Aerodynamics (was: fatality Poland 10 June 2023 WS possible hard opening)

[JoeWeber 2,527]

1 minute ago, billvon said:

Because the shape of an airplane's wing is chosen to maximize lift over drag in a particular flight regime, as well as things like reducing shock wave formation, storing fuel, being strong enough, having a wide speed range etc.  The wing of a Cessna 172, for example, is curved on the front, flat on the bottom and curved on the top, because that gives you fairly efficient flight at around 100 knots with a fairly flat cabin, and still has decent low speed performance and stall recovery behavior.

But that doesn't mean that aircraft generates lift because the top of its wing is curved, any more than a parachute opens because it's made of nylon.  The curvature of the top of the wing is a result of several other decisions, just as the use of nylon is the result of design tradeoffs in weight, material strength and elasticity.

To be more specific, many aerobatic aircraft have symmetric airfoils, and can fly upside down as well as right side up.  The Cessna 150 Aerobat (pretty standard airfoil) can fly upside down, although that's not recommended due to the lack of inverted fuel and oil systems.  The M2-F2 lifting body was curved on the _bottom_ and still landed acceptably well without power.  (The one famous exception became the opening scene for the Six Million Dollar Man.)

What makes a heavier than air aircraft fly at the end of the day is simple physics; the aircraft deflects enough air downwards that the reaction to that pushes the plane upwards.  (Newton's Third Law.)  It can be done by brute force (quadrotor drones) or it can be done via a fixed airfoil that deflects air from the relative wind downwards.  To get fancier, the deflection of air downwards is seen (when looking from the side) as the superposition of a steady state fluid flow (i.e. the relative wind) and a circulation (the deflection of air downwards.)  The superposition of these two produces lift, per the Kutta–Joukowski theorem.

That's a lot to tell a first time flying or skydiving student, so there's a simplification that I often used:

The air on the top of the wing has to move faster than the air on the bottom of the wing due to the curve, and they have to meet up again at the trailing edge* so the air going over the top has less time to press downwards** - so there's less pressure above the wing than below, and the wing gets sucked upwards.

That gets the basic concept across that I want to get across - that lift depends on airspeed, so without airspeed there's no lift, which is why stalling canopies can cause you to drop rapidly.  It's not that accurate but it doesn't have to be for first time skydivers (or pilots.)

And this simplification works OK, but will get you into trouble if you are doing things like designing aircraft or trying to figure out exactly what a ram air wing is doing.

(* - that's called the Kutta condition, and is valid for most flight regimes for standard airfoils)
(** - a really facile explanation of the Bernoulli Effect)

So, what goes up is confusing? I've always believed that without evidence. Thanks.

[Veis 28]

5 hours ago, billvon said:

What makes a heavier than air aircraft fly at the end of the day is simple physics; the aircraft deflects enough air downwards that the reaction to that pushes the plane upwards.

Models and some aircraft can fly this way due to the large relative engine power to mass, but it is very inefficient in terms of fuel consumption. I'm not sure if I can explain this with the help of a translator. For low-speed wings with a thick profile, such as parachutes, the main contribution to lift is different. Air is a very heavy gas, each cubic meter weighs more than a kilogram (at sea level). And this mass is forced by the leading edge to move during flight, transmitting an impulse to the molecules. Basically, the air is diverted upwards, due to the asymmetry of the profile and angle of attack. The mass of air moves along the arc of a circle, experiences centripetal acceleration. And behind the front of the air wave from the leading edge, which has a high density, a "bubble" of low pressure air forms above the rest of the upper surface of the wing.

Thus, the weight of the volume of space, together with the moving wing (and the entire aircraft), is largely balanced by the Archimedes force, as long as the flow velocity and laminarity regime is observed.

5 hours ago, billvon said:

The air on the top of the wing has to move faster than the air on the bottom of the wing due to the curve, and they have to meet up again at the trailing edge* so the air going over the top has less time to press downwards

This "theory" has long been disproved by experiments in wind tunnels, (with smoke and strobe shutter). These flows do not have to meet with the same instantaneous flow rate, and they may not mix behind the trailing edge of the wing for a long time, moving in parallel at different speeds and interacting minimally.

Edited June 16 by Veis

[BIGUN 1,229]

6 hours ago, billvon said:

Kutta–Joukowski

First, thank you. I'd never heard of Kutta Joukowski and now have it bookmarked for additional required reading of all the theorems in the wiki page.  

