The F-35 Thread

[aaron886]

yeah about as smooth as a bag of spanners. The most aerodynamic shape known is the tear drop. The leading edge has no angular surfaces.

 

Or maybe learn a little about what you're talking about.

 

Saying that a teardrop is "the most aerodynamic shape known" in relation to aviation is like suggesting we must put dimples on airplanes because golf balls have them, and golf balls are aerodynamic! :doh:

 

A teardrop shape would create a strong high pressure area in front of it. Fighter aircraft designed to fly at high mach have sharp leading edges, and shapes swept in accordance with careful calculations involving airflow at high mach. They don't have a "teardrop shape."

 

Lolz @ made in the USA sticker. What part of multi-nation do you fail to understand?.

 

First of all, great edit. The only part about the F-35 that is truly "multi-nation" are the sub-contracted lift fan system on the STOVL variant and the AN/ASQ-239. Neither have any bearing on this conversation, in my mind.

 

Here's my edit:

I don't do cryptic sentences, please be more specific.

 

Apparently, you just don't "do" intelligent conversations. I understood him perfectly. It would seem your knowledge of aerodynamics stops at a caveman-esque "it displaces more air!" By your logic, the F-22 would have to be slow because it's a big jet, and it would have to be slow because it has an angular profile, yet we know it is not. Give it up, already...

 

I mean, really? Nobody speaks in terms of "thrust to drag ratios" because that's not even a term. A pilot will call it "thrust required." That big bird sure does displace "alot" of air? Really?

He fails to mention thrust to drag ratios though. physics dicates they're going to be alot higher than a clean F-16 because that big bird sure does displace alot of air

Edited September 15, 2011 by aaron886

[Vault]

Or maybe learn a little about what you're talking about.

 

Saying that a teardrop is "the most aerodynamic shape known" in relation to aviation is like suggesting we must put dimples on airplanes because golf balls have them, and golf balls are aerodynamic! :doh:

 

The most aerodynamic part of an aircraft is the wing and guess what shape that is.

 

A teardrop shape would create a strong high pressure area in front of it. Fighter aircraft designed to fly at high mach have sharp leading edges, and shapes swept in accordance with careful calculations involving airflow at high mach. They don't have a "teardrop shape."

 

Sharp leading edges cause oblique shock which has a far higher impact than drag.

 

http://en.wikipedia.org/wiki/Oblique_shock

 

First of all, great edit. The only part about the F-35 that is truly "multi-nation" are the sub-contracted lift fan system on the STOVL variant and the AN/ASQ-239. Neither have any bearing on this conversation, in my mind.

 

Lolz again. :thumbup:

 

 

Apparently, you just don't do intelligent conversations. I understood him perfectly.

 

Character sniping is low and I'm not going to lower myself to it.

[VincentLaw]

Let's get this straight. The F-35 is a compromise between aerodynamic efficiency and VLO. Any object that moves through air has to displace it's own volume at any given time. The bigger the object the bigger the displacement. The bigger the displacement the higher the surrounding air pressure which equals drag or as you like to say "draggy".

Drag is not simply caused by the amount of air you displace. It mostly has to do with skin friction and flow separation. As long as all of the air comes off of the trailing edge relatively smoothly, the pressure drag can be very low. An example of this is that the P-51 has the equivalent pressure drag of a 0.35 m² flat plate perpendicular to the airflow, but a P-51 is much larger than this flat plate. If you imagine this plate is infinitely narrow, then it has 0 volume, but it would still have a lot of pressure drag.

 

 

yeah about as smooth as a bag of spanners. The most aerodynamic shape known is the tear drop. The leading edge has no angular surfaces.

This is not true at all velocities. anything teardrop shaped will cause bow shocks and a very large amount of drag at supersonic speeds. Supersonic airfoils are angular because a subsonic airfoil can actually start causing negative lift when you get transsonic or above.

