Basic Aerodynamics question
- Thread starter Victor Squawk
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ahw01
Well-Known Member
what can you teach yourself about intersection drag?
Victor Squawk
Well-Known Member
Sure, I learned that interference drag is a sort of intersection drag which appears to be a design consideration in airplane geometry that harnesses advanced aerodynamic science and intricate airflow phenomena to optimize performance. The heuristics of design change depending on the intersection in question but there are some general principles to keep in mind, such as the use of curvature to solve the problem where an intersection in particular is increasing drag. That adds another two terms to my lingo bingo. Hurray!
CFI A&P
Exploring the world one toilet at a time.
I don't know too much about that accident, but I'll take a little bit of time to research it later. What I do know is some details about that engine/ propeller combination and that Rolly Cole was Duane Cole's son. The airframe I'm familiar with that is powered by the R-985 has a redline of 2,300rpm for take-off with max continuous of 2,200rpm and I refer to that later. There are four basic ways to increase horsepower in a recip engine. Increase cylinder pressure (supercharge, turbocharge or chemically with nitrous/ propane but that's more about fuels with higher energy content). Increase length of the stroke, aka stroker engines. Enlarge the bore, such as .10 or .20 oversized cylinders. Lastly, and the most common - increase RPM. Except increasing RPM has complications on airplane engines with a direct drive propeller, since the propeller has aerodynamic limitations. The airframe I am familiar with normally has a two blade propeller option that is limited to a max of 9ft long, minimum of 8' 4". This next part has a few assumptions about it, since I'm extrapolating data from the airframe I know and don't know what propeller was on those 450hp Stearmans. Using a prop tip speed calculator, at 2,300rpm, max propeller length and 15c temperature - the tips are transonic (m.97), creating more noise and losing efficiency. Reducing the propeller blade length to minimum allowable yields almost ideal tip speed of m.90. If we assume the rpm increased to 3,000rpm with min & max blade lengths: m1.17 and m1.26. Both of which would be way too fast. The tips are sonic at that point and all sorts of aerodynamic problems are occuring. Maybe that was part of the plan, more noise for the crowd, but at the cost of losing thrust.
Turning an engine up to 3,000 from 2,300 is a large increase, 30% or so. That creates significant stresses on bearings, and causes valve train issues such as valve float, as well as magnifies the centrifugal force on the propeller hub while the blades are trying to liberate themselves. This was problematic for Hartzell steel hub propellers during normal operations, as they had some cracking issues with blade clamps, a few blades departed the airframe and led to ADs against those propeller assemblies. As mentioned earlier, shot peening of the metal is common to improve strength, and is still the same today. The large Hartzell propellers on Air Tractors, Beech 1900s and Shorts are shot peened. Also, if you look at some of smaller Reno racers with O-200 engines, they're turning those up to 3,200rpm or more but the propeller diameter is far smaller than the range of 67.5-69 inches allowed on the mighty Cessna 150.
Later today I'll try to read up on the 450hp Stearman setup and those accidents, but I suspect that if they are turned up 30% over redline, shedding a blade is entirely plausible with catastrophic results. Even if the blade doesn't hit the pilot or the airframe, the abrupt change in the balance of the rotating mass will have disastrous consequences.
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zmiller4
Well-Known Member
+1 for introducing some of us to “shot peening”. Also, I don’t think I ever had the money to get a pit pass for Reno and didn’t see Rare Bear or Dago up close, but that prop is freaking awesome. Looks like it belongs on a cruise ship.
juxtapilot
Snowflake
This is awesome info.. Well done y’all!
trafficinsight
Well-Known Member
Bill Kerchenfaut once told me "Making horsepower is easy, you just have to figure out how to burn more fuel."
I figured he ought to know.
CFI A&P
Exploring the world one toilet at a time.
Here's a little more for you @WacoFan, as I read about that trio of airshow pilots and their demise, I'm seeing many references to snap rolls. If that is accurate, and not just journalistic embellishment, that is something to think about with the shear weight of these airplanes and the physics involved. Some background on rolls, there are several types of rolls, but I'll stick with the slow roll (which isn't always slow, but consistent technique across any airplane) and the snap or flick roll. In an airplane with a poor slow roll rate, (Cessna Aerobats roll at about 60 degrees per second or 6 seconds for a complete rotation and a few modern composite monoplanes are 400+ degrees second) the snap roll is a good way to get it to roll around without the long wait. This is really condensed for this conversation, but the slow roll uses aileron to generate the rotation whereas a snap roll uses abrupt inputs to exceed the critical AoA then rudder to induce yaw causing asymmetrical lift between the wings and create the rolling moment. Some of the other forces in play during snap rolls are gyroscopic precession, rigidity in space and all that propeller stuff we forgot after private pilot ground school. Those forces are magnified by the mass of the propeller and a two blade Hamilton-Standard weights about 105lbs, whereas the modern three blade composite on an Extra, Edge, MX, Pitts and such only weigh about 30 pounds. During a snap roll or other tumbling type maneuver that we see in today's airshows, it is putting stress on that nose bearing, engine mounts, and internal engine components from the gyroscopic precession, & rigidity in space. By using the lighter weight composite propellers, you do lose out of those forces which impact how those snaps and tumbling maneuvers perform, but you'd gain a savings with less wear and tear on those components.
