totm october 2020 Airplane Design Q&A

[mikegarrison]

So there was a question about winglets in the "questions that don't deserve their own thread" thread.

Winglets are part of a larger class of "wingtip devices". As to why wingtip devices exist, well, it's complicated.

The first thing to understand is the concept of 2D aerodynamics versus 3D aerodynamics. An infinitely long wing is 2D aerodynamics. It has no losses due to the wing ending in a wingtip. But real wings aren't infinitely long. They end. That's where 3D aerodynamics comes in.

3D aero is complicated. The center of the wing makes more lift than the edges of the wing, but if there is a fuselage in the middle that tends to mess up the very center of the wing. The tips have to reduce their lift ultimately to zero, because they are the end of the wing! This is known as "lift distribution", and it's important for efficiency.

The overall wingspan compared to wing area is known as aspect ratio (span^2/area). This non-dimensional number basically describes how long and skinny (sailplane) or short and fat (delta wing) a wing is. The ultimate in small aspect ratios is a lifting body. All else being equal (which it isn't), a higher aspect ratio has better efficiency.

Putting a wingtip device on the end of a wing (winglet, swept wingtip, split winglet, etc.) improves the effective aspect ratio. However, what's the ideal angle for a winglet? 0 degrees from horizontal. In other words, increasing aspect ratio is always better than a winglet of the same length.

Note also that nature understands wingtip devices:

So, if increasing aspect ratio is always better than a winglet, why do we have winglets?

Well, there are limits to aspect ratio. Wings have to end somewhere. And the wider the wingspan, the more "bending root moment", so the structure has to be stronger and heavier. Eventually it doesn't pay anymore to extend the wing even further.

And once a design has been made, changing it is hard. Adding a winglet is not exactly "easy", but it's easier than changing a whole wing.

Also, take the 737. It has obvious *huge* winglets for its size. Why? Well, really this becomes obvious if you know that airports have different categories for gates. One category (the one that 737s have always fit into) is for airplanes with wingspans less than 36 meters. The 737MAX wingspan is 35.8 meters. So if they want to fit into the same gates 737s have always used, they can't make the wingspan any bigger. But they can add winglets. So....

Some airplanes, like those designed for aircraft carriers or the new 777X, have folding wingtips. This is so they get the benefits of that lovely extra wingspan when flying, but can still fit into smaller spaces on the ground. However, that adds weight, cost, complexity, failure modes, etc.

Finally, there are marketing issues. Many people think winglets look spiffy. And they are a great place to paint the airline logo where it can be seen by the passengers from the inside. Branding, branding, branding.

------------

This thread created with the hope of discussing many questions about airplane design, not just winglets.

[JoeSchmuckatelli]

Dude - as the guy who asked the question - thanks! 

This is a ton of info, and I can't wait to chew on it for a while. 

Interim question: 

Given "The center of the wing makes more lift than the edges of the wing, but if there is a fuselage in the middle that tends to mess up the very center of the wing. The tips have to reduce their lift ultimately to zero, because they are the end of the wing! This is known as "lift distribution", and it's important for efficiency" Why don't we see flying wing designs - except in Popular Mechanics? 

[mikegarrison]

28 minutes ago, JoeSchmuckatelli said:

Why don't we see flying wing designs - except in Popular Mechanics? 

Keeping them stable in flight was pretty nearly impossible until the advent of fly-by-wire. The B-2 bomber is a flying wing. (But it has that shape more for stealth reasons than aerodynamic reasons.)

They also have other disadvantages. In order to carry any decent amount of volume the wing would have to be very thick, which is generally enough to offset the aerodynamic advantages of not carrying around a fuselage.

[magnemoe]

47 minutes ago, mikegarrison said:

Keeping them stable in flight was pretty nearly impossible until the advent of fly-by-wire. The B-2 bomber is a flying wing. (But it has that shape more for stealth reasons than aerodynamic reasons.)

