[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18

[Gryphon]

It did not require lift being generated by the bottom of the wing as it was a 1g maneuver, a croscrew pitching up.

My apologies - I wasn't implying anything negative about FAR. I was trying to be funny, but my silly didn't translate through the text, and I didn't include smilies. Yes, the pilot maintained 1g throughout the roll, and it wasn't sustained inverted flight...but he did fly an airliner upside down.

[FourGreenFields]

I have a question regarding the cross section graph

Anybody, do you have any tips on making the yellow curvature-cross-section line on the cross section graph smoother?

Also, what does a smooth yellow curvature-cross-section line look like on the graph?

The most successful design I have in 1.0.2 FAR is the E-2 Hawkeye with FantomWorks, KAX, BD landing gear, yatta yatta. I'm too tired to think: It's midnight in Cali, US.

Try to make the green line look something like this, and bias the maximum rearward if you build for higher Mach numbers: http://upload.wikimedia.org/wikipedia/commons/3/33/Sears-Haack.png So pointy tip, decreasing increase in cross-section, and pointy rear (note that a clean cut-off at the rear hardly adds drag)

Achieve something as close to it as possible using the most importand stuff (wings, engines, cockpit, fuel tanks, etc.), then smooth out using smaller additional fuel tanks, science equipment, air intakes, etc.

Best low-drag design I've built so far: http://steamcommunity.com/sharedfiles/filedetails/?id=444835683

Craft file at KerbalX.com

I'd say that's the kind of thing to discuss here though.

Edited June 15, 2015 by FourGreenFields

[JaredTheDragon]

I remember watching jetliners take off with no necessary angle of attack. Lift being generated by...?

[blowfish]

Jetliners have cambered wings and flaps that generate lift at zero AoA. You can still fly upside down with cambered wings, but you will need a much larger (negative) AoA and thus will generate much more drag.

[FourGreenFields]

I remember watching jetliners take off with no necessary angle of attack. Lift being generated by...?

RL wings accelerate the airflow above the wing -> lower pressure -> plane gets pulled up.

Have fun modelling that properly if there is no visual way to tell which side of the wing is "up".

And then there are flaps + slats ofcourse.

[ferram4]

..by camber and the angle of attack from the wings when the plane is on the ground (which is not 0, most wings have ~1-3 degrees AoA built in to how they're mounted on the fuselage) and the angle of attack created during rotation. You know, the part of takeoff that is the reason why the tail of the plane tapers upwards...

Notice how this video of a plane rotating as it takes off shows it achieving a nice amount of angle of attack:

Angle of attack contributes lift. Camber contributes lift (so long as the flow is subsonic). Both work by the same mechanism though, in that the trailing edge forces the air downwards, creating a lift force. Interestingly, if angle of attack weren't capable of producing lift, planes would be unable to maneuver at all, because they do so by changing angle of attack to create more / less lift in the direction that they need it.

[kcs123]

I come across with this bug. I can reproduce it with provided craft file 100% of times. Minimal required mods used.

Craft file and output log

Mods used:

ModularFlightIntegrator

ModuleManager 2.6.5

FAR dev build 73f947d31c

1) Open SPH editor

2) Load provided craft file

3) Click on CoM to show bubble in editor (might not have influence)

4) Open FAR graph and calculate static analysis - it shows NaN

I hope that it helps to solve this aerodynamic failure FAR bug.

[Naten]

Well, my question got buried by a discussion about upside-down airliners... hehehe

Anyways, do slots, slotted flaps, etc. work in FAR? I know that fixed leading edges can increase camber, but do slots help to prevent stalls?

[ferram4]

@kcs123: Reproduced and fixed, actually just a result of some drag adjustments to try and make subsonic drag a little less stupid in the last build. Completely unrelated to the aerodynamic breaking bugs that people have been complaining about.

@Naten: FAR can't detect slots at all, so no. That said, leading edge flaps can help reduce stall by reducing the local angle of attack at the front of the wing.

[jrandom]

RL wings accelerate the airflow above the wing -> lower pressure -> plane gets pulled up.

My aerodynamics texts say this isn't actually true. Ferram's got it right about the redirected airflow causing lift at 0 AoA for cambered wings.

[blowfish]

Well, my question got buried by a discussion about upside-down airliners... hehehe

Anyways, do slots, slotted flaps, etc. work in FAR? I know that fixed leading edges can increase camber, but do slots help to prevent stalls?

Short answer: yes. Give them some negative AoA deflection and they should improve stall behavior a lot.

[FourGreenFields]

My aerodynamics texts say this isn't actually true. Ferram's got it right about the redirected airflow causing lift at 0 AoA for cambered wings.

Well, RL wings are cambered wings most of the time I suppose... simply more efficient if I am not mistaken.

