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Airplane Design Q&A


mikegarrison
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27 minutes ago, StrandedonEarth said:

Does the reduced wingtip vortices on winglet-equipped planes allow airports to have the planes follow each other down a little closer?

Legally? No.

Physically? Lift creates vortices. That's what it does. That's why there is lift. No vortices, no lift. So a Cessna 150 coming in behind an A380 is always going to have an issue, winglets or no winglets.

Edited by mikegarrison
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Another question, since I seem to be in that kind of mood tonight. The vast majority of aircraft with wing mounted engines have the engines mounted below the wing. Why not mount engines above the wing? It would reduce noise, reduce the risk of FOD, and allow for a smaller undercarriage. The biggest disadvantage to me seems like increased difficulty of maintenance, but the advantages could be worthwhile for an aircraft operating out of minimally prepared areas (such that undercarriage sturdiness and resistance to FOD would be at a premium).

Some examples of designs with an above wing engine mount:

I'm sure I missed some factors here, so have at it.

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1 hour ago, Silavite said:

Another question, since I seem to be in that kind of mood tonight. The vast majority of aircraft with wing mounted engines have the engines mounted below the wing. Why not mount engines above the wing? It would reduce noise, reduce the risk of FOD, and allow for a smaller undercarriage. The biggest disadvantage to me seems like increased difficulty of maintenance, but the advantages could be worthwhile for an aircraft operating out of minimally prepared areas (such that undercarriage sturdiness and resistance to FOD would be at a premium).

Some examples of designs with an above wing engine mount:

I'm sure I missed some factors here, so have at it.

You are correct about maintenance being a big issue. But another big issue is that the airflow on the top of the wing is much more critical than the airflow on the bottom of the wing. Air on the top of the wing is moving faster and has an adverse pressure gradient (it wants to separate away from the wing). Air on the bottom of the wing moves slower and has a favorable pressure gradient. That's why missiles, engines, flap fairings, etc. etc. are located on the bottom of the wing.

Those two jets you mentioned didn't have room under their wing for engines. (That's a big reason why bizjets rarely have wing-mounted engines. They want short gear so the door can double as a staircase.)

Edited by mikegarrison
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Still am puzzled (actually, for decades).

How many wings are here?
images?q=tbn:ANd9GcQFyphbXkWErGp___YBPYk

They say: "wing", with the fuselage in its middle.

How many wings are here?
images?q=tbn:ANd9GcSIxIJDtfbzdB0IDkWMKRE

They say: "wings", and it waves with them.

So, does it mean that a bird has just one wing and cyclically bends it by throwing the fuselage up and down?

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4 minutes ago, kerbiloid said:

Still am puzzled (actually, for decades).

How many wings are here?
images?q=tbn:ANd9GcQFyphbXkWErGp___YBPYk

They say: "wing", with the fuselage in its middle.

How many wings are here?
images?q=tbn:ANd9GcSIxIJDtfbzdB0IDkWMKRE

They say: "wings", and it waves with them.

So, does it mean that a bird has just one wing and cyclically bends it by throwing the fuselage up and down?

If you go to the fried chicken place and say "I'd like a wing", what do you get?

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12 hours ago, kerbiloid said:

If tell to the pax in a plane, that the plane has just one wing, what do you hear?

If an plane has two wings its an biplane. Planes still have an left and right wing. 
Now how many wings does this plane have: 1,2 or many?
300px-Voyager_aircraft.jpg

In short languages often don't make sense they are evolving and that we agree on. 
Back in the start of flight biplanes was most common and you had planes with 3 or more. Once monoplanes took over planes had one wing. 

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43 minutes ago, magnemoe said:

If an plane has two wings its an biplane. Planes still have an left and right wing. 

The biplane has upper and lower wings, not left and right.

43 minutes ago, magnemoe said:

Now how many wings does this plane have: 1,2 or many?

Officially probably 1, as the small plane on the nose of the plane is probably something like canards or so. I.e. an attitude tool, not a lifting force thing.

43 minutes ago, magnemoe said:

In short languages often don't make sense they are evolving and that we agree on. 

In Russian the wings are counted exactly same way.

Upd.
And as now the new remake of Duna is coming soon, a question:

How many wings does an ornithopter have?

As obviously unlike the birds, this is a technical object with official manual. What will they write in the manual: 1 or 2?

