Stall progression

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by pictsidhe, Mar 25, 2019.

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  1. Mar 26, 2019 #21

    BJC

    BJC

    BJC

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    The "stall strips" on the Glasair are made from 1/2" angle and are about 8" long. Their purpose is to create turbulent flow over the horizontal stabilizer and elevators as the critical AoA is approached to create feedback of the impending stall through the stick. Works good.


    BJC[​IMG]
     
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  2. Mar 26, 2019 #22

    Pops

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    Those stall strips looks the same size and location of the stall strips on my F-12. But it never had any pre-stall buffet before or after.
     
  3. Mar 26, 2019 #23

    wsimpso1

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    My friend, you posted on a forum. You are going to get responses. And they were well meaning and written so other folks reading them benefit too.

    As to what foil works well at Part 103 Re, not my area. But you are concerned about keeping good stall behaviour, and I will talk about that.

    Have you been through Harry Riblett's book? He points out that good stall behaviour has been found on airplane designs where the same foil is used from root to tip, whether straight wing or tapered wing or elliptical wing, and with zero washout to boot. The classic NACA report showing poor stall progression was taken from tests where the foils used at the tips was a much smaller percent thickness foil than the root that also had more leading edge reflex - the tip foils used were more prone to sudden stalls than the root foils. NACA had ended up testing taper at the same time they tested the interaction of good root foils with poorer tip foils, it resulted in an erroneous conclusion about taper, and it got propagated. That erroneous conclusion is now inside your head...

    Please read up on this. A bunch of folks have done wings the way Harry wrote about and it works great. Use one foil form that is known to work OK in your Re range and airplanes in your wing loading, use the same foil from root to tip, know that the scheme does work pretty well.

    After that, you get to test and fix any misbehaviour that remains (and is kind of unpredictable before flying):

    Do some spin training before flight;
    First flights with lift off and landing approach at 1.3 Vs;
    Do stalls up high so you can recover from wing drop or a spin entry;
    If it drops a wing, recover, maintain at least 1.3 Vs until the wheels are ready to touch;
    Start in with stall strips and/or VG's and/or other fixes.

    Billski
     
  4. Mar 26, 2019 #24

    pictsidhe

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    Yes, I have read comrade Riblett's book. I am probably going to use Riblett type airfoils. Anecdotally, they work well. I am not convinced by everything he says and I'm going to stick with the everyone else theory on tapered wings being more prone to tip stalls. If you look at the cl distribution on rectangular and tapered wings, you may see why.
    My test plan is do everything I can to survive it. I may be a bit crazy trying to build a plane from coroplast, but I'm sane enough to want to test its limits with a silken plan B and enough air to use it. Something I've never heard about with spin testing, a release-able tail weight. A half gallon of water released from the tail would add around 2% to the static margin. If I'm slowly moving my cg back, that could get me out of an unrecoverable spin. It won't hurt anything on the ground, either.
     
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  5. Mar 26, 2019 #25

    BJC

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    A moving weight in the cockpit has been used for testing deep stalls in canard aircraft. I would prefer a spin chute rather than a weight on the tail.


    BJC
     
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  6. Mar 26, 2019 #26

    Scheny

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    Hi,
    also our project, the Beast One (as can be seen in my avatar) had the exactly same problem of high stall speeds due to the elliptical planform of the less than 18 feet wing. The outmost third would have stalled at landing speeds due to the low Reynolds values.

    This is when I came (by incident) to the work of an aerodynamicist who specialized in the invention of foils for huge scale RC planes and helped me to develop the wing. Originally the wing used NACA 64xxx foils and since the redesign I have 23% more lift at the same drag plus wingtip stalls are gone. VG's and prisms simply were no solution we would be satisfied with.

