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Airplane Design Q&A


mikegarrison
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5 hours ago, kerbiloid said:

And in such rare cases you can use planes, until they build an intercontinental railway, and while there are many employed people having salary to buy a plane ticket to support the surviving airlines.

Once the pax loose their jobs, they will be happy with skype and train, so then you would have to rent a whole plane alone as a lucky one with job.

Fast trains beat planes a lot for some hundred km trips between major cities. 
It does not work Paris to Peking on an realistic budget, and obviously not Paris to NY. 
Fast trains also don't work in low population areas with large distances, again budget but much more so than an Paris-Peking vacuum train. 

Now electric planes will probably become common on shorter flights, one benefit is that they would be very cheap to run. Battery swap would work on planes unlike cars as planes are handled by professionals who are used to weird processes loading cargo. 
Take an 4 propeller plane with one battery pack behind each engine. Swap while people are boarding. Pretty much like rearming an fighter bomber. 
As an bonus you can drop an pack with problems. 

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1 hour ago, Bill Phil said:

Switching to hydrogen could be done. It would certainly be expensive and difficult, but it is doable. The real problem is that something like 96% of hydrogen production is from reduction of natural gas or other hydrocarbons... so it wouldn't be a clean fuel.

And it would only make sense for long distance flights where the weight of the fuel (kerosene) is up to (or more) than half the takeoff weight.  And that really interferes with hydrogen's other huge issue: low density.  You'd have a huge amount of plane that would be nothing but fuel tanks.  But for at least those long flights, it would easily beat kerosene (I think.  Methane is cheap, but I'm not sure how efficient the whole process is in turning methane into liquid hydrogen).

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4 minutes ago, wumpus said:

And it would only make sense for long distance flights where the weight of the fuel (kerosene) is up to (or more) than half the takeoff weight.  And that really interferes with hydrogen's other huge issue: low density.  You'd have a huge amount of plane that would be nothing but fuel tanks.  But for at least those long flights, it would easily beat kerosene (I think.  Methane is cheap, but I'm not sure how efficient the whole process is in turning methane into liquid hydrogen).

Drag would be much more of an issue and hydrogen is hard to handle. 
But hydrogen is nice then you go hypersonic, it burn fast and nice for cooling. 
For an private jet, make an VTOL Skylon. Bezos will fund this.  

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14 hours ago, mikegarrison said:

Hydrogen has its own problems. If it's a gas, it has extremely poor volumetric energy density. And if it's a liquid, it still has bad volumetric energy density plus now you have to keep it at <30K. 

Aviation week has been putting out a lot of articles about hydrogen propulsion, mostly for short haul regional turboprops that will be converted to fuel cell electric. For the ones that use gaseous H2, they tend to have it stored at ~300 bar, from what I understand about Hydrogen its not a matter of preventing leaks at that pressure, its about managing them. Is there any way to have that safely and still allow for the general public to board it?

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Just now, insert_name said:

Aviation week has been putting out a lot of articles about hydrogen propulsion, mostly for short haul regional turboprops that will be converted to fuel cell electric. For the ones that use gaseous H2, they tend to have it stored at ~300 bar, from what I understand about Hydrogen its not a matter of preventing leaks at that pressure, its about managing them. Is there any way to have that safely and still allow for the general public to board it?

I don't know. That's not in my area of expertise.

However, I will say that it's no surprise any interest in hydrogen airplanes is focused on short ranges. You just can't get fuel tanks big enough for long-range airplanes. (And at short ranges the big future competitor is going to be battery-electric.)

But another thing to consider is that airplanes that can only fly very short ranges usually do not sell well. Even if an operator flies mostly short flights, they like to be able to use any of their airplanes on any route in their system, and route systems change, so they tend to prefer airplanes with more range than they currently need over those with just enough range to meet current needs.

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12 hours ago, Nightside said:

I’ll only support hydrogen jetliners if the get to take off in a fireball like the Delta 4.

Agree however boring people will veto it. 
Elevators would be much faster if they only had two modes, 2G acceleration or braking and freefall. 
They would also be much more fun :) 
But it would be less fun it carrying an multi layers wedding cake 

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34 minutes ago, mikegarrison said:

I don't know. That's not in my area of expertise.

However, I will say that it's no surprise any interest in hydrogen airplanes is focused on short ranges. You just can't get fuel tanks big enough for long-range airplanes. (And at short ranges the big future competitor is going to be battery-electric.)

But another thing to consider is that airplanes that can only fly very short ranges usually do not sell well. Even if an operator flies mostly short flights, they like to be able to use any of their airplanes on any route in their system, and route systems change, so they tend to prefer airplanes with more range than they currently need over those with just enough range to meet current needs.

Go talk to the VP of Sales to Southwest.  Although I suspect even he wouldn't try to have a 737 variant designed with shorter range.

