Yes, I can see I'm not very good at explaining what I mean and such as AIM 64 and kilcoo are much better in this respect. The reduction in wing area I illustrated has more to do with compensating for the centre of lift moving aft as the angle is increased, I didn't make that clear. The loss of overall lift (at a constant speed) is a function of the sweep angle increasing and this is counteracted by the increased speed (the 'more lift per sq ft I mentioned earlier).

I did say that it counteracted the loss of area, which it does *as well*, but that is not the *reason* the wing area reduces.

Another way to illustrate aim 64's last point is to hold a sheet of A4 paper about an inch in front of your mouth and turn the edge you are holding as if you are trying to make it horizontal (of course it wont), if you then blow across the top of the sheet it will rise up to the horizontal, lifted by the pressure differential caused by your blowing. The harder you blow the higher it will rise, just like a wing.

[edit on 14-7-2006 by waynos]

Originally posted by Aim64C
You all don't seem to be grasping the concept of a wing through air very well.

I really don't feel this is a fair, or warranted statement. No one on this thread has stated anything that would lead me to believe they don't understand how a wing works. Just because they may not understand the details behind the tricks that can be applied to modify stability and control, or change the aerodynamics of a wing by sweep, doesn't mean they don't understand how a wing lifts.

I commend Figher Master Fin because he wants to know, and then is asking...instead of just staying in the dark.

Can you agree that this...



...is at least non-intuitive - if not downright counter-intuitive for some people? This is probably why this issue is not dealt with in Freshman level aerodynamics, right?

[edit on 7-14-2006 by Valhall]

Good thread guys it was a good refresher for myself to read.
I do have to say though CH the way your site you linked selair.selkirk.bc.ca... (third of the way down) described dihedral is only partly right as most of the realtive airflow is coming form the front of the plane due to the fact that it is traveling faster forwards than into the small slip. One of the reasons linked to what the site says is that since both wings are angled up and not in the best AOA (angle of attack in case you havent caught that short form yet fin) position that when one drops its AOA is better causeing more lift and the wing to rise.
So to recap. Wings that are angled up also called dihedral cause stablity due to the fact that they force the plane to return to level flight when one of its wings drops.

Next point in the case of a high wing aircaft ie C5 or even a cessna 172. Ine these planes the fuselage is bellow the wings and this can act a weight to return the plane to level flight. Say a wing drops at this point the weight of the plane acts like a keel en.wikipedia.org... that hangs bellow a boat that returns to plane to level flight.

All of this information can be backed up the the FTGU (book called From the Ground Up) which is a requirement at pretty much any flight school in north america.

Again I know I may have over simplified but my main focus is just to gte my point across. Feel free to expand apon what I've said.

[edit on 14-7-2006 by Canada_EH]

A simple explanation is quite ofter better than a very complicated one. And I do really appriciate ally our answears, keep up with the good work.

And I was wonderinga bout why the Cessna has top mounted wings to.

Concept Of The Jet Engine

Now, I think I've understood this, but just to make sure. As you can see in my picture there are five or six different parts in a jet engine. First we have the air-intake wich "sucks" air in the engine with the help of the turbine. After the air has gotten in the engine it goes trrhough the compressor wich makes the air more dense (I'am not sure so explain) and after that the air and the fuel is mixed up and set on fire. Now here comes my first question, is the air compressed becaus it burns better? After the air and the fuel burns they start to make the rurbine spinn wich again suck more air in the engine. The "gases" that the fuel and the air make are very expandable, wich means that they take a lot of room in the open wich makes them the perfect fuel because they create lots of thrust. My last question is more about the throttle, how can a pilot actually make a plane go slower and faster with a jet-engine plane. Does it have something to do with the compressor or the turbine or the fuel injector?



[edit on 14-7-2006 by Figher Master FIN]

The more air you compress, the more power you get because you can get more air into the compressor. If you look at the really old pics of B-52s taking off, you see a huge black smoke trail. They used to inject water into the engine to cool the air down. By cooling the air, they could compress it more and get more into the engine and get more power.