That's a long winded explanation to say I'd venture a guess that snap rolls with those big engines and propellers created all sorts of failure modes. The engine is trying to leave the mount, the propeller is trying to pull the crankshaft in ways it shouldn't, the propeller blades are trying to leave the hub. Maybe overspeeding the engine contributes to this, but that 9 foot propeller swinging out there is also working against you when departing controlled flight.
That's a long winded explanation to say I'd venture a guess that snap rolls with those big engines and propellers created all sorts of failure modes. The engine is trying to leave the mount, the propeller is trying to pull the crankshaft in ways it shouldn't, the propeller blades are trying to leave the hub. Maybe overspeeding the engine contributes to this, but that 9 foot propeller swinging out there is also working against you when departing controlled flight.
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trafficinsight
Well-Known Member
WacoFan
Bigly
There are videos of Bill Adams doing multiple-snaps, Marion Cole used to do THREE vertical snaps in a 450 Stearman - God only knows what his entry speed was on the first. They all had crankshaft failures and props (Bill Adams) and the entire engine (Rolly Cole) leave the airframe. They were all also greatly exceeding the 2300 RPM but this is just reading and listening to people hangar fly about 450 Stearmans and the airshow guys. My Grandpa knew a bunch of them - Bill Adams, Harold Krier, and others. Bob Herendeen was a student of his for his primary training and then later my grandpa hired him at TWA. Those guys used to do insane stuff in 450 Stearmans.
Here's a clip of Bill Adams at Rockford (before they moved to Oshkosh).
knot4u
Repeat Offender
When I worked on the airplane we spent a lot of time working on reducing cooling drag, a big air cooled engine making that much power creates an enormous amount of heat. The areas that caused intersection drag were all pretty much stock, maybe a little bondo to smooth it out but no real fairings. Beneath the paint and skin things were done that weren’t visible externally like changing the incidence of the horizontal stabilizer. The flaps were faired over, there was a fairing that covered the area where the tailhook used to sit and that was about it. We made big advances by addressing cooling drag and sealing up as much of the airframe as we could to reduce parasitic 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.
View attachment 72805
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
Repeat Offender
The coolest thing about participating in air racing at that level was the camaraderie amongst the people. Bill was a special person and the world is certainly a poorer place after losing him.
YES! That's the answer.
Any time you move air at speed around an airfoil, that combo will produce both lift and drag (even in the absence of gravity... if that's what you're really trying to get at).
It's a thing (airfoil) in a place (slip stream) with a guy (pilot). Even if, as seems highly probable, it soon becomes just a thing in a place, those first two elements operating together will still produce both lift and drag - coincidentally and effectively.
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Victor Squawk
Well-Known Member
Yeah that's what I was told but rather than simply getting it told to me again, I was looking for more explanatory material, which I got pretty soon after posting my newbie question. Sometimes it's difficult to even ask a question when you're as new to it as I am (it's like you don't even know what to ask).
To be honest with you, though, I'm not really sure what you're saying beyond "making lift produces induced drag" (repeating what any google search could say), nor why you posted.
I get the sense it's all a matter of material, and not just vapid wordplay. Otherwise I might just put a checker on a lingo for my bingo.
knot4u
Repeat Offender
Apropos of nothing I'd like to submit this video from some random person that was some how able to get out to the pylons during a race. I never had a chance to get out there for a race, I was always busy banging things with my fixing sticks, but this guy got out there for a bunch of races including the unlimited gold race (it starts at about the 12:40 min mark). I should also mention that this was after my time working on the airplane. Getting sued for volunteering soured my ambitions and I walked away. This is a horribly low quality recording of the events but it has a Stearman with a big engine doing things a Stearman shouldn't do, T-6's with their engines wound so tight that being in any close proximity was an assault on your sense of hearing, and just the sort of nonsense that will only happen one more time in the desert outside of Reno, this year is the end. I was hoping some of the really fast airplanes would show up, and maybe they will but at this point I'd bet a dollar the final winning speed for the Unlimited Gold will be less than 460mph. I hope I'm wrong, and I hope any of you who were ever interested had a chance to go see it. It was the best of times.
