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Lift %, Upper/lower wing sections

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Lift %, Upper/lower wing sections

Old 2nd Jul 2022, 23:25
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Devil

Originally Posted by PEI_3721 View Post
Hold a sheet of paper in front of your mouth; blow over it (on the top side only). The paper should rise, the pressure falls thus the paper rises.
Now, hold the paper so it hangs vertically. Blow down over one side (ie same experiment but twisted 90 degress so paper hangs down,). Shouldn't the paper curl to the side you blow?

Try it! Very easy to do.

It just hangs vertically, doesn't move towards the "blown" side despite air moving over one side faster.
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Old 3rd Jul 2022, 04:26
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It just hangs vertically, doesn't move towards the "blown" side despite air moving over one side faster.

Indeed, the paper trick is a little misplaced. The driver is the curvature of the paper (think boat sails), which exists when blowing horizontally, and leads to a momentum change (force which is the lift) ... but not if the paper hangs vertically.

For those who have never heard of circulation and Prandtl (and that is 99 % or more of pilots), the first video in Vessbot's post #16 is of a 16 mm film taken of a water tank experiment by Prandtl in 1929. He, and his students, had been working on getting a flow visualisation of the circulation starting/stopping vortices which they knew, theoretically, existed. They experimented with several types of particles to achieve the reflective record - one was just small flakes of aluminium. A neat sideline in the video is that the Reynold's Number suits the formation of Karman (or von Karman) vortex streets - the oscillating vortices in the wake. To demonstrate the small, incestuous nature of aviation, von Karman was one of Prandtl's academic students ...

Providing you don't want to get too involved in the sums (David, and the link to which he referred, can do that sort of stuff for us much easier then we can) the basic circulation model concepts are very easily explained and understood. I use them for my theory class students and no-one seems to have any great problem. However, due to the poor communications in the early days of aviation (when there was not much academic interest) all the enthusiast guesses as to what was going on led to all sorts of half baked ideas which became entrenched in subsequent pilot training. Think about it .. folks still prattle on about Coanda (which is of little relevance) and Bernoulli (his stuff is fine, but used rather out of place to try and explain aspects of lift) when physicists and engineers have been using the circulation model for a CENTURY to work out flow numbers.

Is it any wonder, then, that thousands upon thousands of pilots are totally confused about lift and lifting surfaces ? As a great little tee shirt, given to me last Christmas by a grandson, has it, lift is all about magic. Different bits of the aeroplane have different contributing magical qualities .... and not a word about circulation, Prandtl or fact-based simple explanations.
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Old 3rd Jul 2022, 08:57
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Aaaah! The power of vortices. Thank you John T, Vessbot, tdracer, megan and others for a really 'adult' discussion on lift. Well informed, erudite and slayers of myths!!

Insects are rather good at it too:-
https://www.nature.com/articles/521S64a
https://arxiv.org/pdf/1803.07330.pdf

Mother Nature had it sorted millions of years ago.
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Old 3rd Jul 2022, 10:39
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Going on from the two references, it is useful to review dynamic stall in our world. A significant problem for the design of those fling wing devices so Megan probably can comment ?

Interestingly, as with insects, fish use shed vortices to aid in their swimming motions ...

"The FAA teaches that the air wants to get from A to B over the top, at the same time as the flow goes underneath from the same points. "

That sounds suspiciously like the "equal transit time" idea. Air doesn't work that way if the wing is generating lift. A pet peeve of Holger Babinsky who put together this little video using his classroom portable wind tunnel.


Another video shows a bit about how things vary with alpha

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Old 3rd Jul 2022, 14:17
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Originally Posted by john_tullamarine View Post
Going on from the two references, it is useful to review dynamic stall in our world. A significant problem for the design of those fling wing devices so Megan probably can comment ?
Martin did his PhD thesis in the area of dynamic stall effects and then went on to do the Boeing Vertol study on the VR-7 blade and the 1094 to try and suppress the hysteresis that arises on the coefficients on the spinning rotor. The plots of CL, Cd and CM on a dynamic pitching blade is pretty depressing when you consider how much effort is taken in trying to obtain an elegant Cm for a rotor blade, like the VR7/1094, OA series, Eppler and various other attempts to achieve a Cm that doesn't result in high pitch link and swash plate loads, and then the dynamic pitching swamps by an order of magnitude the steady-state Cm values, and nothing is steady at all. The comment previously on vortex streets comes into play, every pitching event generates a shed vortex that just messes up everything. Add on top SBLI effects near the tips and that messes up the boundary layer, and is itself unsteady too. Babinsky had some success with micro VGs and with bleed for shock-related issues, but conventional VGs massively increase drag as Martin found on the rotor that has dynamic stall conditions. Our own studies showed the same outcome, you can. suppress the vibration, you just need another engine in the UH-60... When we went off with the submerged VG, that changed, drag went down in all cases over the clean blade.