 

Saying that a teardrop is "the most aerodynamic shape known" in relation to aviation is like suggesting we must put dimples on airplanes because golf balls have them, and golf balls are aerodynamic!

The dimples on a golf ball are very specific to the standard Reynolds number of a golf ball. if you tried putting dimples on a giant golf ball, it would not help the aerodynamics, since the flow would have already become turbulent before separating, but I'm guessing that's your point.

[VincentLaw]

Sharp leading edges cause oblique shock which has a far higher impact than drag.

 

You are wrong. Finish reading an article before you link it. Oblique shocks are a supersonic problem only. at M > 1, oblique shocks are the best kind of shocks. bow shocks are much more draggy, and are caused by curved leading edges.

[Vault]

You are wrong. Finish reading an article before you link it. Oblique shocks are a supersonic problem only. at M > 1, oblique shocks are the best kind of shocks. bow shocks are much more draggy, and are caused by curved leading edges.

 

According to Fleeman oblique shock is in the transonic region too. It's not just supersonic.

[VincentLaw]

The reason it is "transonic" has to do with the definition of transonic. Shocks only occur when the airflow reaches sonic speed at some point. This means that if you are traveling Mach 0.9 "Transonic" then the flow that accelerates over your wing actually reaches the speed of sound at a certain point, this is why the shock occurs. This is also why Transonic airfoils look different from teardrops, supersonic airfoils, or subsonic airfoils, the surfaces are designed to delay the drag caused by such effects for as long as possible.

[Vault]

Drag is not simply caused by the amount of air you displace. It mostly has to do with skin friction and flow separation. As long as all of the air comes off of the trailing edge relatively smoothly, the pressure drag can be very low. An example of this is that the P-51 has the equivalent pressure drag of a 0.35 m² flat plate perpendicular to the airflow, but a P-51 is much larger than this flat plate. If you imagine this plate is infinitely narrow, then it has 0 volume, but it would still have a lot of pressure drag.

 

There are many reasons for drag. the main reason for the friction is because of the water vapour in the air. Water doesn't compress well not unless you add a huge amount of energy to it and even then it's neglegible. Flow seperation is important. Hence the tail of the tear drop. The longer the trailing edge the smoother the flow seperation.

 

This is not true at all velocities. anything teardrop shaped will cause bow shocks and a very large amount of drag at supersonic speeds. Supersonic airfoils are angular because a subsonic airfoil can actually start causing negative lift when you get transsonic or above.

 

The shape varies but predominantly it remains a tear drop shape. A larger leading edge followed by a narrower trailing edge.

[aaron886]

...but I'm guessing that's your point.

 

Exactly. :thumbup:

 

The most aerodynamic part of an aircraft is the wing and guess what shape that is.

 

This is wrong in enough ways that I'm not even going to continue discussing aircraft performance with you. It's pointless.

[VincentLaw]

The shape varies but predominantly it remains a tear drop shape. A larger leading edge followed by a narrower trailing edge.

According to aerodynamic theory, the maximum thickness of a subsonic airfoil is at 1/4 the chord length, but for supersonic airfoils it is at 1/2 the chord length. If you want to supercruise around all day, you do not want teardrop wings, but you don't usually want teardrop wings anyway since they don't generate any lift without an angle of attack.

[Vault]

The reason it is "transonic" has to do with the definition of transonic. Shocks only occur when the airflow reaches sonic speed at some point. This means that if you are traveling Mach 0.9 "Transonic" then the flow that accelerates over your wing actually reaches the speed of sound at a certain point, this is why the shock occurs. This is also why Transonic airfoils look different from teardrops, supersonic airfoils, or subsonic airfoils, the surfaces are designed to delay the drag caused by such effects for as long as possible.

 

If air reaches speeds of mach 1 at 0.9 only over the positive pressure side of the wing then the negative side must be moving slower. You need air travelling at mach 1 over both sides of the leading edge to induce oblique shock if it was only at mach, Hence the photo.

Edited September 16, 2011 by Vault

[Vault]

The F-15's wing cross section. There all tear drop shaped.