[Veis 28]

26 minutes ago, BIGUN said:

Kutta Joukowski

It is usually stated in a very simplified form, only for the layer at the surface of the wing, as it could be calculated at the beginning of the 20th century. Hence the discrepancies. If you integrate for the entire volume of air affected by the wing (as is done with modern computers), then there will be no discrepancy. And the "lifting force" considered relative to the perpendicular to the direction of movement, as expected, will decrease sharply in all directions except for optimal horizontal flight.

[olofscience 456]

8 hours ago, billvon said:

But that doesn't mean that aircraft generates lift because the top of its wing is curved

Modern airliners like the A350 or 787 actually have more curvature on the bottom surface than the top surface of the wing, and they work pretty well for low speeds too. I've always wondered how a parachute with a supercritical aerofoil section would behave like.

[billvon 2,684]

3 hours ago, olofscience said:

 I've always wondered how a parachute with a supercritical aerofoil section would behave like.

Rumor had it that the Nova had such an airfoil; it was called "S-shaped" but I have a feeling they were inspired by supercritical airfoils.

[riggerrob 598]

On 6/12/2024 at 2:01 AM, Veis said:

This is far from the truth.

Aerobatic airplanes use minimal lift force, precisely because it limits maneuverability in different directions. They have a very powerful engine in relation to weight and a thin wing profile. Other forces are used more: aerodynamic drag and reactive. And what looks like indifference to the position in space is actually the skill of a pilot who, at high roll angles, uses a vertical tail as a wing.

You have an “independent” understanding of aerobatic airplanes.

While we encourage independent thought, your own theories are so far from fonventional wisdom that’s e have difficulty understanding them.

We also u d’état and that English is your second language.

My understanding is based upon my private pilot license, many years maintains Canadian Air Force aircraft, etc.

I would encourage you to read some of the textbooks used in private pilot ground school (e.g. “From the Ground Up” for Canadian pilots).

Aerobatic airplanes are not radically different than conventional airplanes. Aerobatic airplanes fly based upon the same principles as non-aerobatic airplanes. Any airplane can do a few aerobatic maneuvers (simple loops, rolls and spins) but will only remain structurally intact if flown by a gentle pilot. Many airplanes are placarded against intentional spins because of sluggish recoveries from spins.Any “mere mortal” pilot who attempts aerobatics - in an airplane not approved for aerobatics - is an idiot. See the accident in Belgium where a bored jump-pilot pulled a wing off of a Pilatus Porter because he got bored flying skydivers. Pilatus responded by issuing a Special Inspection to check for cracks in wing strut fittings.

Returning the the notion of conventional airplanes. Let’s start with the aerobatic version of the Blanik glider. It has shorter wings than to reduce loads on the spars in the wing roots. Otherwise it is built from mostly the same parts as the long-wing Blaniks that are popular with gliding students. Blaniks can do most of the gentle aerobatic maneuvers like loops, rolls, stalls and spins. When they fly inverted, Blanik wings work the same way - by deflecting air downwards - the only difference is that their positively cambered wings are less efficient while inverted. When I say “inverted” I mean stable, 1 G flight almost horizontal. Gliders cannot fly perfectly horizontal for more than a few seconds because they are always trading altitude for forward speed … the same way as the Jalbert’s Para-Foil, square, Ram-air parachutes that are currently fashionable among skydivers.

Next let’s look at Cessna’s 150 Aerobat which is a slightly modified version of the 2-seat trainer that many pilots started on. Modifications are limited by to a few local reinforcements (e.g. rudder control horns) windows in the ceiling, quick-release doors and quick-removable seat-cushions. The quick-removable seat-cushions make room for pilot emergency parachutes. 
Cessna 150 can do all the basic aerobatic maneuvers and is only limited when flying inverted until the engine quits for lack of oil and gasoline. Cessna engines depend upon gravity to flow gasoline from fuel tanks - above the engine/in the wings - to the engine.

 Cessna builds 150s using the same materials, tooling and techniques as the 182 and 206 models that are popular with skydivers.

The dedicated competition aerobatic planes built by Extra, Sukhoi and Zlin differ by stronger airframes, symmetric wings, larger control surfaces, more powerful engines and inverted systems in engines. Stronger airframes allow pulling more age in tighter turns and last longer during violent maneuvering.