The following are Star CCM+ DES of a B737 wing section, with shock formation. The section models a slat sitting with a 1mm aft step over a chord of around 5 mtrs, so the mesh used is pretty fine, the solutions were taking around 2 months of continuous supercomputer time to get a single solution concluded.





The next is an absolute pressure plot


The following is a look down the surface over the VG, again, a 1mm aft facing step of the VG on a 5 mtr foil section for Re scaling.


The R22/44/H500/B206/OH58, Safari, UH1H, and the CMRB on the SK61/HH3 showed both improved performance but Craig W's comment on the SK61/CMRB was the removal of vibration that was apparently spooky. The EN280 had an odd outcome, it had the least benefit in required hover power, but had the largest effect in the cruise power required. Then the Chinese went to cheese head land... On the B737, the old classic went from 0.805 to 0.823 for the same flow, but the effect at lower speeds was fairly modest, but shock-related vibration was damped noticeably. For all that, the trailing edge mods are vastly more interesting than the leading edge conformal VGs. Turns out that rotors like TE mods, a lot, like propellers, and fan blades. The V22 and the MH47 remain the most interesting rotors to play with, but fan blades were without a doubt most spectacular. Apart from large changes in Cl and reduction in Cd, tabs on a fan blade reduced vibration in every case we tried, including asymmetric hemispheres of the disk. That led to the observation that lift enhancement tabs also suppress dynamic stall, which occurs any time that the intake does not have axisymmetrical flow, which is pretty much always. The JT15D-1A that we used was pretty daggy, it was beaten up and was puffing out 1800lbs ST instead of 2200 at 100%. We got 1800 lbs at 92% with tabs, and 2400 at 100%, which seemed quite fair. I left before we tried the TFE 731 or our CFM56-3 in flight, but the potential remains pretty interesting. A turbo jet like the CJ-610-8/J85 dry has an TSFC of 1.00 static/SL, and about 1.00 at M0.8/FL350. A TFE731 has 0.35 static, and that degrades to 0.7 at M0.8/FL350... the fan CL collapses, it is after all a fancy fixed pitch prop. Tabs shift Cl by around 0.4-0.6 CL and can delay drag rise considerably but are geometry dependent. In all cases, tabs reduce vibration. On the R22's 63015 section, the mid-span TE tab dropped the stall Nr from the test 82% through 77% for the LE mod only to 68% for the LE & TE mod. The vibration reduction was considerable.
Wendt's study on Gary O. Wheeler's "wishbone" shows how neat Gary's design was. J.C. Lin's parametric study on micro VGs was really neat too. They have issues on a rotor, but they are interesting on a normal wing. Gourdains stuff was neat, as was the flow modeling that was being done on compressors in the Whittle lab a few years back, but compressor effectiveness is nice top have, but the fan blade is losing most of its effectiveness in the cruise and is readily modified, even in the field.

Anyway, bottom line is...
  • dynamic stall is a pain on rotors, blades, props and fans;
  • anything that sticks up on a rotating blade causes separation and the flow flicks spanwise instead of chordwise and then sheds in unsteady vortex structures
  • the conformal vortex generator surprisingly didn't exhibit the spanwise disruption, the wake remains attached as a sub-boundary embedded vortex series.
  • fan blade tabbing, like sub-warp transporters, is actually quite straightforward to achieve, but you need a sense of humor and deep pockets to conclude the STC.
PS: don't try this at home... messing about with propellers/rotors and fans have inherent warning flags.














