Stall strips

[NJA_Capt]

Both of the above arguments are accomplishing the same thing. Stall strips are there to insure control is available to recover from a stall, ie...the root needs to stall first. If a stall strip was added to "create turbulent air/buffet," you can surmise that you needed that buffet because if it were not present you would lose aileron effectiveness. It is the turbulent air at the wing tip that is the problem. By causing this turbulence (air flow separation) at the root first, aileron authority is maintained.

I would say the Dept of the Navy trumps FSI. (Page 56-57)​

"The stall strip is usually a small angular strip attached to the root leading-edge of the wing as depicted in figure 52. By using a stall strip the flow is disturbed near the root such that root area stall is induced first. This method is not preferred because it limits rather than improves CLmax It turns out that the use of stall strips is usually a design "after-thought" to improve stall warning cue characteristics found to be unsatisfactory during flight testing."
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA235994&Location=U2&doc=GetTRDoc.pdf​

STALL STRIPS​

Leading edge stall strips are small, sharp edged, triangular shaped flow trippers that are attached to the front of the wing leading edge close to its stagnation point. (N). These shed a vortex at high angles of attack so as to both selectively initiate stall at that wing station and to send a buffeting vortex aftward to tremble the tail control surfaces and thus provide a stall warning to the pilot.

http://cafefoundation.org/v2/pdf_cafe_reports/localflow2.pdf​

My problem is a lot of people teach that it creates the stall at the root instead of the tip, when the stall strip is installed centered in the middle of the wing. Trying to provide a reference for credibility.

As referenced above, the strip is attached near the stagnation point. It doesn't necessarily mean that it is where the wing joins the fuselage. If the wing has a "cuff" (PA28s, Mooneys and Bonanzas) the stagnation point is farther out. Regardless, as quoted above, they are to create airflow separation to induce stall in a desired location. In other words, they make the inboard(root) stall before the tip.

 

[tgrayson]

wing has a "cuff" (PA28s, Mooneys and Bonanzas) the stagnation point is farther out

The stagnation point isn't a lateral position on the wing, it's a chord position on a wing cross-section, on the lower surface. This is where the airflow comes to a complete stop and the static pressure rises to the total pressure of the airfoil, since dynamic pressure is zero. This point divides the airflow between the top and bottom of the airfoil. Every station along the wing has a stagnation point.

 

[NJA_Capt]

The stagnation point isn't a lateral position on the wing, it's a chord position on a wing cross-section, on the lower surface. This is where the airflow comes to a complete stop and the static pressure rises to the total pressure of the airfoil, since dynamic pressure is zero. This point divides the airflow between the top and bottom of the airfoil. Every station along the wing has a stagnation point.

In English next time...please

In regards to to chord. The cuff area would have a longer chord than the rest of the wing thus a different stagnation point from outboard stations. From what I can remember, most stall strips are outboard of the cuff areas (on small GA a/c). That would stand to reason that the cuff plays a part in the equation.

Moderator: Please remove my duplicate post #18 above. Not sure how that happened. Perhaps if we had a longer time limit I could edit it myself....hint

 

[bLizZuE]

Both of the above arguments are accomplishing the same thing. Stall strips are there to insure control is available to recover from a stall, ie...the root needs to stall first. If a stall strip was added to "create turbulent air/buffet," you can surmise that you needed that buffet because if it were not present you would lose aileron effectiveness. It is the turbulent air at the wing tip that is the problem. By causing this turbulence (air flow separation) at the root first, aileron authority is maintained.

This above summation is predicated on the stall strip installed location along the wing.

As referenced above, the strip is attached near the stagnation point. It doesn't necessarily mean that it is where the wing joins the fuselage. If the wing has a "cuff" (PA28s, Mooneys and Bonanzas) the stagnation point is farther out. Regardless, as quoted above, they are to create airflow separation to induce stall in a desired location. In other words, they make the inboard(root) stall before the tip.

Again, I don't buy that. The Warriors we own have stall strips attached at the MIDDLE of the wing. How do you suppose that makes the root stall before the tip? The stall progression is a characteristic of the wing plan form.

 

[tgrayson]

The stall progression is a characteristic of the wing plan form. .

Don't all Warriors have the semi-tapered wing, even the very first models?

 

[Xiph]

The Warriors we own have stall strips attached at the MIDDLE of the wing. How do you suppose that makes the root stall before the tip? The stall progression is a characteristic of the wing plan form.

Perhaps not the root in terms of strict definition, but it will get the middle of the wing stalling before the rest of it, which still leaves the ailerons in relatively good shape.