[kcs123]

Damm, I was realy thought that is the same thing. Well, have to pay attention too see if zero drag will pop up again.

Good news that that one is fixed.

Found another small issue. Reproducing steps.

- Create craft with cockpit and some parts that makes hull.

- Put some canrads in front and adjust -100% of AoA authority to them.

- Rotate whole craft, so it pitching by 10-15 degree in SPH.

- Sweep AoA in Static Analysis - canrads rotate in one direction along with airflow.

- Next, calculate Data+Stability derivates - canrads rotate in oposite direction.

Does not influence craft behaviour in flight, but people who use AoA% control surfaces might find it confusing in SPH.

Sorry, didn't have time to make some screenshots and upload them. I hope that this one could be easy to reproduce.

[ferram4]

There is no issue. The static anlysis and stability derivs work through the same exact code, and so will move the control surfaces to the same exact AoA. There is no wrong behavior there.

[JaredTheDragon]

Angle of attack contributes lift. Camber contributes lift (so long as the flow is subsonic). Both work by the same mechanism though, in that the trailing edge forces the air downwards, creating a lift force. Interestingly, if angle of attack weren't capable of producing lift, planes would be unable to maneuver at all, because they do so by changing angle of attack to create more / less lift in the direction that they need it.

If this were so, the same wing profile wouldn't work upside down, since the "lift" would be pointing up, not down. If you turn the wing over, it still generates lift. Also, the explanation is post-hoc. It explains that air moves down following the trailing edge, but now how such a motion could push up on a place which is already past that due to the forward vector. It wouldn't have any pressure on something that's no longer there, and a downward vector is the opposite of an upward vector, you see.

If the angle of attack goes negative, in that model, the plane would drop quickly since there would be no lift, thus airplanes wouldn't be capable of shallow descents. Lift is still very positive in shallow descents, as it's counteracting most of the weight of the plane. Most descents don't occur at terminal velocity.

To address your last point, watch any older planes maneuver without changing their angle of attack. Even in the 1920s, planes were highly maneuverable. We've been flying inverted from the early days.

[FourGreenFields]

If this were so, the same wing profile wouldn't work upside down, since the "lift" would be pointing up, not down. If you turn the wing over, it still generates lift. Also, the explanation is post-hoc. It explains that air moves down following the trailing edge, but now how such a motion could push up on a place which is already past that due to the forward vector. It wouldn't have any pressure on something that's no longer there, and a downward vector is the opposite of an upward vector, you see.

If the angle of attack goes negative, in that model, the plane would drop quickly since there would be no lift, thus airplanes wouldn't be capable of shallow descents. Lift is still very positive in shallow descents, as it's counteracting most of the weight of the plane. Most descents don't occur at terminal velocity.

To address your last point, watch any older planes maneuver without changing their angle of attack. Even in the 1920s, planes were highly maneuverable. We've been flying inverted from the early days.

Are you seriously arguing about basic aerodynamics with ferram? o.O

Anyway... he did say that "if this were so" (= if AoA would not contribute lift) planes couldn't fly properly. If you meant "if this were so" (= if AoA and cambering produce lift) then it's BS.

Campering produces a downward force when flying inverted, and the AoA needs to make up for that. I don't see how that wouldn't be possible.

Ofcourse, "the same wing profile wouldn't work upside down", as in: it would be inefficient. But it would be able to keep the plane in the air the way he explained it.

Edited June 16, 2015 by FourGreenFields

[JaredTheDragon]

Are you seriously arguing about basic aerodynamics with ferram? o.O

I'll argue "basic" physics with anyone, thank you very much. If there's a hole in logic or theory, let's attempt to fill it. Ferram has done some great work here and I completely respect and admire that, but that's no reason to call for an appeal to authority while leaving holes in theory and physics. If it's so basic, how can we not answer these questions without dodges, hedges, and the failed textbook answers?

We're trying to explain the vector up with fluid dynamics and angles, and I'm just not satisfied. If Ferram wants me to shut up and start my own thread, all he need do is tell me so. These are questions which seemed critically important to the topic, though - not exhibiting lift on a wing, in the game, which he's definitely done already! But rather, analyzing the root cause of the up-vector.

[blowfish]

A symmetric wing will generate positive lift at positive AoA, and negative lift at negative AoA. If your plane is upside-down and you are flying with negative AoA, then the lift force will be in the correct direction to keep you in the sky. The AoA required to maintain level flight often isn't very large, so it will be quite hard to notice in pictures and videos.

[JaredTheDragon]

A symmetric wing will generate positive lift at positive AoA, and negative lift at negative AoA. If your plane is upside-down and you are flying with negative AoA, then the lift force will be in the correct direction to keep you in the sky.

A symmetric wing should generate no lift, since the top would be the same length as the bottom. But it does.