 

[VincentLaw]

If air reaches speeds of mach 1 at 0.9 only over the positive pressure side of the wing then the negative side must be moving slower. You need air travelling at mach 1 over both sides of the leading edge to induce oblique shock, Hence the photo.

 

There is no requirement for a shock to have a mirror image. It is perfectly possible to have a shock on only one surface, for example, if you are flying a supersonic airfoil at a higher angle of attack than the wedge angle, there will only be a shock underneath the leading edge.

[VincentLaw]

The F-15's wing cross section. There all tear drop shaped.

 

many fighter wings are generally not designed for optimal performance at supersonic speeds. This is because 99% of the time you will be cruising around subsonic since it is more efficient in general, and there is no reason to waste all the gas money, range, or loiter time, except in emergency situations.

[Vault]

many fighter wings are generally not designed for optimal performance at supersonic speeds. This is because 99% of the time you will be cruising around subsonic since it is more efficient in general, and there is no reason to waste all the gas money, range, or loiter time, except in emergency situations.

 

So 99% of the time the teardrop is the most aerodynamic efficient shape for wings?.

 

 

There is no requirement for a shock to have a mirror image. It is perfectly possible to have a shock on only one surface, for example, if you are flying a supersonic airfoil at a higher angle of attack than the wedge angle, there will only be a shock underneath the leading edge.

 

I've never seen an example of oblique shock without a mirror shock wave. The example given was a sharp leading edge, a sharp leading edge like a wedge will produce two oblique shock waves. So am I not correct in my original statement that sharp leading edges cause oblique shock.

 

[VincentLaw]

So 99% of the time the teardrop is the most aerodynamic efficient shape for wings?.

Generally speaking, for subsonic flight, you want a curved leading edge, a pointed trailing edge, and a maximum thickness at the quarter chord. The design can get pretty complex and asymmetric, so if you loosely define teardrop, you are correct (for subsonic flight), but that is a large simplification.

 

The efficiency of a shape is highly dependent on Mach Number and Reynolds Number, and also depends upon how you need it to perform in specific conditions. For example, at low Reynolds numbers, a teardrop shape can actually have more drag than a sphere as seen in the video Fluid Dynamics of Drag Part 1. I can also guarantee you that a standard (symmetric) tear drop does not have the best lift to drag ratio, which is the entire purpose of a wing. If there was one perfect shape for wings, then all airplanes would have the same airfoil, but as with everything in aircraft design, compromises have to be made.

 

With a fighter jet, you need to be able to perform well at subsonic, supersonic, and high angles of attack without breaking or oscillating out of control, all while keeping the radar signature down. The complexity of the problems involved is why aircraft design is an iterative process. The F-35 would not simply have a bad wing design by accident, but there is no possible way to have the best wing or the best airplane for every situation.

 

I've never seen an example of oblique shock without a mirror shock wave. The example given was a sharp leading edge, a sharp leading edge like a wedge will produce two oblique shock waves. So am I not correct in my original statement that sharp leading edges cause oblique shock.

You are right that a wedge can produce an oblique shock, but you were wrong in the way you said it. You said "Sharp leading edges cause oblique shock which has a far higher impact than drag." as if that meant the oblique shocks were what made a teardrop more efficient. In reality, a wedge would only create an oblique shock in a case where the teardrop would cause a bow shock, which is much much more draggy. The biggest problem with a sharp wing or stabilizer leading edge is the possibility of a leading edge stall, which is extremely dangerous while landing or taking off, on other surfaces it does not matter as much.

 

 

The reason you don't see any photos with asymmetric shocks is because almost all of the photos have symmetric objects at zero angle of attack. If you go beyond the angle of attack in this picture, you would actually have an expansion wave on top of the leading edge, which looks similar but is completely different from a shock wave. These expansion waves can occur on the upper surface of transonic airliner wings.