Babinsky, H. (2011). Shock Boundary Layer Interaction Flow Control with Micro Vortex Generators (Issue May).
Babinsky, H. (Cambridge U. (2008). Understanding Micro-Ramp Control for Shock Boundary Layer Interactions (Vol. 298).
Holden, H. A., & Babinsky, H. (2007). Effect of Microvortex Generators On Separated Normal Shock/ Boundary Layer Interactions. Journal of Aircraft, 44(1), 170–174.
Lin John C. (1999). Control of Turbulent Boundary-Layer Separation using Micro-Vortx Generators. 30th AIAA Fluid Dynamics Conference.
Lin, J. C. (NASA). (2002). Review of research on low-profile vortex generators to control boundary-layer separation. Progress in Aerospace Sciences, 38(4–5), 389–420.
Lin, J., Robinson, S., McGhee, R., & Valarezo, W. (1994). Separation control on high-lift airfoils via micro-vortex generators. Journal of Aircraft, 31(6).
Lin, J. C. (NASA La. R. C., Howard, F. G., & Selby, G. V. (Old D. U. (1990). Control of Turbulent Separated Flow Over a Rearward-Facing Ramp Using Longitudinal grooves. Journal of Aircraft, 27(3), 283–285.
Casper, J., Lin, J. C., & Yao, C. S. (2003). Effect of sub-boundary layer vortex generators on incident turbulence. 33rd Fluid Dynamics Conference.
Yao, C., Lin, J., & Allan, B. (2002). Flow-field measurement of device-induced embedded streamwise vortex on a flat plate. NASA STI/Recon Technical Report …, June, 16.
Martin, P., Wilson, J., Berry, J., & Wong, T. (2008). Passive Control of Compressible Dynamic Stall. 26th AIAA Applied Aerodynamics Conference, 1–33.
Titchener, N., & Babinsky, H. (2013). Shock Wave/Boundary-Layer Interaction Control Using a Combination of Vortex Generators and Bleed. AIAA Journal, 51(5), 1221–1233.
Storms, B. L., & Ross, J. C. (1995). Experimental Study of Lift-Enhancing Tabs in a Two-Element Airfoil. Journal of Aircraft, 32(5), 1072–1078.
Ross, J. C., Storms, B. L., & Carrannanto, P. G. (1995). Lift-Enhancing tabs on Multielement Airfoils. Journal of Aircraft, 32(3), 649–655.
Carrannanto, P. G., Storms, B. L., Ross, J. C., & Cummings, R. M. (n.d.). Navier — Stokes analysis of lift-enhancing tabs on multi-element airfoils. Aerospace.
Storms, Bruce L (Sterling Software, Moffett Field, C. 94035), & Jang, Cory S (California Polytechnic State University, San Luis Obispo, C. 93407 E. (1993). Lift enhancement of an airfoil using a Gurney flap and vortex generators. Journal of Aircraft, 31(3), 542–547.
Cavanaugh, M. A., Tech, V., & Mason, W. H. (n.d.). Wind Tunnel Test of Gurney Flaps and T-Strips on an NACA 23012 Wing. Test, 1–18.
Singh, M. K., Dhanalakshmi, K., & Chakrabartty, S. K. (n.d.). Navier-Stokes Analysis of Airfoils with Gurney Flap. Dimension Contemporary German Arts And Letters, 1–6.
Hage, W., Meyer, R., & Schatz, M. (n.d.). Comparison of experimental and numerical work on three dimensional trailing edge modifications on airfoils. Cfd4aircraft.Com, 1–11.Catalano, F. M., Ceròn, E. D., & Greco, P. C. (n.d.). TRAILING EDGE TREATMENT TO ENHANCE HIGH LIFT SYSTEM PERFORMANCE. Icas.Org, 1–10.
Wendt, B. J., & Hingst, W. R. (n.d.). Measurement and Modeling of Flow Structure in the Wake of a Low Profile “Wishbone” Vortex Generator.
Fernández, U., Réthoré, P., & Sørensen, N. N. (n.d.). Comparison of four different models of vortex generators. 1.
Gourdain, N., & Leboeuf, F. (n.d.). Unsteady Simulation of an Axial Compressor Stage with Passive Control Strategies. Cerfacs.Fr.

Last edited by fdr; 3rd Jul 2022 at 14:33.
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Old 3rd Jul 2022, 23:51
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Some interesting stuff there. I never cease to be amazed at what PPRuNe manages to pick up in the way of expertise.