You just said it generated negative lift at a negative AoA. How can a negative lift - by definition, opposite to positive lift - be "in the correct direction to keep you in the sky"? A negative is the opposite of a positive.

And this still doesn't answer the question regarding planes with no adjustable AoA being able to both fly and fly inverted, my friend. Nor the shallow descent problem. Nor the impetus to upward motion problem - the original up-vector. Stating a wing wil generate positive lift is not a description of how, which was my only concern. How does any wing generate an up-vector? What is pushing up? Is the air above cohering somehow, and pulling the very large mass (relatively) of any plane up, then? What is causing the air cohering to pull up, then?

I hate to make a swiss cheese of things, but it's so easy that it can't be ignored. We know there IS a vector up. We utilize it in airplanes, and then oppose it with wings on very fast race-cars (spoilers, obviously). So what is causing this vector up, since it's not AoA, fluid dynamics, or an imaginary cohesion with no reason to pull something below it?

[Bakase]

A symmetric wing should generate no lift, since the top would be the same length as the bottom. But it does.

You just said it generated negative lift at a negative AoA. How can a negative lift - by definition, opposite to positive lift - be "in the correct direction to keep you in the sky"? A negative is the opposite of a positive.

And this still doesn't answer the question regarding planes with no adjustable AoA being able to both fly and fly inverted, my friend. Nor the shallow descent problem. Nor the impetus to upward motion problem - the original up-vector. Stating a wing wil generate positive lift is not a description of how, which was my only concern. How does any wing generate an up-vector? What is pushing up? Is the air above cohering somehow, and pulling the very large mass (relatively) of any plane up, then? What is causing the air cohering to pull up, then?

I hate to make a swiss cheese of things, but it's so easy that it can't be ignored. We know there IS a vector up. We utilize it in airplanes, and then oppose it with wings on very fast race-cars (spoilers, obviously). So what is causing this vector up, since it's not AoA, fluid dynamics, or an imaginary cohesion with no reason to pull something below it?

Well, you've just said you're disregarding exactly what causes any upwards vector.

A symmetric aerofoil generates zero lift at zero AoA. Angle of attack is EVERYTHING to do with lift. A cambered aerofoil flown upside down will produce a downward force. That's basically what a spoiler on a racecar is, like you said.

>You just said it generated negative lift at a negative AoA. How can a negative lift - by definition, opposite to positive lift - be "in the correct direction to keep you in the sky"? A negative is the opposite of a positive.

We're measuring the lift vector relative to the craft. If the craft is upside down, a negative lift points upwards.

>planes with no adjustable AoA

That's what we call a brick. Any plane can adjust its AoA.

There is nothing magical about the upwards direction, in aerodynamics.

>And this still doesn't answer the question regarding planes with no adjustable AoA being able to both fly and fly inverted, my friend. Nor the shallow descent problem.

The AoA required to maintain level flight at the speeds most planes are flown is miniscule, so you probably won't notice it in pictures.

[JaredTheDragon]

A symmetric aerofoil generates zero lift at zero AoA. Angle of attack is EVERYTHING to do with lift. A cambered aerofoil flown upside down will produce a downward force. That's basically what a spoiler on a racecar is, like you said.

You prove my point. Blowfish said the opposite, and he's no fool nor me nor you. We have a disagreement there, a hole needing to be filled.

We're measuring the lift vector relative to the craft. If the craft is upside down, a negative lift points upwards.

The up-vector is an up-vector relative to gravity. How would gravity know the plane had flipped? What is the mysterious mediating particle interaction which would tell the Earth (or Kerbol) that the plane had flipped? If the back edge of the wing is somehow producing thrust up relative to the craft, due to AoA or curvature of the wing body, this would be inverted in an inverted flight. It would be thrusting down relative to the craft, which would be up relative to gravity, and planes would not be able to generate lift while flying inverted, by definition.

That's what we call a brick. Any plane can adjust its AoA.

There is nothing magical about the upwards direction, in aerodynamics.

No, most old planes could NOT adjust their AoA and had no mechanisms at all in their wings to do so. There is nothing magical about it, but there is everything magical about the standard answers here. We need to do better than that. We need real answers, not dodges.

The AoA required to maintain level flight at the speeds most planes are flown is miniscule, so you probably won't notice it in pictures.

But we notice it in flight parameters. Again, old biplanes didn't have adjustable AoAs, but they both flew and flew inverted just fine. That's the problem with the AoA answer, right there. Still no up-vector.

I only intend for people to think on these things. Ferram, you needn't concern yourself with charge physics for the sake of the game. It's a bigger overhaul than anyone is willing to take on. But since the game still represents the best, most accurate physics of ANY game (especially thanks to you, sir), it seems only right that we continue down that course. It's a perfect opportunity to do so. Just exploration, here.

[BudgetHedgehog]

A symmetric aerofoil generates zero lift at zero AoA.

