Stall speed
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Originally Posted by SCaro
... We know that without the stall strips the stall speed would be several kts lower, ...
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UNANSWERED QUESTIONS!
Scaro,
You still haven't replied to a couple of the Questions:
1. What type of ac is it?
2. What are your actual differences in stall speed? (10% at around 30kts would seem virtually insignificant)
An observation:
1. A stall is never defined (to my knowledge) as being defined by the location of the elevator or stick. It is the point where significant boundary layer flow separates from the wing and does not reattach, hence causing a steep decline in the L/D relationship.
Some thoughts:
1. Aerofoil Section: As has been pointed out in several replies the aerofoil section is absolutely critical to stall characteristics. The LE being the most critical element. Any slight deformation causing a rapid loss in chordwise positive pressure gradient could trigger a separation and stall. The surface of your LE is also critical as any difference in smoothness will obviously alter the transition to turbulence and therefore the ability of the flow to remain attached. Is the surface absolutely identical? Are you working at low Reynolds Number where laminar flow is even more critical?
2. Engine Thrust: Can you be certain that the engines produce identical power? Are the props identical to transfer all the energy? A small difference in flow over the inboard wing at 30ins fuselage, where you suggest stall begins could have a huge effect on stall speed.
3. Test Aircraft Plastic Wing Deformation: A long shot but has the ac been subjected to loads that may have plastically deformed the wings during frequent high loading? (The thinner section you mention will effect the pressure profile certainly as well) can you be sure that the aerofoil section does not deform in flight different to the other ac? More info required on the ac structures and flight profiles I think!
4. CofG Vs Mass distribution: An equal CofG does not necessarily mean an equal mass distribution. It just means that the average of the masses is in the same location. You may need to investigate the mass distribution of your ac about the wings and the fuselage. Can you take the wings off to test? For example if the Mass is distributed more outboard on the wings on one ac then you can expect a different wing pressure planform and thus stalling characteristics.
5. Some possible tests and some slightly more crazy ones: I am not sure how much money/time you have or the legalities of these suggestions, but I am sure you can find out if you are that desperate! Take the wings off and find the CofG of the fuselage and Wings separately. Swop the wings over on the 2 ac!!?? Even just to reweigh! Can you swop the engines? Get an accurate readout of thrust at least for the 2 engines at idle.
Regards
You still haven't replied to a couple of the Questions:
1. What type of ac is it?
2. What are your actual differences in stall speed? (10% at around 30kts would seem virtually insignificant)
An observation:
1. A stall is never defined (to my knowledge) as being defined by the location of the elevator or stick. It is the point where significant boundary layer flow separates from the wing and does not reattach, hence causing a steep decline in the L/D relationship.
Some thoughts:
1. Aerofoil Section: As has been pointed out in several replies the aerofoil section is absolutely critical to stall characteristics. The LE being the most critical element. Any slight deformation causing a rapid loss in chordwise positive pressure gradient could trigger a separation and stall. The surface of your LE is also critical as any difference in smoothness will obviously alter the transition to turbulence and therefore the ability of the flow to remain attached. Is the surface absolutely identical? Are you working at low Reynolds Number where laminar flow is even more critical?
2. Engine Thrust: Can you be certain that the engines produce identical power? Are the props identical to transfer all the energy? A small difference in flow over the inboard wing at 30ins fuselage, where you suggest stall begins could have a huge effect on stall speed.
3. Test Aircraft Plastic Wing Deformation: A long shot but has the ac been subjected to loads that may have plastically deformed the wings during frequent high loading? (The thinner section you mention will effect the pressure profile certainly as well) can you be sure that the aerofoil section does not deform in flight different to the other ac? More info required on the ac structures and flight profiles I think!
4. CofG Vs Mass distribution: An equal CofG does not necessarily mean an equal mass distribution. It just means that the average of the masses is in the same location. You may need to investigate the mass distribution of your ac about the wings and the fuselage. Can you take the wings off to test? For example if the Mass is distributed more outboard on the wings on one ac then you can expect a different wing pressure planform and thus stalling characteristics.
5. Some possible tests and some slightly more crazy ones: I am not sure how much money/time you have or the legalities of these suggestions, but I am sure you can find out if you are that desperate! Take the wings off and find the CofG of the fuselage and Wings separately. Swop the wings over on the 2 ac!!?? Even just to reweigh! Can you swop the engines? Get an accurate readout of thrust at least for the 2 engines at idle.