Edited September 16, 2011 by VincentLaw

[tflash]

Take into account that the aerodynamic design of the F-35 is made with today's immense computing power, something not available in the time of Sprey and Wheeler. It is also an unstable, dynamic design.

So I guess their F-105 comparison is a little simplistic. Let alone the teardrop comparison proposed in this thread.

[Vault]

Take into account that the aerodynamic design of the F-35 is made with today's immense computing power, something not available in the time of Sprey and Wheeler. It is also an unstable, dynamic design.

So I guess their F-105 comparison is a little simplistic.

 

Stealth on the F-35 is good but it's not free. There are aerodynamic compromises in any angular suface regardless of how many computers you use, computers can't change physics.

 

Let alone the teardrop comparison proposed in this thread.

 

The most theoretical aerodynamic shape is the sears haack body. The F-15's rear control surface cross section I posted earlier is a sears haack body. It's a teardrop profile albeit with an elongated leading and trailing edge.

[GGTharos]

No, they can't. Instead, they let you do things like apply VLO to an aerodynamic surface, add chines to give you high AoA handling such as found on both F-22 and F-35, not to mention other things. Stealth costs. Money. Unlike the F-117 however, it no longer costs you in aerodynamic capability - case in point proven by the Raptor at minimum, the B-2, and a bunch of UAVs, and the F-35 as well. The F-35 will do just fine in a dogfight, and will make most MiGs cry - it will likely do a number on flankers also, as long as they don't have TVC.

 

So say that an aircraft capable of coordinated 50 deg AoA 'can't turn' is a joke in and of itself.

 

Stealth on the F-35 is good but it's not free. There are aerodynamic compromises in any angular suface regardless of how many computers you use, computers can't change physics.

[tflash]

Computing can't change physics, but smart computer assisted controls can get the optimum out of a wing + aileron +spoliers + slats + tailplanes + rudder + evt. canards.

 

The aerofoil generated in flight is the result of the relative positions of all these controled surfaces, not simply the basic form of the wing.

 

To put it simply: the F-16's wings are just flat, the bernouilli effects are generated by the leading edge flaps / flaperons, through FBW.

 

As it happens, the F-35 has much more powerfull computers on board that calculate the optimal position of each surface continously during flight, as is the case with Typhoon.

 

The flight envelope of modern jets is enlarged by new software releases, not by changes to the planforms.

Edited September 18, 2011 by tflash

[Pilotasso]

F-35B makes first carrier deck landings!

 

http://www.aviationweek.com/aw/blogs/defense/index.jsp?plckController=Blog&plckBlogPage=BlogViewPost&newspaperUserId=27ec4a53-dcc8-42d0-bd3a-01329aef79a7&plckPostId=Blog%3a27ec4a53-dcc8-42d0-bd3a-01329aef79a7Post%3af697a44f-732a-447b-842f-b01523428b9a&plckScript=blogScript&plckElementId=blogDest

[StarHopper]

That was awesome!

[zakobi]

I thought they planned to cancel the B version? Or was that just suggestive talk?

[VincentLaw]

Well if they were to cancel the F-35B, they would either have to keep using the Harrier for years, not use jets on amphibious assault ships anymore, and/or develop an entirely new replacement.

 

I remember reading somewhere that most F-35B landings would be conventional to reduce stress on the lift fan and save fuel. Does this mean they will usually land on the assault ships with higher forward velocity too? My guess is that this just refers to land bases.

[zakobi]

I remember the argument against the B, was something akin to: "It's extremely rarely we are in a situation where we NEED the V/STOL ability. On the other hand we can get the C and use the normal carriers and get less change for delays in the program."

[aaron886]

I'm going to pile on here with what VincentLaw is saying... it's about the amphib assault ships and potentially throwing away years of training and expertise that are involved in a very mobile, very deployable tactical strike force like the Marine Harrier community. It's been proven that the VSTOL concept provides a dispersable/deployable strike and air defense platform, and that better suits the Marines' need than an aircraft designed for long runways.