You prove my point. Blowfish said the opposite, and he's no fool nor me nor you.

Did he?

A symmetric wing will generate positive lift at positive AoA, and negative lift at negative AoA.

He did not. Symmetric wing will provide +lift at +AoA, 0 lift at 0 AoA and -lift at -AoA. There is no hole here, no disagreements.

most old planes could NOT adjust their AoA and had no mechanisms at all in their wings to do so.

You got a source on that? A list of planes that did not have ailerons, elevators or rudders? Because I'm not an old plane expert, but I'm pretty sure planes without control surfaces (and therefore, no ability to adjust their AoA) didn't and do not exist. I'll happily be proven wrong though.

Edited June 16, 2015 by ObsessedWithKSP

[Wanderfound]

A symmetric wing should generate no lift, since the top would be the same length as the bottom. But it does.

You just said it generated negative lift at a negative AoA. How can a negative lift - by definition, opposite to positive lift - be "in the correct direction to keep you in the sky"? A negative is the opposite of a positive.

And this still doesn't answer the question regarding planes with no adjustable AoA being able to both fly and fly inverted, my friend. Nor the shallow descent problem. Nor the impetus to upward motion problem - the original up-vector. Stating a wing wil generate positive lift is not a description of how, which was my only concern. How does any wing generate an up-vector? What is pushing up? Is the air above cohering somehow, and pulling the very large mass (relatively) of any plane up, then? What is causing the air cohering to pull up, then?

I hate to make a swiss cheese of things, but it's so easy that it can't be ignored. We know there IS a vector up. We utilize it in airplanes, and then oppose it with wings on very fast race-cars (spoilers, obviously). So what is causing this vector up, since it's not AoA, fluid dynamics, or an imaginary cohesion with no reason to pull something below it?

There are so many misunderstandings in this is it is difficult to know where to start.

Angle of Attack is a description of the current geometric relationship between the wing and the airflow. It is not an intrinsic property of the aircraft. Some aircraft (in real life most, in Kerbal few) are built with the wings angled up a bit relative to the nose; these aircraft might be said to have a bit of "built-in" AoA, but the actual AoA is still that of the geometric relationship between the airflow and the wings.

The air deflection created by AoA is the primary lift force, and the lift provided by this deflection is always opposite to the direction in which air is deflected. So in normal flight, with the nose a few degrees above the airflow (positive AoA), the aircraft is lifted up. If the nose is moved to a few degrees below the airflow (negative AoA), the aircraft will be pushed down. This all works exactly the same when you invert an aircraft, with the sole exception that "a few degrees above the airflow" now requires you to push the stick forwards instead of backwards. The direction of lift isn't fixed relative to the aircraft, or to the planet; it is the opposite of the direction of air deflection, whatever that direction is. The only thing "negative" about the AoA used for level inverted flight is that it is in the opposite direction (relative to the aircraft) to that used in normal flight.

What a cambered wing does is to enhance that force a bit by increasing the deflection of the air. But unlike the AoA, the camber actually is a physical property of the aircraft. And because wings are only cambered in one direction at a time, that does have a fixed direction relative to the aircraft (not the planet; neither effect here is related to "up" in the gravitional sense). Camber works like an upside-down racing spoiler, and if you invert a normally cambered wing, the camber will start to push down, just as an inverted racing spoiler would push up. But the effect of camber is a small percentage of the effect of normal AoA-driven air deflection, and is easily counteracted by marginally increasing the AoA.

Aircraft with wings, with or without symmetric or asymmetric wing camber, fly inverted just fine.

[Val]

No, most old planes could NOT adjust their AoA and had no mechanisms at all in their wings to do so.

You are confusing terms. What you're talking about is changing the Angle of Incidence, that is changing the angle between the aircrafts fuselage and wings.

Adjusting AoA (Angle of Attack) is done with the elevators/horizontal stabilizer, as in rotating the whole aircraft.

Edited June 16, 2015 by Val

[FourGreenFields]

You got a source on that? A list of planes that did not have ailerons, elevators or rudders? Because I'm not an old plane expert, but I'm pretty sure planes without control surfaces (and therefore, no ability to adjust their AoA) didn't and do not exist. I'll happily be proven wrong though.

I wouldn't call spoilers and slats control surfaces. Both can be used to control a plane (I know that Messerschmitt designed a plane in the 1920s (or very early '30s) with flaps on the entire wing, and spoilers/slats (forgot which, will be able to check in a couple of days) as ailerons).

And then there are twistable wings (one stick to twist left wing, one to twist right wing - basicly changing the AoI).

So a plane without control surfaces could be build if we only call something a control surface if it is a "classical" control surface.

The most Kerbal solution is ofcourse to shift your CoM to control your plane, but I doubt that that was ever used.