Symmetric wings are less efficient in cruise, but have the advantage of lifting equally as well upright or inverted. Aerobatic wings tend to be thicker for strength. 
Larger control surfaces applied ore yaw, pitch, roll for tighter maneuvers. On this issue, you are confusing stability with engine power. Aerobatic airplanes are designed for neutral stability to provide the same handling whether upright or inverted. A disadvantage is that constantly try to wander during cruise flight, so constantly require pilot inputs to keep them upright.

More powerful engines improve climbing back up start the next maneuver. A typical competition aerobat has the same 6-cylinder, 300 horsepower Continental or Lycoming engine as installed in a Cessna 206 jump-plane hauling 6 skydivers. Inverted systems in engines include extra fuel and oil pumps to keep the engine running during extended inverted flight.

The only part of aerobatic flight - that does not involve conventional aerodynamics - is knife-edge flight where wings are vertical (90 degrees from cruise) but the airplane somehow flies horizontal … on its side. If you look closely, you will notice that the propeller is pointed 30 degrees above the horizon and the airplane is trying to constantly climb, but the fuselage makes for a loosy lifting body/wing.

[Veis 28]

57 minutes ago, riggerrob said:

You have an “independent” understanding of aerobatic airplanes.

I don't know if that was intended, but you essentially confirmed what I wrote about.

I will add that in parachutes it is usually required to get the maximum possible lift force. And the scope of application of parachutes in which the lifting force is structurally reduced for the sake of maneuverability or speed is very limited, like CRW rotations, or swoop (and XRW).

[billvon 2,684]

2 hours ago, Veis said:

I will add that in parachutes it is usually required to get the maximum possible lift force.

Nope.  You only need enough lift to counteract the weight of jumper+equipment.  Parachute design tends to focus on:

-L/D - the glide ratio of the system.
-Opening performance - a combination of many things, primarily driven by the reefing system.
-Landing performance - driven, again, by a great many variables, including how the wing responds to an upset (like a high performance landing initiation.)
-Stability - the ability to withstand changing relative wind directions/magnitudes (i.e. turbulence.)
-Weight/pack volume - driven by materials selection.

[kcaero 3]

This debate reminds me of the old Miller Lite commercials...."Tastes great"......"Less filling "..

[mark 107]

3 minutes ago, kcaero said:

This debate reminds me of the old Miller Lite commercials...."Tastes great"......"Less filling "..

Except in the commercial, both sides could be right.

[Veis 28]

12 hours ago, billvon said:

Nope.  You only need enough lift to counteract the weight of jumper+equipment.

The weight is always compensated if the speed of flight is constant. The only difference is how big its values are. By getting maximum lift from the profile, can reduce both the dimensions and the laying volume without losing the landing characteristics.

Eventually, the source of lift force is the displacement of volumes of air having mass.

[GoneCodFishing 24]

22 hours ago, Veis said:

I will add that in parachutes it is usually required to get the maximum possible lift force.

You absolutely do not want maximum lift on a canopy, not even close. You want just enough lift for the canopy to fly a given flight path when combined with its trim. Any more lift is unwanted.

In fact it's quite the opposite, the name of the game is not to increase lift, but to reduce drag. Once you removed drag you can remove lift accordingly to sit on the given gr desired. The more you reduce lift (and drag respectively) the more efficient the canopy becomes. The more efficient the canopy, the better its flying characteristics and the better response to toggles and risers as the laminar flow remains attached throughout the flight and landing.

You do not want a canopy that flies a gr of 5:1 on full flight. You do not want a canopy that will climb when coming out of a turn or a dive. That's the realm of paragliders and military haho jumps, not skydiving wings.

 

[TampaPete 40]

16 hours ago, kcaero said:

This debate reminds me of the old Miller Lite commercials...."Tastes great"......"Less filling "..

I read these discussions under the context of a Monty Python skit on physics. 

[flyingwallop 5]

Am I reading a 'debate' between people that build/design parachutes?  Surely it´s not such a misunderstood field.

[Veis 28]

4 hours ago, GoneCodFishing said:

You absolutely do not want maximum lift on a canopy, not even close. You want just enough lift for the canopy to fly a given flight path when combined with its trim. Any more lift is unwanted.

It doesn't work that way) The drag of a the front part of the wing is a way to make the air move in a certain direction, and create lift. Even the fastest parachutes have a relatively thick profile and a non streamlined front.

[Veis 28]

3 hours ago, flyingwallop said:

people that build/design parachutes?

It seems that you are ready to buy something from us? )

Edited June 18 by Veis

[TampaPete 40]

9 hours ago, TampaPete said:

I read these discussions under the context of a Monty Python skit on physics. 

