nema
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Reducing Stall Speed

Thu Dec 16, 2010 7:42 pm

If an engineer was asked to develop an airframe to have a lower stall speed, i guess the first thought would be a matter of having more wing, although i,m, no engineer so tell me if i,m wrong.

My question more generally though is, could there be any conceivable way that stall speed could be so dramatically reduced that landing becomes less traumatic, (speed wise), than it currently is regardless of cost?
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roseflyer
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RE: Reducing Stall Speed

Thu Dec 16, 2010 8:14 pm

Stall speed can absoulutely be reduced if it was justified. The easy way to do it is to increase flap and slat area. However, increasing flaps has consequences. The extra weight can be heavy and having a lot of flap area causes significant drag. Also it can impact the high speed characteristics of the wing. Fuel burn also goes up with larger flap area.

Airplanes are currently designed to land within a specific runway length. It is virtually impossible to get a 737 down to a landing speed of a Cessna, but at different flap positions, stall speed changes.
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Fly2HMO
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RE: Reducing Stall Speed

Thu Dec 16, 2010 9:04 pm

Quoting RoseFlyer (Reply 1):

Airplanes are currently designed to land within a specific runway length. It is virtually impossible to get a 737 down to a landing speed of a Cessna,

Hmmmm.... I'm sure if you made a STOL kit you could bring a 737 a bit below the landing speeds of a lightly loaded Cessna.... Citation 

A 737 would sure look funny with a STOL kit.
 
PapaChuck
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RE: Reducing Stall Speed

Thu Dec 16, 2010 10:54 pm

There are two main points to keep in mind here. First of all, the name of the game is efficiency. Secondly, every aircraft design is an inherent compromise.

To expand on the first point, consider this: the days of cheap gas (and labor, but that's a topic for another thread) are long gone. Any airliner will be designed to perform its task as cheaply and efficiently as possible in order to keep ticket prices down. Modern jets are happiest flying in the 30-40,000 foot range at around .8 Mach, and their designs reflect that philosophy. Fly high, fly fast, but burn as little fuel as possible.

Now, in order to fly high and fast, you need an efficient high-speed airfoil, i.e. a thin swept wing. This is great during cruise, but not so much during the takeoff and landing phases. A thick, straight wing does much better at low speeds, and this is where the idea of compromise comes in to play. Modern jets use flaps and slats to offset the low speed disadvantage a swept wing has. They increase the wing area, thereby reducing the stall speed to acceptable levels.

Can a high speed airfoil be outfitted with enough high lift devices make it handle like a Cherokee on short final? Sure, but there is a price to pay. As already mentioned, high lift devices and their associated systems add a lot of weight to an airplane. The bigger the flaps, the bigger their actuators, electronics, and hydraulics become. At some point all that extra baggage will offset any gains you've achieved in reducing your stall speed.

Also, you have to consider the maintenance side of things as well. All those extra bells and whistles have to be inspected and replaced from time to time, and as systems become more complex, the risk of a failure increases. At some point it becomes too expensive to fix all that extra stuff you have hanging off the wing.

[Edited 2010-12-16 15:00:08]
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Pihero
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RE: Reducing Stall Speed

Thu Dec 16, 2010 11:26 pm

Quoting PapaChuck (Reply 3):
Now, in order to fly high and fast, you need an efficient high-speed airfoil, i.e. a thin swept wing.

To add just an info on *modern* generation on a very concise and to-the-point post :
The thin well swept wing is now a thing of the past, since the aerodynamicians discovered the *supercritical airfoil*, i.e a wing that's a lot thicker than the previous generation, with a flat top and a curved bottom.
It's quite easier to furnish with all sorts of high-lift devices and , as a by-way quality, allows enormous quantities of fuel to be carried.
All things considered, taking full airplanes on a 12 hr-sector, at destination, the A380 is in the approach some 15 to 20 kt slower than the 744...that's progress.
On the other hand, when the engineers are stuck with mainly commercial considerations, one can really see the penalties involved in a not-so- optimised wing : the A321 has the same wing as the 320 (well, not really exactly) and being heavier and bigger it is the fastest of the whole Airbus family on approach.
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PapaChuck
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RE: Reducing Stall Speed

Thu Dec 16, 2010 11:39 pm

You're absolutely correct about the supercritical airfoil. It was a gross generalization on my part. Simple minds like mine tend to gloss over the details.  boggled 

[Edited 2010-12-16 15:50:09]
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Starlionblue
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RE: Reducing Stall Speed

Thu Dec 16, 2010 11:45 pm

Quoting Pihero (Reply 4):
the A321 has the same wing as the 320 (well, not really exactly) and being heavier and bigger it is the fastest of the whole Airbus family on approach.

