Basic Aerodynamics question

[Victor Squawk]

The question is about causality in scientific terms.

Is wake turbulence a byproduct of the process in which high-velocity air moves across an airfoil (mainly the wing, perhaps) to produce lift?

Or, is wake turbulence simply a coincidence of the fact that the aircraft is lifting?

Is it an effect or a coincidence of flight, that's the question.

The followup question is - if it's an effect, is it due to the phenomena as described in the explanation provided by Bournoulli's principle, or is it due to the shape of the wings/wingtips themselves elevating through the air at a certain angle; or both? (If both, what is the relationship of them?)

I know I'm overthinking it and asking a lot of questions but I just need some clues or hints for how to think about it more precisely.

 

[CFI A&P]

Albeit this photo is about wingtip vortices, it might help understand it as a product of lift.

 

[Fixtur]

I was going to post exactly that picture. Wake turbulence is wingtip vortices (pictured above) which are a product of a wing producing lift. The high pressure air under the wing interacts with the low pressure over the wing at the wingtip, forming a vortex. This vortex is strongest with heavy aircraft at slow speeds, when the pressure differential is greatest, but is present for all aircraft in all phases of flight.

 

[knot4u]

It seems like wake turbulence is always attributed to wingtip vortices. Rightfully so, I was once on downwind for 16L at KVNY and a MD-80 inbound to 15 at KBUR had crossed the field a few moments earlier and I went from “such a nice day to be flying” to “what was that?” in an instant. We all know that WV drop at about 900’/min, but what about the rest of the plane and how it upsets the air? The tail is doing the opposite of the wing and the engine/engines are moving lots of hot air directly behind where they’re pointed. Does that disturbed air go up, down or is it just negligible? Maybe you should look into induced drag and why so many modern airplanes have winglets. Intersection drag is another interesting topic.

 

[knot4u]

If you can figure out how to put all of the air molecules right back next to their neighbor after you’ve driven down their street you’ll not cause a drag, aircraft are the bummer for air molecule block parties.

 

[knot4u]

Wingtip vortices are evidence of how you’ve disturbed the air, they increase with AOA, weight and sometimes wing configuration (flaps/slats). After the In-N-Out crash 757s were referred to, and treated, as “heavies” by ATC because of the wake turbulence they produced when approaching to land. Look up the incident with the Challenger that flew through the wake turbulence of an A380 when both were at cruise altitudes and speeds.

 

[Victor Squawk]

Thanks for all the replies. That helps both in pointing me to the relation of induced drag to downwash / wingtip vortices in the context of my question and also to see a bigger picture of what a sky full of planes are doing + what a safe route means for any given plane in relation to all those other planes along the way.

 

[Richman]

Actually, wake turbulence is what occurs when Taco Bell wakes you up from a sound sleep at 3am.

 

[CFI A&P]

Thanks for all the replies. That helps both in pointing me to the relation of induced drag to downwash / wingtip vortices in the context of my question and also to see a bigger picture of what a sky full of planes are doing + what a safe route means for any given plane in relation to all those other planes along the way.

There is a wide range of material about wake turbulence, from primary studies at the private pilot level and all the way up to NASA papers from aerodynamicists. In a nutshell, wake turbulence is present when lift occurs, as air is really a “fluid” which means disturbances are present in the same way vessels in water create wake.

To answer the follow up question from your first post; it depends. All airfoils create wake, but each airfoil (think C172 wing versus B737) has different characteristics. Each aircraft’s wake is also dependent on other variables that are present: angle of attack, weight of the aircraft, speed, etc.

With regard to “safe routes”, you might see an airplane flying in trail of another on the same course, but staggered to avoid wake. This is called Strategic Lateral Offset Procedures, or SLOP.

Clear as mud?

 

[Victor Squawk]

There is a wide range of material about wake turbulence, from primary studies at the private pilot level and all the way up to NASA papers from aerodynamicists. In a nutshell, wake turbulence is present when lift occurs, as air is really a “fluid” which means disturbances are present in the same way vessels in water create wake.

To answer the follow up question from your first post; it depends. All airfoils create wake, but each airfoil (think C172 wing versus B737) has different characteristics. Each aircraft’s wake is also dependent on other variables that are present: angle of attack, weight of the aircraft, speed, etc.

