totm october 2020 Airplane Design Q&A

[magnemoe]

17 hours ago, mikegarrison said:

High-bypass turbofans. It would probably look more like a 777 than a B-52.

The B-52 replacement was first supposed to be faster planes, then stealthier planes. But the current plan is to keep B-52s flying for 100 years.

Yes, think main problem making an bomber version of an plane is the wing box don't like bomb bay doors. You could put an bay in front and behind, think the B1 has this so its not an new issue. 

[mikegarrison]

2 hours ago, magnemoe said:

Yes, think main problem making an bomber version of an plane is the wing box don't like bomb bay doors. You could put an bay in front and behind, think the B1 has this so its not an new issue. 

True, it might be more like a high-wing version of a 777.

Bombers are often mid-wing or high-wing. Low-wing is especially favored by planes with passenger cabins.

[mrfox]

Would it really be a large twin like a 777 vs a quad? With 3 or 4 engines, an engine failure allows enough margin for the mission to continue (fail operational), vs a most likely mission abort in the case of losing an engine in a twin.

Of course, the 4 engine P-3 is being replaced by the twin P-8. KC-135s by KC-46s, and so on. So I guess it’s no longer seen as such a big deal to go with 2. 
 

I must be getting old. The latest gen of aircraft being fielded - both civil and military, just doesn’t seem generate as much intrigue nor excitement as they used to.

[mikegarrison]

11 minutes ago, mrfox said:

Would it really be a large twin like a 777 vs a quad? With 3 or 4 engines, an engine failure allows enough margin for the mission to continue (fail operational), vs a most likely mission abort in the case of losing an engine in a twin.

Of course, the 4 engine P-3 is being replaced by the twin P-8. KC-135s by KC-46s, and so on. So I guess it’s no longer seen as such a big deal to go with 2. 
 

I must be getting old. The latest gen of aircraft being fielded - both civil and military, just doesn’t seem generate as much intrigue nor excitement as they used to.

I don't really know. I'm much more familiar with the design requirements of civil airliners.

[Pds314]

On 9/26/2020 at 9:51 PM, mikegarrison said:

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.

Ehhh... Not sure that's a correct characterization of lift. Where is the vortex on a wing with infinite spanwise efficiency? Certainly 2D wings generate lift. Indeed, they generate the largest possible amount of lift. But there's no wingtip vortex. Winglets might not *eliminate* wingtip vortices but if they increase the spanwise efficiency they should reduce it. 

On 10/16/2020 at 12:24 AM, mikegarrison said:

Keeping an airplane stable by computer is fine for a fighter jet. If anything goes wrong -- out goes the pilot! But it's a bad idea for an airplane that doesn't have ejection seats.

Didn't the MD-11 have pitch instability?

[mikegarrison]

17 minutes ago, Pds314 said:

Ehhh... Not sure that's a correct characterization of lift. Where is the vortex on a wing with infinite spanwise efficiency? Certainly 2D wings generate lift. Indeed, they generate the largest possible amount of lift. But there's no wingtip vortex. Winglets might not *eliminate* wingtip vortices but if they increase the spanwise efficiency they should reduce it. 

There is vorticity, but it's bound to the wing. It never sheds because the wing is infinite. Of course, since such a thing only exists in theory, it's not a real-world concept.

[Spica]

2 hours ago, mikegarrison said:

Of course, since such a thing only exists in theory, it's not a real-world concept.

 

3 hours ago, Pds314 said:

Where is the vortex on a wing with infinite spanwise efficiency? Certainly 2D wings generate lift. Indeed, they generate the largest possible amount of lift. But there's no wingtip vortex.

A wing whether finite or infinite in span still sheds a starting vortex upon the onset of lift generation, and therefore there still must be real bound circulation about the wing.

Of course there isn't a little tornado attached to the wing in flight, but if you were to photograph the induced flow around a wing section near the root you would still see an overall vortical flow pattern.

I am fairly sure you could photograph this in a water tunnel if the camera and water were stationary, and the airfoil was moved down the test section. Tracers (dust, glitter, bits of dye...) could be used to visualize the induced flow. I have never found such a photograph though, only renders.

[mikegarrison]

40 minutes ago, Spica said:

A wing whether finite or infinite in span still sheds a starting vortex upon the onset of lift generation

Yes, I was trying not to get into the time dimension, but this is also true. https://en.wikipedia.org/wiki/Starting_vortex

The idea is that in potential flow (that is to say, a flow that can be defined with the velocity as the gradient of a scalar "velocity potential"), net vorticity must always be zero. So whenever the lift changes, the circulation changes, so the vorticity around the wing changes, so an equal and opposite amount of vorticity must be shed. This is not shed from the tip of the wing, it is shed from the trailing edge. It's linked to the Kutta condition. This is true with spatial dimensions (3D wing) but is also true with time (starting vortex).