FADE IN:

EXT. PEASANT TOWN SQUARE – DAY

A PEASANT GROUP runs toward the TOWN SQUARE dragging a YOUNG WOMAN dressed as a WING. SIR BEDEVERE, tending to physics experiments, releases a nuclear powered ornithopter tied to a coconut.

PEASANT GROUP

(Shouting)

"We found a wing, she’s a wiiing fling her."

BEDEVERE

(addressing the peasants)

"How do you knoooooow she’s a wing?"

PEASANT GROUP

(Shouting)

" ‘Cause she looks like one."

"FLIIIIIING her."

YOUNG WOMAN

"I’m not a wing. They did this to me."

"These aren’t my A lines."

Bedevere addresses the peasants.

BEDEVERE

"Did you do this to her?"

PEASANT GROUP

"No… no… no… yes… no."

PEASANT 1

"We did do the A lines."

"And the d-bag."

(pause)

"But she does have an air lock."

PEASANT GROUP

(Shouting)

"FLIIIIING her!"

BEDEVERE

"There are ways to tell if she is a wing."

The peasants look confused.  

BEDEVERE

"What else can be flung like a wing?"

PEASANT GROUP

"Churches… marshmallows… navel lint… Mozhaisky’s airplane…"

KING ARTHUR watches the goings on from the side.  

KING ARTHUR

"A piano… flung from a trebuchet."

All turn to look at Arthur. Bedevere, in great amazement, exclaims…

BEDEVERE

"Exactly!"

PEASANT 1

"So… if she can be flung…"

Peasant 1 strains to find the logic.

"As far as a piano……"

PEASANT GROUP

(Shouting)

"She’s a wing… fliiiing her."

BEDEVERE

"We’ll use my largest trebuchet."

Unfortunately, at this point the illogic of the physics takes hold causing the film to be quickly burned by the bulb and break. Thus, just like how many licks it takes to get to the center of a Tootsie Roll Pop the world will never know how far a Wing can be flung from a trebuchet.

[sfzombie13 317]

13 hours ago, Veis said:

It seems that you are ready to buy something from us? )

not this guy.  just following the silliness.  trying to use human terms and science to explain away the magic.  what a fool's errand that is...

 

 

[billvon 2,684]

On 6/18/2024 at 1:46 PM, Veis said:

It doesn't work that way

He's exactly right.

Let's take an example.  A student pilot is departing an airport.  He wants to climb faster - he wants more lift!  So he pulls the yoke back as much as he can.  His climb rate increases - but his speed falls off.  When the wing reaches about 15 degrees angle of attack, he feels the buffeting of an incipient stall.

What should he do?

If he wants to get "the maximum possible lift force" as you mentioned before, he should pull back further.  That will maximize his lift.  Up to about a 40 degree angle of attack, more pitch up equals more lift.

If he wants to survive, he reduces the angle of attack.  This decreases lift slightly but GREATLY decreases drag, so that he can keep climbing with the thrust he has available in the engine.

You almost never want "the maximum possible lift force" in airplanes OR parachutes.

Quote

The drag of a the front part of the wing is a way to make the air move in a certain direction, and create lift.

Nope.  Drag does not "make the air move in a certain direction" and it does not "create lift."  Drag is the result of the aerodynamic forces acting to slow the wing.  The front of a ram air parachute is open because the dynamic pressure caused by the motion of the parachute with respect to the relative wind keeps the canopy slightly pressurized, which causes it to keep its shape.  That happens due to the momentum of the air, the pressure behind that air and the stagnation point in front of the wing - not because of drag.

If you sealed off the front of the wing completely and pressurized it from another source (a compressed air bottle for example) you'd get a very similar amount of lift.

[pchapman 275]

Getting aerodynamic principles just right, and explaining them correctly,  is quite tricky. There are plenty of ways to explain something that are "sorta kinda right in some circumstances, for the example being given, but not sufficiently correct to really explain most of the possible situations". And this particular discussion has been messed up by one participant not having really good command of the English language. Not his fault, but makes explanations and interpretations even more confusing & sketchy & vague.

I'll certainly disagree with Veis on a lot of his aerodynamics. Although I do agree that the Kutta condition or Kutta-Joukowski don't imply that neighbouring air molecules, one going over and one going under an airfoil, need to meet up again at the trailing edge. That just isn't true. So, billvon, I don't think that is a useful part of how to describe lift.

