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8 Answers

Stall Propagation/Wing twist

Asked by: 2999 views , ,
Aerodynamics

Background Info: Alright, so yesterday I came upon an aircraft at the local FBO; a cherokee 140. It had a few modifications, only two worth mentioning for this question: vortex generators & drooped wing tips.

 

For those student readers I'd like to define these two items:

 

Vortex generators: Small (1 inch long by 1/4-1/2 inch tall) strips placed a couple inches back from the leading edge on top of the wing. These act to create vortices on the upper portion of the wing. By disturbing any airflow we can help to keep it attached to a surface, in other words, prolonging stall.

 

Drooped wing tip: Like any end plate attached to the wing, many lear jets have these, it acts to increase the effective span of the wing. The end result is lower induced drag.

 

The Question: Now back to the question with one final piece of background information: the vortex generators were located ONLY on the inboard portion of the wing. WHY!?!

 

I'd believe this to promote a dangerous characteristic, that is, the outboard portion of the wing stalling prematurely and ailerons becoming useless. It would seem to completely midigate the designed wing twist.

 

(Definition for the students again) Wing twist - the process of quite literally twisting the wing. The end result is the inboard wing being a few degrees higher AOA than the outter portion so the inboard wing will stall. Essencially keeping aileron effectiveness through stalled flight.

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8 Answers



  1. Nathan Parker on Dec 14, 2011

    I presume that the 140 has a rectangular planform like most of the older Pipers, so the stall naturally occurs at the root, even in the absence of washout, so there may be no twist at all.
     
    If you’re going to reduce the stall speed of an aircraft, you need to delay separation where the stall first occurs, which is the root in this case, so I don’t see any option for where the vortex generators go.  If this were to make the aircraft uncontrollable in a stall, the STC wouldn’t have been approved.

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  2. Brian on Dec 14, 2011

    Nathan,
     
    Yes, the wing is a square, very thick, and has a long chord when compared with the Cessna. I’ll have to look again and pay more attention to it, but I thought the 140s we have do have some twist. That’s neither here nor there as it’s not the main point.
     
    It is favorable to have an aircraft designed to stall at the root first. Wouldn’t it be undesireable, possibly dangerous, to make this change? Obviously he would need them on the inboard, for reasons you mentioned. Though wouldn’t it be wise to put them across the entire span? In essence keeping the designed margin between root and tip stall in check.
     
    Without proper checks and balances it would seem to me that this kind of mod is akin to the comparison of abritrarily deciding you can fly at a farther aft cg. Both design changes would promote unfavorable conditions. What stops a guy from going to far? Maybe piper put out an…(STC is it?) that claims this to be safe? 

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  3. Best Answer


    Nathan Parker on Dec 14, 2011

    “Maybe piper put out an…(STC is it?) that claims this to be safe? ”
     
    The STC (Supplemental Type Certificate) probably isn’t Piper’s. But whoever owns it would have to ensure that it doesn’t degrade the stall characteristics.  Part 23 requires that lateral control be maintainable up until the stall.  Looks like the 140 was mostly a CAR 3 airplane, but some 23 requirements were applied to it.  CAR 3 probably had a similar requirement to Part 23. 
     
    It’s fine to argue how this or that would affect an airplane’s handling characteristics, but in the final analysis, you have to flight test the airplane to know for sure.  The stall characteristics of Pipers are so benign that it probably takes a great deal of effort to make them unsafe.  
     
    As for putting them along the entire span, consider that vortex generators add drag to the airplane, so you don’t want to scatter them all over the aircraft.  Still, vortex generators in front of the control surfaces are common, so if there had truly been a problem with aileron control on the 140, you might have found some there.

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  4. Bill Trussell on Dec 14, 2011

    I have flown a 140 with some of the mods mentioned.  The intention is to increase performance a slow speeds in order to qualify it ( as closely as possible ) for STOL or short take off and landing capabilities.  As such, delaying the onset of a stall at the wing root is important for both take off and landing at lower than “normal” speeds for this aircraft.  The vortex generators at or near the wing root would accomplish this, but only to a point.  So long as the wing root stalls before the area containing the ailerons experinences separated airflow over the controls all is good.  In my case it made for an unusual shape for the aircraft in general but with some increase in performance in only one aspect, that being low speed takeoff and landing distances.

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  5. Brian on Dec 21, 2011

    Nathan, “but in the final analysis, you have to flight test the airplane to know for sure.”
     
    That’s what I was looking for: to answer, “how is this safe?”. The answer being that there are requirements that must be met before an STC is approved. I did some googling after reading your reply and discovered Title 14 Part 21: Certification Procedures for Products and Parts (linked here for convenience). Seems that answers all my other questions. Thank you.

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  6. BC on Feb 05, 2012

    I’ll take a guess: the vortex generators are on the inboard part of the wing to help keep the airflow attached when the flaps are lowered, making the flaps a bit more effective.
     
    Most often you’ll see vortex generators upstream of control surfaces (usually on jets, because they have lots of stringent handling requirements). This helps keep airflow attached as the control surface is deflected. Take the B-727 for example. Oops, not enough rudder authority. OK, lets add vortex generators forward of the hingline. Airflow stays attached longer and more rudder authority. It’s a cheap fix.
     
    Capt BC

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  7. BC on Feb 05, 2012

    Let me add part 2: Above i assumed the VG’s occupy about the same span as the flaps. If there are just a few near the very inboard part of the wing, the VG’s are probably to control the airflow (keep it attached) near the fuselage/wing junction. This area is notorious for airflow separation which causes a lot of drag. In this situation, the VG’s serve to reduce drag not only at high AOA (low speeds), but probably in cruise as well.
     
    In all cases, these inboard VG’s won’t delay the stall propagation to where the outboard part of the wing stalls first (thats a no no). They are there for other reasons. Likely for one of the reasons i mentioned.
     
     

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  8. BC on Feb 05, 2012

    As long as I’m at it, an extreme example of my part 2 reason above would be the F-16 (and pretty much most fighers since). The wing strake (the first couple feet of wing span adjacent to the fuselage, where the leading edge angles back from the fuselage) is really a giant vortex generator that lays two (one on each side) big vortices over the wing roots and upper fuselage. These two miniture tornados really stick to the wing at high AOA and allow the wing to fly at much higher AOA’s than it otherwise could. You can see the effect on humid days when an F-16 is pulling G’s: the vortex will cause condensation and be clearly visible.

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