The significant difference vs. the 318-320 is that the 321 has double-slotted flaps as opposed to single slotted. And still it is fastest.
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vikkyvik
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RE: Reducing Stall Speed

Fri Dec 17, 2010 12:03 am

Quoting PapaChuck (Reply 3):
They increase the wing area, thereby reducing the stall speed to acceptable levels.

Just to add detail: perhaps more importantly, flaps and slats increase the camber (curvature) of the wing.

So basically, you have 3 airfoils in use during different phases of flight:

1.) Takeoff/Initial Climb - slats deployed, flaps usually deployed part-way (results in a moderately cambered airfoil)
2.) Cruise - slats and flaps stowed (results in a relatively straight and clean airfoil)
3.) Approach/Landing - slats deployed, flaps deployed at or near maximum (results in a very cambered airfoil)

Essentially, on takeoff and landing, you're making up for your lack of speed (which reduces lift) by cambering the airfoil (which increases lift).

Slats, in particular, serve the important function of increasing your stall angle-of-attack, allowing you to fly slower and remain un-stalled.
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rwessel
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RE: Reducing Stall Speed

Fri Dec 17, 2010 7:56 am

Quoting Fly2HMO (Reply 2):
Hmmmm.... I'm sure if you made a STOL kit you could bring a 737 a bit below the landing speeds of a lightly loaded Cessna.... Citation

A 737 would sure look funny with a STOL kit.

Can we put tundra tires on it too?   

As a side effect, that ought to solve that pesky ground clearance problem keeping Boeing from putting higher BPR engines on the thing!
 
Pihero
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RE: Reducing Stall Speed

Fri Dec 17, 2010 4:28 pm

Quoting Starlionblue (Reply 6):
The significant difference vs. the 318-320 is that the 321 has double-slotted flaps as opposed to single slotted. And still it is fastest.

     
However, the main problem with the A321 was body attitude and body angle at rotation. The double-slotted Fowler flaps maintained the nose attitude at an acceptable value. A very expensive solution, IMHO, but necessary in order to keep the same attitudes as the other members of the family.
It did the work, as I and thousands of pilots can testify
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PapaChuck
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RE: Reducing Stall Speed

Fri Dec 17, 2010 7:23 pm

I don't want to imply that jets with good low speed characterisitics don't exist. They do, but this is the unfortunate result:


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Photo © Viktor Laszlo - Budapest Aviation Photography



Don't expect to see her burning up the airways.

Oh, by the way, 737 + STOL package + tundra tires = Awesome! I'll take three!
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Pihero
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RE: Reducing Stall Speed

Fri Dec 17, 2010 8:19 pm

Not many people know that there was (still is ?) an advanced 732 optimized for shortish/unprepared runways.
Toundra tyres as well.
Identified by the perch-like appendage protruding from under the engine : some bled air was blown through some holes in order to disperse thestones and debris (one could call it FODavoidancegismo).
See the difference with a normal 732 :

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Photo © Robert Jones
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Photo © Kai Lebert

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SchorschNG
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RE: Reducing Stall Speed

Sat Dec 18, 2010 10:56 am

A B737-700 has a V_ref of approximately 135 knots.
Wing area is 122m².
Assume 60t gross weight at landing.

Lift coefficient: W/(V_ref2 *0.5 * Air Density * Wing Area) = ~1.7

A more complex high lift system could achieve a CL of 2.4. Of course with a weight penalty.
Additionally we increase the wing area by 20%.
We assume a 4t increase in gross weight.

Our new approach speed: 105 KIAS.

Getting lower would be more difficult. Yet more wing area. CL above 2.5 is tough to achieve and requires stuff like blown flaps and direct lift.

The proposed 105 KIAS B737 will be a wasteful cruiser.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
mandala499
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RE: Reducing Stall Speed

Sun Dec 19, 2010 3:48 pm

Quoting nema (Thread starter):
i guess the first thought would be a matter of having more wing, although i,m, no engineer so tell me if i,m wrong.