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29 minutes ago, mikegarrison said:

What point are you trying to make here?

Just that somebody figured out that they could make a ton of money on short range jet transport.  But I doubt they significantly influenced jet design all that much, I doubt there is much point in making the fuel tanks significantly smaller on anything that doesn't already cross continents/oceans.

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2 hours ago, wumpus said:

Just that somebody figured out that they could make a ton of money on short range jet transport.  But I doubt they significantly influenced jet design all that much, I doubt there is much point in making the fuel tanks significantly smaller on anything that doesn't already cross continents/oceans.

I'll just say that every generation of 737 has carried more payload and had longer range than the previous generation did. And 737s are by no means unusual in that regard.

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2 hours ago, JoeSchmuckatelli said:

I think I just found the antithesis to the flying wing 

 

https://www.cnn.com/travel/article/celera-500l-plane/index.html

 

How about this design? 

Mentor pilot over at Youtube had an video about it. 
Its an very low drag plane making it cheap to operate, more so as it uses an IC engine rather than a jet engine while maintaining turboprop speed. 
You can however not scale this up much, they work on an 10 man version. 
 

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1 hour ago, magnemoe said:

Mentor pilot over at Youtube had an video about it. 
Its an very low drag plane making it cheap to operate, more so as it uses an IC engine rather than a jet engine while maintaining turboprop speed. 
You can however not scale this up much, they work on an 10 man version. 
 

I think you mean it uses a reciprocating engine (with pistons). Turbine engines are also "internal combustion". External combustion engines are like steam engines, where the "engine" part (pistons or turbines) uses heat that was created somewhere else (a boiler, a nuclear reactor, etc.).

I don't know much about this plane, but it has straight wings, which indicates it's not intended to fly at normal airliner mach numbers. Regional turboprops often have straight wings, so I would guess it flies at most no faster than those.

Laminar flow aerodynamics has always been attractive, but hard to achieve in service. It's easy to trip a boundary layer into turbulent flow, and once you do there is no "untripping" it. All you can do is scrape it off and start over (by using suction, for instance).

2 hours ago, JoeSchmuckatelli said:

Goldangit - Mike said that way back, and I did not get it.  That thing... surely it did not take off on its own?

I can see it landing, shuttle (brick) like... but that's about it

That's what it did. They were flown in drop tests. One of them had a spectacular crash that became famous for its appearance in the credits for the 1970s television show The Six Million Dollar Man. (The cause of the crash was rather mundane. The pilot became overly concerned about possibly colliding with a nearby helicopter and forgot to extend the landing gear.)

Shuttle-like is appropriate, because they were being studied mainly as re-entry vehicles. The Dream Chaser is a lifting body. The X-37B is nearly a lifting body, but has stubby delta wings.

Blended Wing Body airplanes are sort of hybrids between lifting bodies and flying wings and fuselage/wing airplanes.

 

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2 hours ago, mikegarrison said:

I think you mean it uses a reciprocating engine (with pistons). Turbine engines are also "internal combustion". External combustion engines are like steam engines, where the "engine" part (pistons or turbines) uses heat that was created somewhere else (a boiler, a nuclear reactor, etc.).

I don't know much about this plane, but it has straight wings, which indicates it's not intended to fly at normal airliner mach numbers. Regional turboprops often have straight wings, so I would guess it flies at most no faster than those.

Laminar flow aerodynamics has always been attractive, but hard to achieve in service. It's easy to trip a boundary layer into turbulent flow, and once you do there is no "untripping" it. All you can do is scrape it off and start over (by using suction, for instance).

I'd imagine that laminar flow might be a little easier for this project since:

  • It's a small aircraft
  • The wing looks like it has a pretty high aspect ratio (so low chord)
  • The lack of sweep also keeps the effective chord seen by the relative wind short

All of these keep the length scale, and thus Reynolds number, down across the fuselage and wing chord, respectively... so perhaps it's possible? Still, the boundary layer is a fickle beast, and easily tripped by even small imperfections (icing conditions would likely be problematic).

The manufacturer is claiming, "cruise speeds of up to 460 mph," (which corresponds to Mach 0.67 assuming an outside air temperature of -40 deg C at altitude) on an engine which puts out 500 - 600 bhp with turbocharging (so I'm assuming that the engine power is flat rated up to cruising altitude). Even with beautiful aerodynamics, 460 mph seems difficult to believe with those power numbers... maybe I'm just too used to WWII fighter aircraft behemoths which need ~1,800 bhp to reach such a speed.