I guess fdr has let Megan off the hook re dynamic stall problems in the rotary world.
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Old 4th Jul 2022, 00:15
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The utilisation of the lift phenomena can show up in unexpected places. There is not a helicopter pilot in the world who doesn't under stand what the phrase "running out of pedal" means. For the incognisant it is when the pilot has demanded a level of power from the main rotor that the tail rotor is unable to counter, so the helo begins an uncontrollable yaw. The pilots of UH-1C gunships flown in Vietnam didn't have the power to sustain a prolonged hover when loaded for bear. To get from the revetment to the runway involved a series of jumps. Pilot would pull into the hover and start hover taxiing, the rotor would begin to bleed RPM, when reaching the point that you ran out of pedal, chop the throttle, thus regaining pedal control, perform a hovering autorotation, wind throttle back on after landing, rinse and repeat. On reaching the runway a running take off would be made, at times the two back seat crew would run along side to ease the load and jump back on board when the aircraft reached translational lift. An enterprising individual some where realised that the rotor wash passing over the tail boom generated lift in a horizontal direction and could be tailored to aid the tail rotor. Strakes fitted to the side of the tail boom trip the airflow into turbulence, effectively stalling that side of the tail boom. Now standard fit on some types.

PS: Just after we left the UH-1C was upgraded to UH-1M standard by simply replacing T53-11 engine with the T53-13, bliss, you could hover.


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Old 4th Jul 2022, 00:55
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Now, how do we keep this thread running forever, just getting better and better ?
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Old 4th Jul 2022, 13:05
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Originally Posted by megan View Post
The utilisation of the lift phenomena can show up in unexpected places. There is not a helicopter pilot in the world who doesn't under stand what the phrase "running out of pedal" means. For the incognisant it is when the pilot has demanded a level of power from the main rotor that the tail rotor is unable to counter, so the helo begins an uncontrollable yaw. The pilots of UH-1C gunships flown in Vietnam didn't have the power to sustain a prolonged hover when loaded for bear. To get from the revetment to the runway involved a series of jumps. Pilot would pull into the hover and start hover taxiing, the rotor would begin to bleed RPM, when reaching the point that you ran out of pedal, chop the throttle, thus regaining pedal control, perform a hovering autorotation, wind throttle back on after landing, rinse and repeat. On reaching the runway a running take off would be made, at times the two back seat crew would run along side to ease the load and jump back on board when the aircraft reached translational lift. An enterprising individual some where realised that the rotor wash passing over the tail boom generated lift in a horizontal direction and could be tailored to aid the tail rotor. Strakes fitted to the side of the tail boom trip the airflow into turbulence, effectively stalling that side of the tail boom. Now standard fit on some types.

PS: Just after we left the UH-1C was upgraded to UH-1M standard by simply replacing T53-11 engine with the T53-13, bliss, you could hover.



Good stuff Megan.

While I was doing the slat trick on the B737, I got asked to help a UH-1H/T700 that was doing some interesting work over water, at low speed, and with one very large mass appended at the end of the sling. The guys were having some excitement when operating below ETL, at high power, lots of barf bags needed, and expensive splashes were issues... lots of torque, same ol' TR system... I had previously done some testing on a B206BIII and an OH58 with different blades... and they had been able to come and see what we were doing on wings and props. The UH1 with the big donk gets lots of available torque, but the TR is standard. We placed a mod on the MRB LE and that worked well, but they wanted as much TR authority as could be gained. While i have modded TRBs before, I expected that they would get enough out of 2 foam rubber strips on the port side of the boom, at the 7 and 10 positions, and a couple of rows of foam rubber VGs on the RHS. The 3 remaining options, tabbing the MRB, putting CVGs on the TRB and tabbing the TRB were not needed.

The trick with tabbing a propeller, rotor or fan blade is doing that and not killing yourself at the same time. Most of the outcome is benign, remarkably so, but I recall being taught that adding mass to a TE could define bad days, and having half a dozen torsion and bending modes happening seemed like a bit of an issue to put anything in there that would alter the moment of inertia of the area with some of the highest strains... seemed like a good way to cause a super quick failure of the skins, or to cause the bonding to fail instantly, or to have the tab blow apart, or all 3 options. Upside was that fluids don't care what a device is made of, just if it will cause the desired flow modification, and for most tabs, that's a set of 3 transverse vortices and a couple of stagnation points. 30+ years on props gave some confidence on the rotor to try that out, and golly, if it isn't quite an interesting outcome.