Edited by kerbiloid
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1 hour ago, kerbiloid said:

The biplane has upper and lower wings, not left and right.

Officially probably 1, as the small plane on the nose of the plane is probably something like canards or so. I.e. an attitude tool, not a lifting force thing.

In Russian the wings are counted exactly same way.

Upd.
And as now the new remake of Duna is coming soon, a question:

How many wings does an ornithopter have?

As obviously unlike the birds, this is a technical object with official manual. What will they write in the manual: 1 or 2?

Agree its probably one wing, now having an forward stabilizer let it create lift who is very nice then flying around the world without refueling. 

And again language is not accurate, pair of trousers well trying to pick up an second one after paying will not work out well :) 
It might have an backstory  way older than biplanes but way way weirder I say.

And to counter myself, an plane landing with one wing.

 

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A biplane is really just one wing with multiple airfoil elements. Biplanes were mainly a response to the structural capabilities of the day. They needed that truss structure to make wings that were light enough to fly and strong enough to carry the weight of the airplane. While there were some famous tri-planes, remember that WW1 was only a bit more than 10 years after the flight of the first powered airplane. People hadn't yet figured out that stacking more airfoils directly on top of each other is very inefficient.

It does give very low wing loading, though. Wing loading is the total lift divided by the wing area, and with three times the wing area, that's three times less wing loading. Wing loading is an important characteristic for low-speed flight (this is why flaps extend and increase wing area for takeoff and landing) and also maneuverability. Bi- and tri-planes were less efficient but more agile, and in the skies of WW1 it was the latter that was important for the newly discovered tactic of "dogfighting".

Edited by mikegarrison
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They should implement a proper termilnology for the wing-like elements.

Like they did.
topgallant-sail.jpg

 

Say, the thing above
300px-Voyager_aircraft.jpg

should have mainwing, topmainwing, and jib)

The same for biplanes, triplanes, etc.

***

A strange fact.
The Russian names for sails are copypasted from Dutch, so I was expecting that the English ones sound similar, as they even don't need to copypaste, two cognate languages and same Northern Sea.

But instead of simple and clear "bramzeil" and "boom-bramzeil" they call it "topgallant sail" and "royal sail". And others also have nothing common at all.

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On 9/26/2020 at 8:21 PM, mikegarrison said:

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?

The problem is typically described as "fuel leaking due to need for expansion in the fuel tanks", so I really don't know how hot the interior of the plane was (probably still top secret) nor the fuel tanks.  But designing a cooling system with no external heat sink is a challenge and likely means that only the cockpit and other absolutely critical areas get cooled (possibly just the flightsuit).  I wonder if it would make sense/even be possible  to have "clean" intakes just for cooling air?  I think that is central to the Sabre engine... but I'm not claiming much about Sabre's effectiveness.

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30 minutes ago, wumpus said:

The problem is typically described as "fuel leaking due to need for expansion in the fuel tanks", so I really don't know how hot the interior of the plane was (probably still top secret) nor the fuel tanks.  But designing a cooling system with no external heat sink is a challenge and likely means that only the cockpit and other absolutely critical areas get cooled (possibly just the flightsuit).  I wonder if it would make sense/even be possible  to have "clean" intakes just for cooling air?  I think that is central to the Sabre engine... but I'm not claiming much about Sabre's effectiveness.

Well, the flash point of Jet-A is 38C. The flash point of JP-4 is only -18C. The flash point of JP-7 is 60C. The US military generally used JP-4, but had JP-7 developed specifically for the SR-71.

For reference, gasoline has a flash point of -40C and diesel fuel has a flash point of 50-100C. This is why gasoline engines use spark ignition but diesels use compression auto-ignition.

That's not to say the fuel in the tanks only got up to 60C, though. According to the Wikipedia article for the J58 engine, the fuel was at about 600F (316C) when it was delivered to the fuel nozzles.

Edited by mikegarrison
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On 9/26/2020 at 4:41 AM, mikegarrison said:

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

hydrogen may be a better option for energy storage in aircraft than batteries. lets ignore the problems with hydrogen manufacture for a second. you need electrical power to recharge batteries as much as you do to make hydrogen though the latter is definitely going to be less efficient. when compared with fossil fuels, those have to be trucked from the refineries to the airports, where as hydrogen manufacture can take place close to the tarmac using local grid and water supply. the fossil fuels can be burned at the power plants where you get carbon capture and waste heat scavenging. there is a lot we can do to clean up the power grid in both the short and long term. meanwhile jet engines dump raw exhaust directly into the atmosphere. im curious what airbus can do with its hydrogen concepts. 