    Andreas
     
  7. Mar 26, 2019 #27

    Pops

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    I believe there was an old article in Sport Aviation about using a weight that was moved by the pilot by turning a hand crank and a bicycle chain to move the sliding weight. Would make the rear CG and spin test safer
     
  8. Mar 26, 2019 #28

    pictsidhe

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    I've heard about moveable weights, and they seem a great way to move the cg during flight testing. But when six turns into a spin I am failing to get out of, and with altitude rapidly decreasing, I'd much rather pull a release lever than crank a weight forwards. For varying cg, pumping water back and fore appeals, especially with an emergency dump facility on the rear tank. Maybe I'm being overly cautious, that's only going to cost me time, though...
     
  9. Mar 27, 2019 #29

    Heliano

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    Most interesting postings.
    I just want to call your attention to something we've used in some designs here in Brazil to have a more benign stall, including the Embraer Tucano, which hasn't been mentioned yet: slotted ailerons. Slotted ailerons do increase parasite drag to some extent, but very little. The positive side of it is: retards the wing stall in that portion of the wing and do increase the overall CLmax. A leading edge stripe is in fact a way of improve stall propagation, but at a cost. We use the leading edge "angle bar" as we call it here on one side only, to compensate for asymmetries, very small.
    Wood wings may have a higher washin-per-g, as wood has a lower elastic modulus. My advice to those intending to design a wood wing from scratch is: do investigate the load factor effect on the wing. It depends heavily in the chordwise structural shear center of the design.
     
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  10. Mar 28, 2019 #30

    pictsidhe

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    I will look into slotted ailerons. That sounds interesting. Do you have any NACA links?

    Meanwhile, rather than rewrite the FORTAN IV computer program in the paper at the start of the thread, I thought I could cheat by examining the cl of root and 70% sections and attempting to get the same relationship. At my taper ratio, having the 70% around 6% less than root lift is going to give an 'interesting' stall if not Fw190. If it is 10% higher, it should be very tame. The same lift at root and 70% should be acceptable, if not a pussycat. The full size root airfoil doesn't mind 103 Re much, but the tip airfoil trips over and falls down a hole... I am working on the assumption that while xflr5 doesn't model stalls too well, it's going to be a similar % out on similar airfoils.
    I played around in xflr5 last night and have a pair of airfoils, but with mismatched Cm, so I need to check the overall wing Cm to see if they are usable or need more tweaking. I did find that I need to thicken the root to 21% to get a good margin at the high end of my speed range. Oh dear, that will make my spar caps 10% lighter. Compromises, compromises!
     
  11. Mar 28, 2019 #31

    blane.c

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    A friend of mine had similar looking stall strips on the inboard section of his O-470 powered Navion. A little "burble" a few knots above the stall is good.
     
  12. Mar 28, 2019 #32

    radfordc

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    No kidding! Look at the airfoils used on typical ultralight airplanes...a flat bottom and a curved top. It's said that the TEAM Minimax uses a #12 airfoil (curve drawn with the side of a size 12 shoe). At ultralight wing loadings airfoils aren't super critical.
     
  13. Mar 28, 2019 #33

    blane.c

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    Stall Progression; Stall, spin, crash, dowsed with gas, fire, burnt, first responder mentions "magneto", reporter hears, news proclaims man dies in aircraft accident caused by magnesium fire.
     
  14. Mar 28, 2019 #34
  15. Mar 29, 2019 #35

    pictsidhe

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    Well, the DH53 has a similar wing area, similar gross weight, similar speeds. One crashed in 2012, killing it's very experienced pilot after a nasty stall. That has a rectangular wing, but a vintage 'sharp' airfoil.
    I can design in what should be a good combination of airfoils now, it will make zero difference to construction if I used PXE1 and PXE2, or clark YH. xflr5 really is not impressed with the clark at my speeds, though...
     
  16. Mar 29, 2019 #36

    radfordc

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    Reference the DH53 crash...it's not apparent that a "nasty stall" was the conclusive cause.

    "Conclusion to the official AAIB report into the accident: The aircraft departed from controlled flight for reasons that could not be fully determined. Technical failure of the aircraft and pilot incapacitation were considered, but ruled out as causal factors. Given the prevailing weather conditions and the challenging operating/handling characteristics of the aircraft, it is considered that the most probable cause of the accident was handling related."