With regard to “safe routes”, you might see an airplane flying in trail of another on the same course, but staggered to avoid wake. This is called Strategic Lateral Offset Procedures, or SLOP.

Clear as mud?

Clear as....air.

Just a bit of a joke, apparently air has some counterintuitive qualities to it. I guess when an old guy said he can't explain something to me the way a fish would not understand water, he wasn't just talking hot air.

Okay it steams the puns are uncontrollable now and dropping in quality, jokes on fumes ha ha ha

On a serious note, simply seeing that air is fluid as water is fluid is a helpful pointer. It gives me many things to imagine that help me visualize what I am trying to understand.

 

[Victor Squawk]

It seems like wake turbulence is always attributed to wingtip vortices. Rightfully so, I was once on downwind for 16L at KVNY and a MD-80 inbound to 15 at KBUR had crossed the field a few moments earlier and I went from “such a nice day to be flying” to “what was that?” in an instant. We all know that WV drop at about 900’/min, but what about the rest of the plane and how it upsets the air? The tail is doing the opposite of the wing and the engine/engines are moving lots of hot air directly behind where they’re pointed. Does that disturbed air go up, down or is it just negligible? Maybe you should look into induced drag and why so many modern airplanes have winglets. Intersection drag is another interesting topic.

What can you tell me about intersection drag?

 

[CFI A&P]

On a serious note, simply seeing that air is fluid as water is fluid is a helpful pointer. It gives me many things to imagine that help me visualize what I am trying to understand.

Oh yeah, it is all around us. There’s a reason the Toyota Prius is shaped the way it is, and not because it wants to be a F1 car, although both of those vehicles are shaped the way the are for the same reason - aerodynamic efficiency. Even the feathers on an arrow have aerodynamic qualities, in the same sense a scuba divers fins have hydrodynamic qualities (Think shirt and stubby for snorkeling and long fins for scuba or free diving).

 

[WacoFan]

There is a wide range of material about wake turbulence, from primary studies at the private pilot level and all the way up to NASA papers from aerodynamicists. In a nutshell, wake turbulence is present when lift occurs, as air is really a “fluid” which means disturbances are present in the same way vessels in water create wake.

To answer the follow up question from your first post; it depends. All airfoils create wake, but each airfoil (think C172 wing versus B737) has different characteristics. Each aircraft’s wake is also dependent on other variables that are present: angle of attack, weight of the aircraft, speed, etc.

With regard to “safe routes”, you might see an airplane flying in trail of another on the same course, but staggered to avoid wake. This is called Strategic Lateral Offset Procedures, or SLOP.

Clear as mud?

The conversation is centered around "wings" regarding the wake turbulence/wingtip vortices conversation BUT...you mentioned "airfoil" and so I will bring up "propeller" and @knot4u brought up winglets as a mitigation for wingtip vortices and I'd be interested in his thoughts on q-tip propellers or any kind of more efficient propeller and such. Just saying it would be interesting but only if y'all want to discuss it.

 

[CFI A&P]

The conversation is centered around "wings" regarding the wake turbulence/wingtip vortices conversation BUT...you mentioned "airfoil" and so I will bring up "propeller" and @knot4u brought up winglets as a mitigation for wingtip vortices and I'd be interested in his thoughts on q-tip propellers or any kind of more efficient propeller and such. Just saying it would be interesting but only if y'all want to discuss it.

Sure, propellers are one of my specialties. Blade, face, camber, pitch… Is there anything in particular you’re curious about?

 

[WacoFan]

Sure, propellers are one of my specialties. Blade, face, camber, pitch… Is there anything in particular you’re curious about?

Just any new'ish thinking on propeller design - perhaps a look back at propeller evolution, and @knot4u talking about propeller tech as related to Reno air racers and Rare Bear in particular. That'd be great. Thanks in advance!

 

[CFI A&P]

Just any new'ish thinking on propeller design - perhaps a look back at propeller evolution, and @knot4u talking about propeller tech as related to Reno air racers and Rare Bear in particular. That'd be great. Thanks in advance!