Well... there are other methods available. More wing is one, more slotted flaps is another. But both have penalties. Boeing came up with the SFP for the NG: the ShortField Performance.
From: Could The 737NG Be A True Stol Airplane (by OyKIE Dec 12 2006 in Tech Ops)

Quote:
The 737 design enhancements allow operators to fly increased payload in and out of airports with runways less than 5,000 feet long. The design enhancements include a two-position tail skid that enables reduced approach speeds, sealed leading-edge slats that provide increased lift during takeoff, and increased flight spoiler deflection on the ground that improves takeoff and landing performance.

Two position tailskid increases protection/buffering on when one scrapes the butt of the 737 if it rotated or flared with too much nose up on low speed   
Sealed LE slats means less "aerodynamic leakages"... so it reduces the stall speed maybe by a few knots?
Increased spoiler deflection enables faster aerodynamic braking... helps rejected take offs coming to a stop faster... hence reduces the "accelerate stop distance" requirement (in addition to accelerate - engine fail - continue) for the same given weight in comparison to a non SFP aircraft.

Quoting Pihero (Reply 11):
Not many people know that there was (still is ?) an advanced 732 optimized for shortish/unprepared runways.

It's still in service in Canada...

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Click here for bigger photo!

Photo © Jason Pineau

Quoting Pihero (Reply 11):
some bled air was blown through some holes in order to disperse thestones and debris (one could call it FODavoidancegismo).

You mean the intake vortex dissipator? Don't forget the nosewheel gravel deflector option that's also available... see them in action...   

View Large View Medium
Click here for bigger photo!

Photo © Jason Pineau
View Large View Medium
Click here for bigger photo!

Photo © Jason Pineau



Long live the thunderpig ! (732!)

Mandala499
(coincidentally... the MDL499 flight number was only ever operated by the 732!)
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Pihero
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RE: Reducing Stall Speed

Sun Dec 19, 2010 9:06 pm

Quoting SchorschNG (Reply 12):
A more complex high lift system could achieve a CL of 2.4. Of course with a weight penalty.
Additionally we increase the wing area by 20%.
We assume a 4t increase in gross weight.

Our new approach speed: 105 KIAS.

Careful ! You're coming awfully close to Vmca...!...even on a JT-8D
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SchorschNG
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RE: Reducing Stall Speed

Mon Dec 20, 2010 2:14 pm

Quoting Pihero (Reply 14):
Careful ! You're coming awfully close to Vmca...!...even on a JT-8D

This was a theoretical exercise.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
wingscrubber
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RE: Reducing Stall Speed

Tue Dec 21, 2010 4:24 am

I'm an engineer... If I were faced with STOL as a primary requirement for an airliner with no cost constraints I would implement variable sweep, variable camber wings with fully blown flaps, then give it a huge pair of engines.
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Pihero
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RE: Reducing Stall Speed

Tue Dec 21, 2010 6:20 pm

Quoting Wingscrubber (Reply 16):
I would implement variable sweep, variable camber wings with fully blown flaps, then give it a huge pair of engines.

Bold, bold and brutal !    
And very expensive, too !
We're only talking civilian airliners, here, and IMHO, the future will be about ecology, sustainable development and a lot fewer nuisances from airliners.
Everything above translates into 1) new engines with higher by-pass ratios and lower pressure, 2) slower cruise, I reckon in the region of .70 to .72 M at higher altitudes... and the resulting solutions will be about very high aspect ratio wings, with some morphing qualities, with a lot less sweep angle...there are somewhere in the WWW. pictures of projects from both main manufacturers along those lines.
The weight penalties for the variable sweep airplane are well known and totally impractical for a civilian airline economy.
Ditto for the blown flap solution, noit counting the lost efficiency - hence SFC - for the engine.
A noce dream, though.
BUt, as you said, with no constraint....

Regards
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dimik747
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RE: Reducing Stall Speed

Tue Dec 21, 2010 7:30 pm

Heck,, if there were no money constraints couldnt we go to an overwing, V-22 style system and go very STOL or basically VTOL.  
 
2H4
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RE: Reducing Stall Speed

Tue Dec 21, 2010 7:41 pm

Quoting PapaChuck (Reply 10):
this is the unfortunate result:

The only unfortunate thing about the PZL M-15 Belphegor is that there are no longer any flying.   

Someday, when I win the lottery, I will acquire one, fit it with amphibious floats, and use it for an Oshkosh Partymobile.
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