Edited by Nuke
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35 minutes ago, Nuke said:

hydrogen may be a better option for energy storage in aircraft than batteries. lets ignore the problems with hydrogen manufacture for a second. you need electrical power to recharge batteries as much as you do to make hydrogen though the latter is definitely going to be less efficient. when compared with fossil fuels, those have to be trucked from the refineries to the airports, where as hydrogen manufacture can take place close to the tarmac using local grid and water supply. the fossil fuels can be burned at the power plants where you get carbon capture and waste heat scavenging. there is a lot we can do to clean up the power grid in both the short and long term. meanwhile jet engines dump raw exhaust directly into the atmosphere. im curious what airbus can do with its hydrogen concepts. 

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. 

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56 minutes 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. 

the airbus concepts use cryofuel, storing it in the fuselage. it has 3x the energy density of jet than jet fuel by weight despite being lower in volume, which provided you dont lose it all in additional weight of the tankage is better suited to an aircraft. the coldness of the fuel can also be use in a precooler if you wanted a hypersonic aircraft or as the cold side of any thermodynamic components, its effectively another form of energy storage.

Edited by Nuke
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59 minutes ago, Nuke said:

the airbus concepts use cryofuel, storing it in the fuselage. it has 3x the energy density of jet than jet fuel by weight despite being lower in volume, which provided you dont lose it all in additional weight of the tankage is better suited to an aircraft. the coldness of the fuel can also be use in a precooler if you wanted a hypersonic aircraft or as the cold side of any thermodynamic components, its effectively another form of energy storage.

Rockets already have some trouble making use of hydrogen's high energy density due to the reduced mass ratios inherent in hydrogen tankage. One could argue that volumetric density is even more important for aircraft than spacecraft because aircraft spend all their time fighting drag, and such large fuel tanks would add substantially to an aircraft's wetted area (hydrogen would lower both mass ratio and L/D). Hydrogen also leaks profusely and can cause embrittlement in certain metals. I'm certainly no expert in this area, but I think that a hydrocarbon based biofuel is a better bet than hydrogen for aircraft... though I wouldn't mind being proven wrong! Flying on LH2 sounds very cool (pun intended :D).

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Tu-155 with 1 cryoengine of 3.
100 flights, including 5 on LH2, others - on natural gas.

https://translate.google.com/translate?hl=ru&sl=ru&tl=en&u=https://ru.wikipedia.org/wiki/Ту-155

Also requires LN2 (to replace the air in the dedicated compartments) and LHe systems.

Afair,  liquid propane-buthane mixture is considered more actual due to its storage characteristics, at least for the helicopters.

 

P.S.

Suggestion: Let's put a bag on top and turn into ballon as emergency option.

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I assure you that everybody has studied cryoplanes, but they just don't pencil out. Commercial airliners just don't have the available volume they would need to use it.

Kerosene is at a really sweet spot for energy density -- most other potential aviation fuels are either less energy dense by volume (needs bigger tanks) or less energy dense by mass (more weight for the airplane).

These factors are currently driving the industry very hard toward trying to develop sustainable "drop-in" bio-jet fuel that would be available in large quantities. As a bonus, existing airplanes could also use it ("drop-in" means that it could be freely used instead of or mixed in with current jet fuel).

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Which in turn brings us to the question: will the civil aviation stay self-sustaining in the perspective of 2040s with the current trends of electrification of the transport infrastructure, or they will be replaced with the matured high-speed railways in all significant directions, making the problem of cryoplanes obsolete.

Edited by kerbiloid
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2 minutes ago, kerbiloid said:

Which in turn brings us to the question: will the civil aviation stay self-sustaining in the perspective of 2040s with the current trends of electrification of the transport infrastructure, or they will be replaced with the matured high-speed railways in all significant directions, making the problem of cryoplanes obsolete.

Let me know when I can get that train ticket from Seattle to London. It ought to be an interesting journey!

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4 minutes ago, mikegarrison said:

Let me know when I can get that train ticket from Seattle to London. It ought to be an interesting journey!

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.

Edited by kerbiloid
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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.

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