In a quick and easy explanation, the q-tip had two purposes. Reduction in drag at the tip, similar to a winglet, and noise. A object rotating up to 2,700 rpm creates considerable noise. Which is why you see many GA airplanes in EASA land with 4 blade propellers (more blades reduces overall diameter ergo less noise). Such as the Cirrus and Extra. Sidebar, there's a group of seaplane pilots that are under the impression of turning up their governors to 2750, 2800, or more think their plane "pulls better" when in reality all they’re doing is allowing their tips to go sonic, creating unnecessary noise and losing efficiency. New materials, such as composite are replacing aluminum for the weight savings.

One of the more surprising facts about propellers is that the one bladed propeller is actually quite efficient because the second and subsequent blades are not passing through the disturbed air (wake turbulence) of the previous blade.

Hartzell does or did have this for Comanches, with a trailing edge tab that caused a mild rise in manifold pressure.

 

[knot4u]

Just any new'ish thinking on propeller design - perhaps a look back at propeller evolution, and @knot4u talking about propeller tech as related to Reno air racers and Rare Bear in particular. That'd be great. Thanks in advance!

The three blade prop on the Bearcat was the brain child of a retired Lockheed aerodynamicist named Carl Friend, a lot of people helped to make it a reality but it was his idea. At that point in time the engine program was probably at its peak and the engines were, at least on paper, making north of 4200 HP for at least very short periods of time. This all happened before my time working on the airplane. Carl had an idea for a more efficient way to turn that power into thrust, the engines already had the slow nosecase that reduced the propeller RPM down to roughly 1/3 of the engine RPM resulting in a prop RPM of about 1000/minute. Slowing the prop down kept the 13’ diameter tips from going transonic. Carl figured a P-3 engine/prop combo was putting out similar power at around the same RPM so he put on his thinking cap and slide rule and came back with with data to support his theory. Carl had already been involved heavily with cleaning up the airplane aerodynamically and at that time they’d just set a 3K speed record that would stand for decades so his ideas were not considered frivolous and ignored. His idea was a blade with a thin cross section and a fat chord would be more efficient at the speeds and angles in that environment than the Aeroproducts 4 blade prop they’d been using. A bunch of very talented people modified P-3 blades and mounted them in a Super Constellation hub, machined a one piece aluminum backplate, built a composite spinner, set the engine and controls up to use a governor instead of a regulator, and made sure the whole thing wasn’t completely out of the CG limits. And it worked, it flew and it won races. It was also intimidating and beautiful at the same time. It also had a tendency to beat up the airframe, when that airplane was running well and it flew by at at low altitudes it would shake everything in close proximity, people, the ground, everything. Only one was ever built. As far as I know it’s hanging on a wall in a hangar in Texas. There are legal reasons why it will probably never be reinstalled. It was a beautiful thing for as long as it lasted. Who’s that handsome guy in the cockpit?
Edit to add: Carl had also done lots of thinking about a V-tail for the airplane and at one point it was a real possibility.

 

[WacoFan]

Hey @CFI A&P and @knot4u - didn't want to quote y'alls posts but - WOW - thanks for the great info, write ups and pictures. Really appreciate it.

Question about the seaplane guys turning the governors to run the higher RPM. I believe Bill Adams, Rolly Cole, and another guy - Mel something - all 1960's airshow pilots of some measure of fame were all running 450 Stearmans. The story is that those guys were all turning those Pratts around 3,000RPM and they suffered crankshaft failure, prop seperates and takes out the N-struts, wings fold up and they all died in the wreck. Does that sound right/plausible? Wouldn't turning those seaplanes up like that put increased load on the crankshaft and cause bad things potentially, or does it have to be extremely more and not just that 50 or 100 RPM?

 

[inigo88]

What can you tell me about intersection drag?

Since you asked this earlier, intersection drag aka interference drag is a type of parasitic drag that occurs due to airflow mixing at the intersection between different parts of the airframe. Common examples are at the wing root where wing and fuselage meet, in the empennage where horizontal and vertical tail meet, where engines intersect with wing etc.

I bet @knot4u has some cool war stories about shaping custom one-off composite fairings for intersection drag on Rare Bear, am I right?

 

[knot4u]

An odd question many would ask about the prop in the picture I posted was why wasn’t it polished all of way down to the cuff. The part that looks “grey” is actually shot peened aluminum. Shot peening broke the surface tension of the metal and reduced the chance of failure. We were nuts but Lockheed and Hamilton Standard had figured that out and we weren’t daring enough to question it. I always liked it, it seemed like evidence that it wasn’t just a vanity project.