They also have other disadvantages. In order to carry any decent amount of volume the wing would have to be very thick, which is generally enough to offset the aerodynamic advantages of not carrying around a fuselage.

This, now add safety factors. The reason why modern planes gas an so large tail is because engine out at low speed, less an issue with 4 engines but most modern planes has two large engines and if one dies during takeoff tail has to compensate. 
Not an issue for the B2 as its an military plane with other safety concerns like not getting shot down and the pilots has ejection seats. 

Related, how do you evacuate an flying wing fast? Assume belly landing. 

[Silavite]

What literature and rules-of-thumb are available for fuselage shapes, wing fairings, and general streamlining?

[mikegarrison]

21 minutes ago, Silavite said:

What literature and rules-of-thumb are available for fuselage shapes, wing fairings, and general streamlining?

Any basic aerodynamic textbook would do, probably. A lot of research was done on this about 100 years ago, and it's all in the open domain. All the NACA work for instance.

There are also lots of codes for this. https://en.wikipedia.org/wiki/XFOIL comes to mind for me, because it was written by one of my former professors, Mark Drela.

[Silavite]

30 minutes ago, mikegarrison said:

Any basic aerodynamic textbook would do, probably. A lot of research was done on this about 100 years ago, and it's all in the open domain. All the NACA work for instance.

There are also lots of codes for this. https://en.wikipedia.org/wiki/XFOIL comes to mind for me, because it was written by one of my former professors, Mark Drela.

You'd be surprised at the lack of such information in the basic aerodynamic textbook my university uses (Fundamentals of Aerodynamics by Anderson), though my understanding of what a "basic aerodynamic textbook" entails may be off. What other aerodynamics books would you recommend? Thanks for the suggestion on NACA (I never cease to be amazed by the breadth and depth of research they did). This looks like a good start.

[kerbiloid]

How many wings does a bird have?

[sh1pman]

Is this a realistic design? It looks like it would be a nightmare to pilot. What are the advantages of this shape vs. traditional shape?

[Hannu2]

It is realistic if they will build it commercially. Probably almost anything can work technically but if they do not give significant benefits, they fail commercially and be never used.

I think piloting is not a problem nowadays. It will have computer assisted fly by wire system in any case. Computers can fly unstable planes which are impossible for human pilots. If it can be developed and produced at reasonable costs, save significant amount of energy (or be otherwise environmentally friendly)and be safe to travel it will probably come in production. It may take couple of decades because aviation officials are so extremely conservative but at the end money talks. But I will say it realistic after all this have realized.

 

[mikegarrison]

1 hour ago, sh1pman said:

Is this a realistic design? It looks like it would be a nightmare to pilot. What are the advantages of this shape vs. traditional shape?

You have to realize that everybody wants to figure out the best replacement for the "tube and wing" airliner, but so far nobody has anything except concepts that look interesting in cgi renders. A lot of people have been looking at engines mounted where those are. I've been told the CFD studies say there are some advantages to that. I think the idea is that the engines can sort of "fill in" the momentum wake that is left behind the fuselage.

See also, for instance the "D8" concept:

https://en.wikipedia.org/wiki/Aurora_D8

Also, everybody wants to design electric airliners, but battery technology isn't anywhere close to being good enough yet.

[kerbiloid]

This wide fuselage looks great for the paired turrets like on B-36

Spoiler

or in the animation I'm fond of.

Spoiler

 

Also it looks like a place for the bomb bay like in the latter.

Though, I'm not sure if that horizontal bar on top is safe for the rear shooting, wouldn't it lock the guns all the time.

[Guest]

3 hours ago, mikegarrison said:

You have to realize that everybody wants to figure out the best replacement for the "tube and wing" airliner, but so far nobody has anything except concepts that look interesting in cgi renders. A lot of people have been looking at engines mounted where those are. I've been told the CFD studies say there are some advantages to that. I think the idea is that the engines can sort of "fill in" the momentum wake that is left behind the fuselage.