I see, so does this have something to do that the more compressed the air is, the better it burns with the fuel, and the more thrust you get?

[edit on 15-7-2006 by Figher Master FIN]

ValHall - I wasn't really meaning to insult anyone. I guess I fail to realize how some things that are intuitive and obvious to me are completely beyond other people. And it's hard for me to determine, often, whether someone is just being ignorant or truely doesn't understand.

now - onto the jet engine....

The jet engine is really a somewhat simple principle. what you are doing is heating air at one end of an enclosed space, causing the air to expand and be forced out the closest side of the tube. This causes cooler, more dense air to rush in to fill the gap.

Compression is there to increase the airflow to increase efficiency (taking less temperature difference and generating a greater flow of air). However, the concept has been overly complicated with the standard approach.

Tesla designed a 'jet engine' of sorts that takes a radically different approach - using only a single and oddly formed disk, compression is achieved and it can operate pretty efficiently. I'm not sure why this design has not been adopted into fighter craft - since it's much smaller, lighter, and maintainable than any present jet engine. Unless there is a catch-22 that I'm unaware of.

Another way of looking at a jet engine is to take the concept of a tornado and put it in a 'bottle'. In a tornado - you have warm air on the surface that has been overtaken by cold air above it. Since warm air rises, it begins to attempt to punch a hole in the cold air. Due to the rotation of the Earth, this causes the updraft to begin to spin. However, a new effect begins to take place - as the air rises, it also cools (and contracts), this causes a suction that draws more and more warm air into the area of low pressure that has been generated. .... So I guess a jet engine can be looked at as an inverted tornado.....

not sure if that helped.

Although to answer another question, yeah - different compressions yeild different efficiencies with fuel.

Originally posted by Figher Master FIN
After the air has gotten in the engine it goes through the compressor wich makes the air more dense (I'm not sure so explain)


Now here comes my first question, is the air compressed because it burns better? After the air and the fuel burns they start to make the turbine spin which again sucks more air in the engine.


The "gases" that the fuel and the air make are very expandable, wich means that they take a lot of room in the open wich makes them the perfect fuel because they create lots of thrust.


My last question is more about the throttle, how can a pilot actually make a plane go slower and faster with a jet-engine plane. Does it have something to do with the compressor or the turbine or the fuel injector?

- Yeah, the compressor basically keeps shoving the air into a smaller and smaller volume, with conservation of mass dictating a constant mass flow rate:

mass flow rate = rho * A * V

the area [A] decreasing, velocity [V]... well, it effectively chokes and cannot increase further, thus making the density [rho] have to increase.



- The turbine rotors have to be spinning to start with to turn the compressor, there is no clutch on a turbine. Yes, the air is compressed because it burns much, much better. Current turbofans have a operating pressure ratio of around 30, but at such high pressures, there are problems with emissions, specifically NOx [IIRC]



- The mixing of the air and fuel is very important to minimise unburnt combustant, and alot of R&D goes into combustor designs. Yes, as you say, when the gases do combust, they produce a large expansion of air, which is key to the operation of a turbine [and an internal combustion engine for that matter]



- The turbine is balanced with the compressor [for anyone wondering about fans etc I'll use a single spool turbojet design], more power to the turbine = more power to the compressor = more air in the front.

If the pilot increases the throttle by say 5%, more fuel is dumped into the combustor, more energy is extracted by the turbine blades, leading to more energy for the compressor, which drags in more air, leading to a further improvement in power. This will eventually balance out through drag on the compressor.

Certain engines must be treated with care though, if the pilot increases the throttle by 50% too quickly, too much energy can be absorbed by the turbine, spinning the compressor too fast - causing it to stall as there is not enough mass flow through the engine. The engines on the F-14A were notorious for having stall happy engines.