Regards
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Solved!
We finally got the stall speed down to where it should be. It turned out to be a combination of two separate issues that influenced each other. The wing leading edge contour was not correct and the flap leading edge contour was not correct either. The latter deviation was not visible even with templates, but showed itself by tuft tests. The flap upper surfaces had completely turbulent flow at full flap deflection. We sanded the flap leading edges until they were nice and round, and it got the flow reattached.
Do a Hover - it avoids G
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Originally Posted by GOLF_BRAVO_ZULU
What I wan't to know is what IAS am I going to depart from controlled flight
Sadly you will first have to let the 'techies' know the present weight, bank angle, g, IMN and rate of change of IAS (to name just the main variables)
Which is why you should demand to be given an AoA gauge. No automatic control system will agree to work without such information so why men (and women) do is beyond me.
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Originally Posted by barit1
But--if what Mad (Flt) Scientist proposes is true, removing stall strips MAY NOT decrease the stall speed.
further thoughts...
An interesting thread & a good point John - data is required to be completely sure.
Going back to the original question about differences between aircraft of the same model having different stall speeds maybe it's due to manufacturing tolerances ? a few thou' here & there on the overall
aerofoil shape etc plus different paint finishes / polish etc must a difference -Gliders I've flown were more efficient when polished & the
same must be true for power too - an old instructer at southend I knew used to swear by mr sheen on his 150's & they did indeed fly very well - far better than the 'matt & dirty' finish competitors !
I wonder what effect these things have on fast jets too - it must be even more important to have a good finish - any thoughts folks ?
I remember mr Boeing advising that even small imperfections make a huge difference. (although that was with regard to icing - similar except potentially worse as it increases in bad conditions ! )
All Interesting stuff anyway...... :-)
Going back to the original question about differences between aircraft of the same model having different stall speeds maybe it's due to manufacturing tolerances ? a few thou' here & there on the overall
aerofoil shape etc plus different paint finishes / polish etc must a difference -Gliders I've flown were more efficient when polished & the
same must be true for power too - an old instructer at southend I knew used to swear by mr sheen on his 150's & they did indeed fly very well - far better than the 'matt & dirty' finish competitors !
I wonder what effect these things have on fast jets too - it must be even more important to have a good finish - any thoughts folks ?
I remember mr Boeing advising that even small imperfections make a huge difference. (although that was with regard to icing - similar except potentially worse as it increases in bad conditions ! )
All Interesting stuff anyway...... :-)
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Old timer is right about wing surface smoothness.
My motorglider would roll to the left when flying in rain and not when dry.
I determined the reason was the difference in the way rain beaded up on the left wing which had been painted and the right wing wing that had old paint and the rain laid smooth. Very minor difference, but it scared me the first time it happened. Both wings painted now and it flies straight and level.
Also, if the root gap tape is missing the aircraft will shake at low speed. The root gap is only about .060" without tape, but it does make a difference!
My motorglider would roll to the left when flying in rain and not when dry.
I determined the reason was the difference in the way rain beaded up on the left wing which had been painted and the right wing wing that had old paint and the rain laid smooth. Very minor difference, but it scared me the first time it happened. Both wings painted now and it flies straight and level.
Also, if the root gap tape is missing the aircraft will shake at low speed. The root gap is only about .060" without tape, but it does make a difference!
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Boeing has determined that the 787 nacelle should be one color only (no trim features) to prevent disruption of laminar flow at the step of a new paint layer. 
gaps
Interesting subject - thanks for feedback folks,
ref the root gap this reminds of when I used to fly Condors, (FUN !)
there is (was ? - post CAA-now EASA/PFA) an AD to ensure that the gap between the elvator & tailpane was taped over - if not, the gap adversely affected the elevator effectiveness due to bleed through airflow - not quite sure of the exact terminology but I know of another Condor operator who chose to ignore this AD which very nearly resulted in a tragedy.
I believe the 'bleed through' airflow caused loss of elevator authority ?
ref the root gap this reminds of when I used to fly Condors, (FUN !)
there is (was ? - post CAA-now EASA/PFA) an AD to ensure that the gap between the elvator & tailpane was taped over - if not, the gap adversely affected the elevator effectiveness due to bleed through airflow - not quite sure of the exact terminology but I know of another Condor operator who chose to ignore this AD which very nearly resulted in a tragedy.
I believe the 'bleed through' airflow caused loss of elevator authority ?