To be fair, there were more than a few military designs which looked that radical. In fact, except for transports, most new military planes have a more interesting shape. I think that the biggest problem is that most aviation regulations were designed with the assumption that the aircraft regulated were of the "tube and wing" design. Same with airport facilities. Anything but a slight variation on the concept will face serious regulatory and infrastructure challenges.

[Meecrob]

19 hours ago, mikegarrison said:

Note also that nature understands wingtip devices:

 

Its so cool you can see the airflow with the small feathers close to the owl's wingroot. The owl seems to be turning camera left by looking at the (small) amount its feathers are deflected. It feels weird talking about a living creature as if it was a mechanical object.

[insert_name]

why have ramjets and scramjets not seen much use on manned aircraft?

[Nightside]

54 minutes ago, Meecrob said:

Its so cool you can see the airflow with the small feathers close to the owl's wingroot. The owl seems to be turning camera left by looking at the (small) amount its feathers are deflected. It feels weird talking about a living creature as if it was a mechanical object.

Remember also that owls are optimized for silent flight. I wonder what features of the wing allow this?

[Meecrob]

Presumably reducing their induced drag as that results in noise...and looking at their high aspect ratio wings, the loud owls died out.

[wumpus]

1 hour ago, insert_name said:

why have ramjets and scramjets not seen much use on manned aircraft?

The Blackbird needed ramjet action to hit mach 3.  I'm not even sure the compressors were running at that speed, but most of the compression was thanks to aero.  Aside from that, I can't think of a single reusable ramjet.  Scramjets are still in the research phase: I'm pretty sure that X-43 was reasonably successful in the mach 6.8 range, and could presumably be a third stage of an effective rocket.  But that will be used if and only if it is worth designing a rocket around a few tens of thousands of dollars (per launch) in fuel savings.  Right now that is some really high fruit.

Ramjets have seen much more use, but need some other means of getting them up to the mach .9 or so they need to function (and of course crewed aircraft would preferably not deadstick it back home).  As far as I know, only the Blackbird had a reusable ramjet engine, typically they aren't, and probably get away with much cheaper ablative engine liners.  Not to mention just how hard the Blackbird engine design was (no idea if computers would make the design easier.  They certainly won't help at all in making the thing easier to machine).  I'd like to think a modern blackbird could be made that used normal jet fuel, and possibly a steel [stainless?] body, but most of the problems would be just as hard as in the 1960s.

[DerekL1963]

4 hours ago, insert_name said:

why have ramjets and scramjets not seen much use on manned aircraft?

Ramjets want high speeds and high altitudes for maximum efficiency (or even decent usability), and that's a pretty niche application.

Scamjets are still in the process of consuming flaming truckloads of money in hopes that they'll someday be a) practical, and b) useful.  Given that they also (like ramjets) want extremely high speeds, I have my doubts about "b".

[mikegarrison]

7 hours ago, insert_name said:

why have ramjets and scramjets not seen much use on manned aircraft?

They are useless unless you are already moving. That means you need to carry two sets of engines. Or you need some kind of complicated multi-mode engine. And generally they only work better than turbojets at supersonic or hypersonic speeds.

[Silavite]

I understand that the vast majority (all?) of gas turbine engines today use fixed pitch compressor/turbine blades. Would it be practical to make a variable pitch system?

[mikegarrison]

5 hours ago, wumpus said:

I'd like to think a modern blackbird could be made that used normal jet fuel, and possibly a steel [stainless?] body, but most of the problems would be just as hard as in the 1960s.

The basic problem of heat build-up is just physics. True today as it was 70 years ago. How are you going to keep kerosene from cooking in a plane that is sustaining Mach 3+ for hours at a time?

4 minutes ago, Silavite said:

I understand that the vast majority (all?) of gas turbine engines today use fixed pitch compressor/turbine blades. Would it be practical to make a variable pitch system?

Not really needed.