I'll wimp out from wading too far into this, as writing a good textbook explanation of lift is hard. But I'd say:

1. Lift comes from pushing air down. [Circulation in other words, if doing Kutta-Joukowski integration around the whole airfoil stuff]
Edit: That's simple Newton's laws stuff, or force diagrams. Pushing air down means you have pushed up on the wing.

2. You can do that with a flat plate, the so-called barn door, at some angle of attack, catching the air. No curved airfoil needed. But, it is a very inefficient way to make lift, as there's a ton of drag for the lift being produced.  (At the sizes and speeds we are talking about. Things are different at the sizes & speeds of paper airplanes and insects. [Small Reynolds numbers ])

3. Airfoils happen to be a shape that can make lift, while still being really low drag. Very efficient, that's  why we use them.

4. There is some pressure pushing up on the bottom of airfoils if at enough angle of attack. [This gets complex and depends on exact shape & angle]

4. But most of the lift comes from the top: The way air works is that it speeds up and lowers pressure when moving around a gently curved surface. That's where the Bernoulli stuff come into play.  That provides most of the lift of an airfoil.

5. But the air can't turn too sudden of a corner. So at too high an angle of attack, the air starts to separate from the surface of the wing and turns into a swirling chaotic mess of waves and vortexes. The airfoil has stalled. Lift starts to decrease* but drag is massively up, so you no longer have an efficient airfoil and your airplane or parachute is probably going to start dropping suddenly (assuming we are talking about roughly horizontal flight), leading to even higher angle of attack and even lousier flying.
 

*[Edit: Duh somehow I said it was still increasing. Which isn't true for a regular airfoil after the stall. What I should have said is that one can have very low aspect ratio aircraft, or aircraft with strakes, or a wingsuit, or something, where there isn't a simple clean stall. Parts of the wing may not be working efficiently, and there's increasing drag, but lift from say vortexes can keep on rising past say 15 degrees to say 40 degrees angle of attack. The concept of "the stall" isn't as clear cut any more.]

Edited June 20 by pchapman

[Veis 28]

4 hours ago, billvon said:

He's exactly right.

Let's take an example.  A student pilot is departing an airport.  He wants to climb faster - he wants more lift!  So he pulls the yoke back as much as he can.  His climb rate increases - but his speed falls off.  When the wing reaches about 15 degrees angle of attack, he feels the buffeting of an incipient stall.

What should he do?

If he wants to get "the maximum possible lift force" as you mentioned before, he should pull back further.  That will maximize his lift.  Up to about a 40 degree angle of attack, more pitch up equals more lift.

You're both wrong. I do not know the source of your fantasy, where did you read that an increase in the angle of attack leads to an increase in lift? In such a situation, the lift is increased by the release of the front and/or rear wing mechanization. Slat - makes the flow around the upper surface of the wing more dynamic, and creates a larger bubble of low pressure air. And the flap increases the surface, which perceives this low pressure from above and increased pressure from below.

[olofscience 456]

14 minutes ago, Veis said:

In such a situation, the lift is increased by the release of the front and/or rear wing mechanization.

When you pull the yoke of an aircraft, nothing happens on the (front) wing. If I were to use your words - no release of mechanization happens.

 

Lift coefficient would still increase though.

[billvon 2,684]

20 minutes ago, Veis said:

You're both wrong. I do not know the source of your fantasy, where did you read that an increase in the angle of attack leads to an increase in lift?

I'm a pilot and I know that from experience; it's how airplanes work.  You can also read it in any book about aerodynamics.

But if you want a nice chart see below.  (Note that this airfoil maxes out lift at about 20 degrees, not 40 degrees.)

 

Quote

In such a situation, the lift is increased by the release of the front and/or rear wing mechanization

Nope.  During normal flight, lift is increased by increasing the angle of attack.  This is done by deflecting the elevators on the back of the horizontal stabilizers using the yoke/stick.  Nothing to do with "mechanization" on the wing.

What you are talking about - flaps and slats - are only used during takeoff and landing, to reduce stall speed so that the aircraft can take off/land at a slower airspeed.

But don't take my word for it.  Take a flying lesson and see for yourself.  Or just look out the window of an airplane next time you are in one.

[Veis 28]

19 minutes ago, billvon said:

I'm a pilot and I know that from experience; it's how airplanes work.  You can also read it in any book about aerodynamics.

You have read familiar words, and you are arguing with the theses that you put forward yourself. What does the dependence of lift on the angle of attack have to do with it? I was talking about a profile that provides maximum lift for a given wing area, and speeds acceptable for a landing human body.