View: https://www.youtube.com/watch?v=o7EIxneD4f8
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Well, the central function of work here is the airfoil (and all its aspects as described -roughly- by the equation of lift) in the presence of airflow. The input to that function is the airflow. The shape and size and coefficient of lift of a fixed-design airfoil it fixed; You know, immutable. The amplitude and viscosity of the airflow are the variables. Yet, no matter how those variables vary, lift and drag will always happen when an airfoil meets an airflow. And not to be snide, but seriously - both coincidentally and functionally. (Perhaps, for ease of understanding, replace airfoil with "foil" and airflow with "fluid" flow.)
Victor Squawk
Well-Known Member
I don't know much math but I think I got the answer at the end there - it's both. Thank you for helping me understand.
That said, there are different kinds of drag. There are induced and parasite drags. I suspect your question has more to do with induced drag, because parasite drag is very intuitive. To intuit parasite drag, just think of trying to push a refrigerator through air at high speed... If you can't imagine a refrigerator, think of an SUV or a semi-tractor.
Induced drag is the trickier one... the one that has to do with a more nuanced understanding of aerodynamics. Induced drag happens as a function of lift. When you think of the work accomplished by a wing, you can kinda sorta - but not precisely - think of the induced drag as the resultant exhaust of that work. The more lift, the more induced drag. Induced drag is the drag that causes oddities like wing vortices and situations like "being behind the power curve."
While parasite drag is largely a function of object form and speed, induced drag is largely a function of wing shape and angle of attack. (Be careful with angle of attack. For an airline pilot, AOA is largely a function of airspeed. But properly understood, AOA has little to do with airspeed, per se.)
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Victor Squawk
Well-Known Member
So that does provide some logic for integrating more information to the function of lift (or, lift as a function). Yet it still takes for granted that lift and induced drag coincide (and thus produce the various consequences being discussed throughout this thread, even to include concerns for designing an experimental airplane).
Yes, they must indeed coincide and I cannot doubt that, since - planes fly and pilots use the assumption to fly them well. But defining lift as a function is more like "redefining" it to protect the assumption. It's just another way of "taking it on faith" (as our instructor in PPGS says from time to time, implying that we will gain the understanding later on).
The part that stands out to me is the emphasis on airflow as the input for the function. This ascribes to it causality. It begins with a pressure differential. Then stuff happens to the airfoil like developing lift? Do the air molecules gain velocity above the wing because the wing moves through the same absolute space at a constant speed, while the molecules are forced to cover a longer distance due to the shape of the wing?
Like, why do they gain velocity just because an airfoil is there, man. I guess I can intuit the pressure differential since the engine produces both suction and blast. But what about the velocity of air above the wing? I kinda think I got it, but then I think about it some more and I probably dont.
So, you are banging on the door of one of the most commonly misunderstood - and often not at all understood - assumptions in aviation.
Most pilots have little idea how a wing actually produces lift, nor how that lift is actually related to drag. But to blame them for their ignorance is likely akin to blaming a crack baby for being addicted to crack. It's not much their fault for misunderstanding a concept that was poorly explained to them.
Here's the thing. Fluid dynamics are a female dog. Even very clever physicists find the topic highly challenging.
While I very much appreciate your intuitive sense of WTF? and your curiosity to learn more, you will be hard pressed to find binocular vision in the kingdom of one eyed men.
At some point, we mere practitioners of aeronautical appliance operation must accept the abstract of the actual physics and take the practical outcome of those facts at some level of "faith". If pilots actually understood physics, most of them would be working for NASA or, more likely, working as quants on Wall Street.
Sadly, the physics of lift production by an airfoil is typically abstracted to an absurdity that the molecules of air impacting the leading edge of a wing split - with half of them moving above the wing and half of them moving below. It should already be obvious that we have already entered fairy tale land. But now it gets even better. The tale continues with a bunch of big words to explain how the top molecules are moving faster than the bottom molecules. Then it goes full crack-head and suggests that baby Jeebus demands that all those molecules MUST meet up at the trailing edge of the wing...
All of that is pure bunkum.
Three are several complicated factors that create lift. And yes, some of them have to do with the Bernoulli effect. But at the end of the day, just think of lift using a basic understanding of the 3rd law of Newtonian mechanics. The wing diverts the air molecules smashing into it into a net downward force. That net downward force creates an equal and opposite upward force that we chain smoking drunks refer to as "Lift".
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