While rotors suck as far as dynamic stall effects go, they really do, they are also rather sensitive to the addition of any disruption to the boundary layer, which just doesn't seem fair; there is no silver lining. Nevertheless, things can be improved, although lordy, the Enstrom and its 0015 section is darn hard to make happier in the hover without getting serious. Still, a rotor head that permits you to take your hands off the cyclic and fold them has some drawcards. Don't try that on an R-22...

anyway,

for some background, for a certain Brand X rotor, a vanilla 63105 club, the following are recorded data outcomes... It is consistent with the 2 D RANS CFD analysis done well before and published in AIAA J. Aircraft, and is also consistent with an EN done in early '90s (IIRC) looking at the BO-105 MR in autorotation, and certain modifications to same; using Wayne Johnsons CAMRAD II code.

Putting a tape on the LE increases the stall Nr of the blade, and takes more ergs to hover. Representative stall clean at 82%, 68 units of torque, add tape to the last 3 ft of the blade, and the stall Nr goes to 85%, and torque to 72u.
Place a conformal VG tape (a weird variant of a zig-zag tape although doing a different resultant flow...) on the same location, and the stall drops to 77%, and torque goes to 65u... add 3 feet of tab mid-span, and the stall Ne goes to 68%, and (seriously) the torque drops to... 34u. that is thirty-four, relative to a NACA clean section, 68... Look closely at the effect of an LET and it can be seen that the outcome is geometry dependent, a matter that caused major delays with development of LETs anywhere on planes, but in the right spot, it is possible to both get massive changes to drag at the same time as lift gets a large boost, of around 0.4 to 0.6 CL. Works on props, works on rotors, and it worked on the static test of the fan blade. LETs can impact the trim condition for auto rotation, and that was, without doubt, the most concerning flight that I ever flew, and that did include a parachute and a means to remove blades and not heads from the helo.... In comparison, playing with props and fans is much less exciting.

To prove the point on the effectiveness of the LET being actual, and not observer bias apart from the instrumentation of the performance and engine instruments, and straining the rotor root and pitch links, the flight controls were measured and recorded as well s displayed in real-time. The "standard" NACA clean rotor gets to stall Nr about the time the pedal hits the stop, For the conventional HS8663/8671 linear blade tape, a stall occurs as LTE has commenced, gets interesting. For the conformal VG tape, around 25% pedal remained at stall. for the tabbed blade, the pedal required was less than 50%, and turns in the anti-torque direct had essentially no constraints, other than ear plugs for the blaring underspeed (underspend?) warnings. From idle, the collective could be raised fully up, the throttle increased, and the rotor would accelerate. (don't do that at home, certainly not with anything that uses TT straps...) increasing the Nr to 68% resulted in liftoff, hover and some weird and uncomfortable flight. Rotors should be at normal Nr at all times, but from a standpoint of weird science, not sure that any blade at full collective will accelerate from idle without a tab. It suggested that the reduction in torque demands with a tabbed blade are quite real. They are repeatable and are consistent with published works with AIAA and on NASA NTRS. Just happens that tabs can be done.

The V-22 and the MH-47 are the poster children of systems in need of a bit of tabbing... look at the video of the U.S.S. Green Bay bingle... But, as much fun as rotors are, the most interesting application, other than ships props, C-130 props etc, are fan blades... I bought a jet just to do that, and circumstances intervened, but darn if that isn't more interesting than the effect on a rotor.

Just to be clear, fan blades are fancy fixed pitch props, they are a little more, once the overall effect of intake geometry has occurred. Thereafter, the TSFC collapses as TAS changes. The TSFC for a pure jet more or less is constant, nearly so, whereas the TSFC of the fanjet degrades by a factor of at least 2 to M0.8/FL350 from SL/static. That is where the parametric effects of tabs gets interesting. If anyone has a spare CFM56, TF-30, JT9, RB211, GE90... (or TFE731... mine are spoken for which is a necessary nuisance)

End of the day gotta go stop a large cat from snoring. Maine coons are an acquired taste

Aerodynamic cat plan form:






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Old 17th Jul 2022, 07:13
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Coanda effect has relevance indeed ; it effect shows in its purest form the Newton law, thanks to
which the lift is generated. In fact , even without any pressure difference between upper/lower side, the curved shape generates lift,
thanks to imparting a deviation to the fluid flow - try with a spoon under tap water flow, where pressure is the same
on the upper and lower side of the spoon (it is the athmospheric pressure, both for the water side and the air side ) .