Now in the real air, viscosity exists, so real airflow is not actually potential flow. Starting vortices don't really sit there spinning forever. But boundary layer theory allows us to treat most of the flow as potential flow, with all the complicated stuff that violates potential flow theory assumed to be happening in the boundary layer.

I know potential flow can be applied subsonically or supersonically, but I can't remember if it is valid across a shock. A shock is entropic, and I have a feeling that violates potential flow theory. But maybe somebody can remind me.

 

Edited November 6, 2020 by mikegarrison

[Spica]

Shocks are highly nonlinear and definitely not smooth processes, so potential flow solutions should not be valid across a shock. The flow across an oblique shock isn't irrotational as far as I am aware, which would also break any potential flow solutions across one.

[Entropian]

What are the advantages and disadvantages of pusher vs. puller propeller configurations?  This has bugged me for a long time. 

[mikegarrison]

4 hours ago, Entropian said:

What are the advantages and disadvantages of pusher vs. puller propeller configurations?  This has bugged me for a long time. 

If the propeller is in front, it gets nice clean air for good propeller efficiency, but the airflow over the wings is disturbed. If the propeller is behind the wing, then the blades chop the wing wake, but it has less effect on the wing.

There have also been attempts to do both, like this:

That typically means they needed the power of two propellers but wanted to keep the engines in a central fuselage. This particular airplane was optimized for an unobstructed view from the windows, which is probably why they went with a high wing and no wing-mounted engines.

[starcaptain]

The Otto Celera 500L is supposed to be a design which optimizes for close-to-laminar flow all over, maximizing efficiency through minimizing aerodynamic drag. Minimized drag informs every exterior design choice, to the extent that it dominates the overall structural shape. Its general cross section is purely circular, and propeller diameter is actually smaller than its overall frontal size. Later versions are supposed to improve aerodynamics further by eliminating its large air ducts. Its final power plant may yet change to BEV or HFC-Hybrid.

I got a question for the nerds on aircraft design:

If this thing's plan is going for laminar flow all over (which it is), how would it fare in, say, rainy conditions, or turbulent air?

Golf-ball dimpling doesn't work on airplanes (normally) because skin friction is a bigger deal than pressure drag. (The opposite is true for golf balls, magnus effects notwithstanding.)

Would dimpling help at all for a plane like this?

[mikegarrison]

We've already talked about this plane, but your question does bring up something that airplane manufacturers have been working on for some time now: riblets. (This is, by the way, another example of trying to Google something about airplanes and getting links to food items.)

Riblets are one of those things that are going to be seen on airplanes "soon", and they have been that way since the 1980s. Which goes to show that bringing things from the lab to the in-service environment is never a sure bet. They are also called "sharkskin" by some people.

They are basically very small groves that run parallel to the airstream and have a measurable reduction for skin drag. But applying them to an airplane and then keeping them in good shape are easier said than done.

Here's a NASA report from 1986: https://ntrs.nasa.gov/citations/19880005573

Here's an article about them from 2019: https://simpleflying.com/shark-skin-paint-technology-aircraft/

The basic problem is that microscopic ribs are quite hard to apply to the skin of an airplane and quite easy to damage.

 

Edited April 12, 2021 by mikegarrison

[magnemoe]

7 hours ago, mikegarrison said:

If the propeller is in front, it gets nice clean air for good propeller efficiency, but the airflow over the wings is disturbed. If the propeller is behind the wing, then the blades chop the wing wake, but it has less effect on the wing.

There have also been attempts to do both, like this:

That typically means they needed the power of two propellers but wanted to keep the engines in a central fuselage. This particular airplane was optimized for an unobstructed view from the windows, which is probably why they went with a high wing and no wing-mounted engines.

Still with the two tail booms putting the engines in the booms lighting style you could go for an glass nose for very high visibility, but yes then the booms has to be much thicker so more drag, and having both engines in the same frame give you an very low drag two engine plane. 

Looks like outside the B-36 front mounted propellers are most common for multi engine transports or bombers including modern turboprop planes. 
Some design like WW 2 fighter planes tended to have massive propellers who required the body to tilt up. 

[starcaptain]

The characteristics of airships (aerostats, rotostats, zeppelins, blimps) have such major differences from heavier-than-air aircraft that i wonder if they are considered more similar to submarines than airplanes.