The compressors are paired rotor/stator/rotor/stator/etc., and it's much easier to make the non-rotating stators variable pitch. So they just handle all the adjustability in the stators.

[Silavite]

 

8 minutes ago, mikegarrison said:

Not really needed.

The compressors are paired rotor/stator/rotor/stator/etc., and it's much easier to make the non-rotating stators variable pitch. So they just handle all the adjustability in the stators.

Ah, that's a good point. What about the fan at the front? I'd imagine that air bypassing the core would benefit from a variable pitch fan.

[mikegarrison]

27 minutes ago, Silavite said:

Ah, that's a good point. What about the fan at the front? I'd imagine that air bypassing the core would benefit from a variable pitch fan.

It's not been felt to be necessary, I guess.

My only experience working blade aero was in the turbine. And that was over 30 years ago, now. Maybe somebody else here can answer that. In the turbine all the blades and nozzles have to be cooled, so you aren't going to find a bunch of fancy adjustable pitch mechanisms back there. (Nozzles being the stationary airfoils and blades being the rotating airfoils.)

Edited September 27, 2020 by mikegarrison

[Exploro]

On 9/25/2020 at 5:44 PM, mikegarrison said:

So there was a question about winglets in the "questions that don't deserve their own thread" thread.

Winglets are part of a larger class of "wingtip devices". As to why wingtip devices exist, well, it's complicated.

The first thing to understand is the concept of 2D aerodynamics versus 3D aerodynamics. An infinitely long wing is 2D aerodynamics. It has no losses due to the wing ending in a wingtip. But real wings aren't infinitely long. They end. That's where 3D aerodynamics comes in.

3D aero is complicated. The center of the wing makes more lift than the edges of the wing, but if there is a fuselage in the middle that tends to mess up the very center of the wing. The tips have to reduce their lift ultimately to zero, because they are the end of the wing! This is known as "lift distribution", and it's important for efficiency.

The overall wingspan compared to wing area is known as aspect ratio (span^2/area). This non-dimensional number basically describes how long and skinny (sailplane) or short and fat (delta wing) a wing is. The ultimate in small aspect ratios is a lifting body. All else being equal (which it isn't), a higher aspect ratio has better efficiency.

Putting a wingtip device on the end of a wing (winglet, swept wingtip, split winglet, etc.) improves the effective aspect ratio. However, what's the ideal angle for a winglet? 0 degrees from horizontal. In other words, increasing aspect ratio is always better than a winglet of the same length.

Note also that nature understands wingtip devices:

So, if increasing aspect ratio is always better than a winglet, why do we have winglets?

Well, there are limits to aspect ratio. Wings have to end somewhere. And the wider the wingspan, the more "bending root moment", so the structure has to be stronger and heavier. Eventually it doesn't pay anymore to extend the wing even further.

And once a design has been made, changing it is hard. Adding a winglet is not exactly "easy", but it's easier than changing a whole wing.

Also, take the 737. It has obvious *huge* winglets for its size. Why? Well, really this becomes obvious if you know that airports have different categories for gates. One category (the one that 737s have always fit into) is for airplanes with wingspans less than 36 meters. The 737MAX wingspan is 35.8 meters. So if they want to fit into the same gates 737s have always used, they can't make the wingspan any bigger. But they can add winglets. So....

Some airplanes, like those designed for aircraft carriers or the new 777X, have folding wingtips. This is so they get the benefits of that lovely extra wingspan when flying, but can still fit into smaller spaces on the ground. However, that adds weight, cost, complexity, failure modes, etc.

Finally, there are marketing issues. Many people think winglets look spiffy. And they are a great place to paint the airline logo where it can be seen by the passengers from the inside. Branding, branding, branding.

------------

This thread created with the hope of discussing many questions about airplane design, not just winglets.

The use of winglets has a lot to do with the reduction of induced drag caused by wing tip vortices, improving efficiency and performance overall. Dryden states winglets allow for 7% increase in mileage.