Obviously , for Coanda effect to take place, fluid shall not be inviscid,
otherwise it will not remain attached to the "upper" side, and consequently will not be forced to deviate from its initial direction.

When I tought basic aerodynamic to pilots, I started with water skis ; there, the lift is generated purely by deviating
a mass of water, creating an opposite force which lifted the ski. Independently from the difference in static pressure above and below the ski
(which is nil, since above the ski there is the static sea level atmospheric pressure , and below there is the static sea level water pressure,
which is equal to the atmospheric pressure !), lift is created just by deviating, the incoming water flow, which obviously creates a dynamic pressure.

The beauty of the wing profile is that , thanks to specific pressure distributions obtaine throug a peculiar shape, it is able to deviate the incoming mass flow with much less drag
than a water ski, even at a zero angle of attack, wsomething that the water ski is not able to do.
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Old 18th Jul 2022, 11:10
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(a) Coanda effect has relevance indeed .... even without any pressure difference between upper/lower side, the curved shape generates lift

I don't think so. Else, how do you explain Jonkster's last comment in Post #21 ? If you don't have a pressure delta, you don't have a force. You really don't need to appeal to Coanda, the effect is explained with reasonable ease by Newtonian thoughts.

While you can impose a force directly on a solid due to its ability to withstand shear stresses, that is not feasible with fluids, either liquid or gaseous. In the case of a fluid, there must be a pressure gradient or delta over a region for a force to be developed. With a moving fluid turning a corner, there is a transverse pressure gradient which does the honours - hence the usual graphics associated with flow over an aerofoil.

Can you offer any objective evidence (say, pressure tapping data) to support your theses that
even without any pressure difference between upper/lower side, the curved shape generates lift and try with a spoon under tap water flow, where pressure is the same on the upper and lower side of the spoon (it is the athmospheric pressure, both for the water side and the air side ) .

(b) lift is created just by deviating, the incoming water flow, which obviously creates a dynamic pressure. But this is not very relevant to what an aerofoil might be doing ?
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Old 18th Jul 2022, 18:17
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Playing with lift. Talking to a delta wing fighter driver who had a penchant for low flying related how if you had the aircraft sitting on the ground cushion you could push the stick forward and the nose would lower, but that's all that happened. Amount/degree of 'push' not elaborated upon. The only explanation I could come up with later when giving it some thought was the increased camber by the elevons being deflected down counter balanced the "loss" of lift due to the reduced angle of attack, so the status quo remained. Not a chap inclined to stretch things for a good story.
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Old 18th Jul 2022, 21:46
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Angel

My father held a B.S. in Mechanical Engineering from the Georgia Institute of Technology and a M.Sc. in Marine Architecture from the United States Naval Academy. Dad was the lead designer of the Essex Class aircraft carrier hull; he was also a true gentleman with a superlative sense of humo(u)r and the heart and patience to be a wonderful parent.

I had lengthy conversations with him regarding the possibility of hydrofoil sailing vessels. He claimed that such a ship was an impossibility - I felt that the design was practicable. I lost my father in 1975, but wish for myriad reasons that he had been with us when the 75-foot beauty shown below made its debut in 2012. Had he seen it, Dad would have clapped me on the back and said "You were right all along, son. Let's go get a beer!"

I wonder how lift production in a hydrofoil "wing" traversing an incompressible fluid (water) varies from that of an aircraft wing encountering compressible airflow. Extra credit for an explanation of that beautiful sail/wing....

- Ed


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Old 18th Jul 2022, 23:28
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Playing with lift

An interesting thought, I shall have to ponder it a while. Or, perhaps enlist the expertise of a couple of our aerodynamicists on the matter. Ah, looking back, again, I see Dave has been here already, so I have sent an email off to Clive to see if he might have some thoughts to offer for the other end of his professional speed spectrum.

He claimed that such a ship was an impossibility .... I wonder how lift production in a hydrofoil "wing" traversing an incompressible fluid (water) varies from that of an aircraft wing encountering compressible airflow.

What a lovely shot. A bit above my little RL24 trailer sailer toy class, I'm afraid.

For lower speeds, we can treat air as incompressible with little error. While I have never done any work on hydrofoils, I don't see that they would be doing anything much different to a wing, so far as flow and forces are concerned. Some area, some camber, and a good dose of speed and away we go ?