Originally posted by Zaphod58
If you look at the really old pics of B-52s taking off, you see a huge black smoke trail. They used to inject water into the engine to cool the air down. By cooling the air, they could compress it more and get more into the engine and get more power.

No, this isn't why they injected water. They injected water to increase the mass of the eflux. More mass in the eflux, the more thrust for the same amount of fuel.

My mistake then. I was told a long time ago by someone that was the reason they injected it. Probably someone trying to talk down to a kid not knowing that I probably already knew more than he thought I did. heh. I used to love it when they'd talk down to me, and I'd get all technical, and they'd be like "huh?"

Well, they do use water injection on some OTHER aircraft for the reason of cooling the air down to prevent turbine overheating, but in the case of the B52 it was not for that. It was a brute-force, "bigger hammer" way of increasing mass eflux (and therefore thrust) on take-off. Because the B52 could carry more than it could take off with if it didn't have the water injection.

Yeah, I remember launching B-52Gs out of Hickam with water burning. It's not a pretty sight. After the first two go, you won't see another one until they're climbing out and turned to avoid overflying Honolulu.

We had a KC-135A (the only one to use water injection) taking off in the late 70s that was damn glad they hadn't built up one side of the Ala Wai Canal area yet. He lost water on take off, and was looking UP at buildings on the other side of the canal. That was always the big danger on take off with it. If you're airborne then you can keep enough speed to STAY airborne, you just aren't climbing for awhile.

Thanks for the replies kilco zap and valhall


Tail And Tailless Design

I have often wondered why planes have tails? It can't create lift so it has to be something about the stability right? Does it work like a keel of somesort that tries to stabilize the plane?

Different tails. The second question is about the different tails. Some planes have one tail, som have two. And of those planes that have two tails can also have tails that are not straight up but sort of bent. Why is this?

And those planes that have no tails, otherwise called planes with tailless design (like the X-36) what does a plane acchieve by having no tail, stealth I believe but how can a plane be stabile without a tail.





[edit on 16-7-2006 by Figher Master FIN]

1 Yes

2 Once an aircraft designer has deduced the amount of vertical side area required it is all down to choice, the Tornado has one huge fin, the MiG 25 had two smaller fins plus two more smaller still ventral fins underneath. This feature is often added as an afterthought if the flight testing shows that the fin isn't big enough (ie Jaguar) or the fin itself can be extended forwards (Nimrod) or completely redesigned (BAC Lightning F.3).

It can even be down to fashion, in the early 1980's the RAF was adamant that it (wanted the Typhoon to have twin fins (look at the TKF-90 and ACA designs) until it was pointed out that this would unnecessarily increase the weight (and cost!) of the plane and the Typhoon was born with a single fin.

When discussing the 'tail' though you should be clear what you mean. There is the vertical tail (fin) which is what I have just been describing, and the horizontal tail (or tailerons or tailplanes - the former is all moving, the latter has 'flaps' attached but is fixed)

The horizontal tail is there because a normal plane always wants to pitch nose down (ask someone more technical than me why) and the tail pushes the back of the plane back down to balance the aircraft in level flight. Delta's can get away without one because the inner wing flaps do the same job as the tail while the outer ailerons do their normal job.

Doing without a fin (your Q.3?) is a recent development and it is only possible with complex FBW systems which make thousands of adjustments every minute of the wing controls to compensate for the lack of a tail. This is not a practical solution for anything away from combat aircraft however as the plane is completely unflyable if there is a glitch in the FBW. Of course these planes are not stable, that is the point, they are extremely agile because their is a complete lack of 'keel action' anywhere on the airframe, without FBW they would be as erratic as a piece of paper in a breeze.

I see. However, is there any clear reason why the F/A-18 and F-22 vertical tails aren't straight as for exampel the F-15.

And could somebody explain why the nose of the plane wants to pich downwards.

Twin fins are required because of the vortex coming off the wing. Having two fins makes it more stable. Canting it outward affects the airflow more as it passes over the tails.