[mrfox]

On 12/28/2020 at 11:42 AM, Entropian said:

What are the advantages and disadvantages of pusher vs. puller propeller configurations?  This has bugged me for a long time. 

With air cooled piston pusher designs, the persistent problem was engine cooling - especially at low speeds. Often the rear mounted engine would overheat during climb out, or during extended ground operations.

[JoeSchmuckatelli]

@mikegarrisonhow does a needle shaped plane like this generate enough lift to take off?   

https://www.flightglobal.com/systems-and-interiors/aerion-selects-more-suppliers-for-in-development-supersonic-business-jet-as2/135933.article

(The concept designs for planes like these look reminiscent of some of the x-planes I've seen that were dropped from another plane to test supersonic /hypersonic flight) 

[mikegarrison]

1 hour ago, JoeSchmuckatelli said:

@mikegarrisonhow does a needle shaped plane like this generate enough lift to take off?

By going fast. (And probably by having some high lift flaps and or slats you don't see in the picture.)

I will also note that last year Aerion started showing a different render:

One of the problems with the F-104 was its high landing speed, required because of its high wing loading.

According to Wikipedia, Aerion is aiming for a balanced field length of 7500 feet. That's pretty long for a business jet. Many business jets try for shorter field lengths so that their operators can fly into smaller airports.

Edited January 13, 2021 by mikegarrison

[YNM]

10 minutes ago, mikegarrison said:

One of the problems with the F-104 was its high landing speed, required because of its high wing loading.

Wouldn't a private jet be preferably capable of landing on smaller airports ? I mean if it needs a long runway to takeoff / land then it'd limit the airports they could fly to/from.

[mikegarrison]

Just now, YNM said:

Wouldn't a private jet be preferably capable of landing on smaller airports ? I mean if it needs a long runway to takeoff / land then it'd limit the airports they could fly to/from.

I actually just updated my answer to mention this.

[YNM]

17 minutes ago, mikegarrison said:

7500 feet

About 2300 m (2.3 km)... pretty long.

So I presume that the only way you'd be able to fly with their plane to smaller airports is by taking the fuel to near exhaustion (less landing weight so less stopping distance), then load only a bit of fuel (less takeoff weight so less takeoff distance), to then refuel again on a preferably nearby larger airport...

IDK but kinda takes the edge off the point of using supersonic airplanes.

Edited January 13, 2021 by YNM

[mikegarrison]

17 minutes ago, YNM said:

About 2300 m (2.3 km)... pretty long.

So I presume that the only way you'd be able to fly with their plane to smaller airports is by taking the fuel to near exhaustion (less landing weight so less stopping distance), then load only a bit of fuel (less takeoff weight so less takeoff distance), to then refuel again on a preferably nearby larger airport...

IDK but kinda takes the edge off the point of using supersonic airplanes.

7500 feet isn't super-long as airports go. It probably just means the operators would have to fly into and out of commercial airports rather than the smaller general aviation airports.

Many operators of bizjets like to fly out of smaller airports so their passengers can more easily avoid road traffic and maybe land closer to wherever they are really trying to go. A G650 has a balanced field length of 5858 feet (according to Google). Also according to Google, the smaller Cessna Citation Latitude has a balanced field length of 4030 feet.

Aerion operators would be somewhat restricted in that respect, but would have shorter flight times.

Edited January 13, 2021 by mikegarrison

[YNM]

25 minutes ago, mikegarrison said:

7500 feet isn't super-long as airports go.

Depends on where you are I guess. But if most airports you have are remnants of WWII fields then 2.3 km runway is pretty long. But yeah, 2.3 km runways would see commercial aircrafts like 737s or A320s or Embraers or A220 / Bombardier CS in.

25 minutes ago, mikegarrison said:

land closer to wherever they are really trying to go.

Exactly the point of private jets... Though I suppose if you really need to go to somewhere that's really remote using air transport is already considered quick enough compared to other options.

So this is the plane for those who manage like operations in two large cities or something. Not if you have to go all over the world to remote places.

Edited January 13, 2021 by YNM

[mikegarrison]

Since I was tossing the term around -- "balanced field length" refers to the length required to reach takeoff speed, then reject the takeoff and come to a full stop again before running out of concrete.

Edited January 13, 2021 by mikegarrison

[YNM]

18 minutes ago, mikegarrison said:

come to a full stop again before running out of concrete.

Wonder if EMAS would be more common... They can magically add stopping length without actually having all the space. (there's a limit to the magic ofc.)

Edited January 13, 2021 by YNM