    "Synopsis The pilot lost control of the aircraft in gusty wind conditions during a re-familiarisation flight. There was insufficient height in which to recover and the aircraft impacted the ground, causing the pilot to receive fatal injuries."

    "The Chief Pilot had considered that the conditions were unsuitable for some of the flying planned for the day and he had cancelled some aircraft and less experienced pilot combinations."

    "The Luton Airport TAF issued at 0500 hrs on the day of the accident gave a forecast wind from 230° at 12 kt, with a 30% probability, between 0900 hrs and 1500 hrs, of wind temporarily from 240° at 17 kt, gusting 27 kt."

    If you plan to fly your Hurrican in 27kt gusts you better have the best airfoil you can get!!
     
  17. Mar 29, 2019 #37

    Chilton

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    The DH53 wing was constant chord but tapered in thickness from the strut to both the root and tip, the unfaired strut joint to the wing was reported to cause some interesting flow effects and the ailerons were very large and according to the test pilot who flew it for permit approval after rebuild tended to snatch.

    The wing section was also heavily under cambered as was common at the time it was designed and many of these sections have interesting stall behaviour.

    As a combination the taper in thickness, undercamber, aileron behaviour and a light weight underpowered aircraft loosing speed rapidly in manoeuvre I believe it is easy to see the machine being very unpleasant in this situation.

    The Hurricane wing with almost any normal flat bottom section is unlikely to have a fraction of the bad tendencies of the DH 53 wing.
     
  18. Mar 29, 2019 #38

    Aesquire

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    Sliding weights that can jam under sideways G loads as a way to improve the safety of stall/spin testing, scare me.

    A gallon jug with a high reliability release systen, for example, a skydiving "3 ring circus" single cable release, is intriguing, but may violate FAA regs on dropping stuff from airplanes. A fancy valve system for CG changes has killed a lot of people in very expensive bombers just in my lifetime. Iirc the B-58, B-1, and B-47? all had crashes related to CG shifts with fuel system systems to move mass around. Please correct me if I'm wrong!

    Re: NACA airfiols.

    The dirty little secret of NACA airfoils, and.... Full disclosure, I obsessed over "Theory of Wing Sections"...... is that they were never intended to be a catalog of great airfoils for airplane designers to use.

    They were used that way a lot between their creation and today, most WW2 aircraft have variations of those shapes lifted from the files to save time and work. ( and why not? When reinventing the Wheel, everyone uses Ogg's patent#3 aka, "it's round" )

    No....those endless numbers generated by the NACA were a brute force approach to data creation/collection. To create a database that charts could be derived from, theories made & tested, equations compared to.

    A "kickstarter" to the creation of an actual science of aerodynamics, to replace TLAR and inspiration with algorithms and mathematical precision.

    Ideally, you'd pick the characteristics you wanted, look up shape "x" see that shape "y" had some better aspect, and create a new shape that gave you the best of both for your application. I don't think we ever quite got there. :)

    Harry Riblett spent many hours in a wind tunnel making changes, testing, change, test..... Real Olde School experimental science. I'm told he goes off the rails on some political aspects of the reactions to his work from "the establishment", so I take his opinions of PEOPLE with some salt, but the work is both scientific and inspired.

    And he may have been right about the establishment and the "not invented here" mind set. I wasn't there for that. ;)
     
  19. Mar 29, 2019 #39

    BJC

    BJC

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    Yup, that is why they designed the installations to avoid that potential problem.
    The data that I have, Riblett's GA Airfoils, states that his work was with computer models, not wind tunnels.
    He loses lots of credibility with his comments about the 23000 series, and how it is a dangerous airfoil, in spite of the exemplary performance of that series on multiple GA and larger aircraft.


    BJC
     
  20. Mar 29, 2019 #40

    blane.c

    blane.c

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    Having flown behind the NACA 230xx airfoils, I found them to be honest airfoils that would carry a buttload of ice. Sometimes collecting ice on approach you would get a buffet, add another notch of flaps and a couple inches of power and drive it on in.
     

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