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32 minutes ago, Silavite said:

I'd imagine that laminar flow might be a little easier for this project since:

  • It's a small aircraft
  • The wing looks like it has a pretty high aspect ratio (so low chord)
  • The lack of sweep also keeps the effective chord seen by the relative wind short

All of these keep the length scale, and thus Reynolds number, down across the fuselage and wing chord, respectively... so perhaps it's possible? Still, the boundary layer is a fickle beast, and easily tripped by even small imperfections (icing conditions would likely be problematic).

The manufacturer is claiming, "cruise speeds of up to 460 mph," (which corresponds to Mach 0.67 assuming an outside air temperature of -40 deg C at altitude) on an engine which puts out 500 - 600 bhp with turbocharging (so I'm assuming that the engine power is flat rated up to cruising altitude). Even with beautiful aerodynamics, 460 mph seems difficult to believe with those power numbers... maybe I'm just too used to WWII fighter aircraft behemoths which need ~1,800 bhp to reach such a speed.

I don't remember noticing them give any weights, but the lighter the airplane, the less lift they will need and thus the less drag they will create and thus the less thrust they will need.

Anyway, I guess we'll see.

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1 hour ago, Entropian said:

Can these new hydrogen-fueled planes store LH2 and combust it with oxygen from the atmosphere to get a better fuel fraction in the tanks?  Does the extra cryogenic equipment weight cancel out the fuel density?

If you are asking whether they are airbreathing, the answer is yes.

As for the tanks, yes, that is one of the issues. Fuel today is not pressurized. So fuel tanks can be any old shape. If the fuel is pressurized then the tanks have to be pressure vessels. And if it is cryogenic, they have to be well-insulated pressure vessels.

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Let's talk about lift. (But keep in mind that it's been about 30 years since my last aerodynamics class, so there may be some small errors mixed in here....)

Lift happens because the airplane pushes down on the air. The basic idea of a wing is that if you move this long line of air-pushing through enough air, it can hold up the airplane. The faster you move it, the more air it moves through, meaning the more air it pushes down, meaning the more lift it makes. Also the faster you move it, the harder it pushes the air. So conceptually, this is why lift is proportional to velocity squared -- one of the velocities is for how much air you move through, and the other velocity is for how hard you push that air.

When aerodynamicists calculate lift, they often think in terms of circulation. It looks complicated, but it's actually not so bad. https://en.wikipedia.org/wiki/Circulation_(fluid_dynamics) 

The air on the top side of the wing is moving faster than the air on the bottom side of the wing. If you subtract out the average speed of all the air, you see that the difference from average is made up of air moving from front to back over the top, and from back to front over the bottom. So the airflow past the wing is made up of the sum of the average speed past the wing plus this additional circular velocity (the "circulation"). The lift is directly proportional to the circulation.

Circulation-theory-of-lift-inviscid-flow

There's a simplification that can be made about the air flow that says vortices can never stop or stop. They just go on forever. That's obviously not really true, because viscous effects will eventually damp them out, but it's true enough for our purposes here.

And now you see why lifting wings always leave a vortex. That circulation that is moving around the wing is making the lift, and since it can't just stop at the wingtip, it "sheds" and leaves a vortex in the air. When it sheds it doesn't keep going out sideways, though. It sort of turns the corner, and behind the airplane you get a vortex on either side pushing down in the middle and lifting up on the outsides. (This is why geese fly in V pattern, so they can always be flying in the updraft of the lift vortex from the goose in front of them.)

But ... since lift is proportional to circulation, and since lift is higher at the root than the tip, that means the circulation has to be decreasing from root to tip. And since the vorticity creates the circulation, that means the vorticity has to be decreasing from root to tip. Thus, the vorticity sheds as the lift changes on the wing. So in fact it doesn't all shed from the tip. It sheds all along the wing, leaving a "sheet" of vorticity. Then this vortex sheet all along the wing on either side sort of joins together (we usually say "rolls up") and becomes one pair of large vortices.

Anyway, the point here is that while wingtips are important, vorticity is actually shed all along the wing.

Another consequence is that because the airplane is leaving this swirling vortex of air behind it, that must mean it is leaving energy in the air. And it is. So the energy left behind in these vortices is what "lift induced drag" is.

One of the ways that CFD is used to calculate lift and drag from a wing is called the "vortex lattice method". It uses this idea of a constant shedding of vorticity along the wing to do computational CFD. https://en.wikipedia.org/wiki/Vortex_lattice_method

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There are actually two kinds of vorticity going on, both to do with circulation. Wingtip vortices is one, but you also have a vortex sheet layer. A totally laminar flow cannot generate lift, because it cannot produce circulation in the air. In order for there to be net circulation, there has to be a layer of turbulent air separating flow above and bellow the wing. So even a theoretical infinitely long wing, the kind that can't possibly have wing tip vortices, because it lacks wing tips, and one that produces even lift along its entire infinite length, would still produce a turbulent vortex layer behind it.