Indeed, much work is done in water tanks ... the earlier link to Prandtl's 16mm movies relate to work done in a water tank. Air (at a suitably low speed) and water ... same, same ?
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Old 19th Jul 2022, 01:34
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Far higher Reynold's number for water at similar speed; also, look at the relative areas for carrying the opposing forces.
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Old 19th Jul 2022, 02:38
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Ah, once again, I get pulled up for being a bit brief in a story. Consider my wrist to have been slapped.

For those who might be wondering a bit at MechEngr's comments, it may be worth having a play with the following links

Reynolds number and dynamic similarity of fluid flows – Flow Illustrator

Similitude - Wikipedia

If it all gets a bit too confusing, don't worry about it. Just accept that the boffins can play with toys to get useful measurements in ways which don't always make immediate sense to the casual observer.

I recall a tale as an undergrad student - a bevy of politicians was being given a Cook's Tour of a water model of a local beach structure in the Mech Eng lab which had been set up to investigate flow patterns to figure out a way to reduce some erosion problems. One of the pollies was aghast - the model was nothing like the dimensions and depths of the real deal so, quite clearly, the whole thing was a bit of nonsense. Likewise, I can recall a Senate Enquiry with which I had some involvement. A couple of the worthy Senators just couldn't get their heads around what we were talking about in respect of some flutter stuff with a particular aeroplane's empennage - Dave will recall that one, I'm sure.

Sometimes, it gets a tad difficult to get folks to understand the protocols used when playing with similitude and flow modelling.
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Old 19th Jul 2022, 03:52
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No slap intended - just a couple of neurons waking up to recall the Fluid Dynamics class and recalling seeing dye flow tests on small scale models for supersonic designs operating at low speed and high angle of attack in water tunnels to visualize vortex flow.

The one that people get wrong all the time is thinking that rubber is compressible; it is stiffer in compression than steel is. However, it's more elastic and can, if allowed, flex enough to get out of the way. Confine it and the difference becomes really clear. People think air is compressible because they can use an air pump and compress it - but that's only because it is confined. Let it have freedom to get out of the way and compressing it is really difficult.
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Old 19th Jul 2022, 03:57
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it is stiffer in compression than steel is

I learn something new every day ..

Let it have freedom to get out of the way and compressing it is really difficult.

A useful tale which highlights the confusion in many minds regarding the need for a pressure gradient /delta to generate forces in fluids. It is very hard to belt a fluid into submission, in the manner of a lump of hard stuff, when it just slips and slides out of the way.
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Old 19th Jul 2022, 16:46
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Thank you for your answers, john tullamarine and MechEngr! I found the site hyperlinked below to be interesting. It includes some brief but breath-taking video of hydrofoil vessels under full sail. Interesting that the foil itself is a blade two meters in l.o.a. and a width of 0.7 meters. I continue to search for a cross-sectional representation. Astounding to me that these beasts can make in excess of 60 knots!

America's Cup Hydrofoils

- Ed
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Old 20th Jul 2022, 07:21
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John,
I wasn't going to post anything as it has all been said, but since you ask:
Concorde, like many supersonic aircraft, has wings swept so that the flow normal to the LE is subsonic. That being so, the usual explanation fits, that is to say:-

An airfoil passing through a block of static air changes the state of that block in that some of it is left with a substantially downwards velocity

Newton tells us that to change something's state there must be a force applied

In this case that force is the mass flow affected per second times the incremental velocity

Newton can tell us nothing about the affected mass or the velocity, so is useless for calculating lift

Newton also tells us that every action (force) has an equal and opposite reaction

That reaction is the force we call lift acting on the airfoil

That’s Newton’s contribution

The magnitude of the force can most conveniently be calculated using the circulation hypothesis; Lift equals fluid density times circulation times velocity.

Circulation is effectively the strength of the vortex flows around the airfoil

Vortices can only be generated in a viscous fluid, and on a finite wing form a closed loop.

That loop is formed by vortices rotating around a spanwise axis (bound vortices), trailing (wingtip) vortices and way downstream, a starting vortex.

So much for lift generation

Force can only be applied to an object immersed in a fluid by changes in pressure of the fluid in contact with the body.

The distribution of those pressures and forces is best calculated using Bernouilli’s theorem

That covers Bernoulli

That's it really
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