This is also why the trailing edge is sharp. It's not actually the most aerodynamic shape, as you lose energy to that vortex layer, but you need vortex sheet for lift, and sharp trailing edge is necessary for Kutta condition which you need to shed the vortes sheet.

Far behind the airplane, the wingtip vortices and the vortex sheet merge into a single wake.

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6 hours ago, K^2 said:

This is also why the trailing edge is sharp. It's not actually the most aerodynamic shape, as you lose energy to that vortex layer, but you need vortex sheet for lift, and sharp trailing edge is necessary for Kutta condition which you need to shed the vortes sheet.

Yes, the Kutta condition is illustrated in the picture I showed. With no Kutta condition, the air does not leave the wing at the trailing edge, and there is no lift (first diagram, labeled FL'=0). But add the circulation (second diagram labeled Γ)  and that satisfies the Kutta condition in the final diagram. The label for that one reads "lift = air density * freestream velocity * circulation".

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A topic that has been coming up due to SpaceX: landing gear.

The main purpose of landing gear is to safely land the airplane. This is obvious, right? But some planes don't have landing gear. Seaplanes have floats. Flying boats have hulls. (Old joke: a flying boat is not a very good boat and it's not very good at flying either.) Some drones often don't have gear at all, and are just caught in a net. The Wright Flyer just had skids.

Some sailplanes have a single wheel (or just a skid). There have been airplanes with "bicycle gear" -- two wheels in line with each other. They usually have small outriggers to keep the wings off the ground bit otherwise don't support the weight of the airplane. (The bicycle configuration is not very popular because it is quite difficult to rotate the plane for takeoff.)

The B-52 had bicycle gear, 2 posts up in front and 2 posts in the back. It also had long, flexible wings. This made it vulnerable to wingstrikes if it tried to use the usual yawing technique to "de-crab" when landing in crosswind. So they just used crabbable landing gear. See how the airplane in this video rolls down the runway while yawed.

The most popular landing gear configuration for a long time is still known as "conventional" gear, even though it is no longer very conventional to use it. It's also called "taildragger", because it involves two main wheels ahead of but near the CG and one small tail wheel just to keep the tail off the ground. It does give a lot of clearance for a nose prop, but it can be hard for the pilot to see over the nose when taxiing. Also, it is vulnerable to this sort of classic accident (picture from the best airplane museum in the world, the National Museum Of The US Air Force):

150316-F-IO108-005.JPG

The most common configuration now is the tricycle gear, where the main gear is a little behind the CG and a steerable nose wheel is up front. This configuration is easier to steer on the ground and also keeps the airplane cabin level. It can be a little vulnerable to tailstrikes on takeoff rotation, especially if the airplane is long and the gear is short.

Some planes, like the 747 or A380, have more than two sets of main gear. These are functionally basically the same as regular tricycle gear -- they just spread the weight out a bit so as to not damage the runway and taxiway.

Landing gear can be wheels, skis, or even caterpillar treads. The main concern is spreading out the weight of the airplane appropriately for the surface that is being landed on.

Landing gear is draggy, so it's usually worth the weight and complexity and cost to make it retractable. But finding room for it is difficult. Many airplanes have found themselves restricted by their landing gear design. Short gear is light and keeps the airplane close to the ground (especially favored by general aviation, making it easier to get in and out of the airplane without external support equipment). But it doesn't leave much room under the wings if the airplane is a low-wing design. So that takes away a favorite location for the engines, or at least limits that location. The 737 was famously designed around short gear and low-bypass JT-8D engines. They have had issues fitting high-bypass engines under the wing ever since.

South_African_Airlink_Boeing_737-200_Adv

Landing gear are often so integral to the basic structural design that they simply can't be changed much. It requires complicated engineering to try to fit bigger, longer gear into the same space.

So what else does the gear have to do? It has to stop the airplane. A lot of wheel braking technology used today was invented for airplane landing gear. Anti-lock brakes, for instance, were first developed for aviation. Disc brakes were first widely used on airplanes. Carbon-carbon disc brakes have been widely used in aviation since being introduced on the Concorde. Airplanes are heavy, moving fast, and need to slow down quickly, which means a *lot* of energy has to go into those brakes. But landing is not actually the design case for airplane brakes. Their toughest test is a refused takeoff, where the airplane is almost at rotation speed and then has to stop without running off the end of the runway. And you can't assume any thrust reverse, either, because probably the reason you aren't taking off is that something bad just happened to an engine.

Many airplanes have brake cooling fans to help cool the brakes after landing. This is in order to have reasonably fast turnaround times. If the brakes are still hot by the time you want to take off again, then they have less energy capacity available, and so they can't fully perform in a refused takeoff scenario. So they want the brakes cool for takeoff, which leads to brake fans.

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