It's been a while since I've flown a Warrior, so I don't remember exactly how the wing is shaped, but there could be other factors at work like washout that would get the root area stalling before the tips.

 

[bLizZuE]

Perhaps not the root in terms of strict definition, but it will get the middle of the wing stalling before the rest of it, which still leaves the ailerons in relatively good shape.

It's been a while since I've flown a Warrior, so I don't remember exactly how the wing is shaped, but there could be other factors at work like washout that would get the root area stalling before the tips.

Again, not the case. The stall strip is actually installed in a location to provide a buffet on the inside edge of the aileron. This is to enhance the buffet feeling, and provide that early stall indication.

To me, the plan form, the washout, those are items that create stalls at the root. The stall strips job is only to provide an early buffet, and enhance the stall warning capability of the wing. HOWEVER, I will relent that in certain aircraft, the Traumahawk for example, the stall strip installations were used to modify the stalling pattern.

 

[bLizZuE]

Don't all Warriors have the semi-tapered wing, even the very first models?

To be honest I'm not sure. I've only flown Warriors with tapered wings.

 

[Crockrocket94]

I thought the PA28-140s had the Hershy Bar wing.


Let me check.

 

[Crockrocket94]

Here is an older Arrow, but you can see a its a fairly rectangular wing with just a bit of bend in the begninning.

But its not the same wing that was on the later cherokees.

 

[tgrayson]

I've only flown Warriors with tapered wings.

They didn't make any of those. The Warrior has a semi-tapered wing; the section around root is rectangular, but mid-span, it starts to taper, hence semi-tapered. You don't get the root stall like you would in a Hershey/rectangular plan form. Our late-model Arrow with the semi-tapered wing also has stall-strips.

Here is a section from "Flight Testing of Fixed-Wing Aircraft", p. 377:

The stall strip, discussed in Chapter 4 [lie], is a very powerful device for fixing stall characteristics problems. It is a triangular strip on which the height depends upon the size of the airfoil and the length depends upon the severity of the stall problem. It is primarily used for fixing problems of roll-off, but can be used for deep stall problems by forcing the wing to stall at the center section first.

It is normally best to use stall strips on both wings so that they may be adjusted up or down to correct roll-off problems. For light airplanes, a triangular stall strip of about 0.25 in. in height and about 6 in. long is a good place to start. The strips should be located on the inboard portion of the wing initially but can be added to the outboard portion as necessary. The strips can be increased in length on the wing opposite the roll-off, moved up to hasten the stall, or both. On the wing in the direction of roll-off, the strips may be moved down, or shortened. The strips may be used to tailor the stall to the desired characteristics. On some airfoils it may be necessary to tailor the location of the stall strips for each individual airplane. In this case, rubber adhesive stall strips are used during the production flight test in order to tailor the stall characteristics and then once their location is determined the permanent stall strips are installed. For airplanes with deicing boots, the rubber adhesive strips are used on top of the boots. Figs. 33.1, 33.2, and 33.3 show stall strips and their location on several different airplanes.
​

 

[bLizZuE]

Excuse me, semi-tapered. That is what I meant.

 

[NJA_Capt]

To me, the plan form, the washout, those are items that create stalls at the root. The stall strips job is only to provide an early buffet, and enhance the stall warning capability of the wing.

Washout (twist) is there to increase control at higher AoAs, and doesn't have anything to do with making the root stall.

If someone wants extra credit: Figure out why the Lear 31s leading edge is mounted with alternating flush and round headed screws.

 

[NJA_Capt]

They didn't make any of those. The Warrior has a semi-tapered wing; the section around root is rectangular, but mid-span, it starts to taper, hence semi-tapered.

To clarify, Piper used two different wings on many of their planes, from the Warrior to the PA32. I have flown both wings on most of those planes. The last digit of the model number denotes the wing shape. On the PA28-150, the "0" denotes the straight wing. The "1" in PA28-151, has a tapered outboard section.

 

[tgrayson]

Washout...doesn't have anything to do with making the root stall.

Indeed it does, according to every book I have. For instance, "Aeronautics, Aerodynamics, and Flight Mechanics", McCormick, page 117:

Most wings employ a negative twist referred to as "washout." Generally, ε is of the order of -3 or 4 degrees to assure that the inboard sections of the wing stall before the tip sections.
​

But I see ensuring that the root stalls first as being synonymous with maintaining control.

 

[bLizZuE]

What he said.

 

[400A]

If someone wants extra credit: Figure out why the Lear 31s leading edge is mounted with alternating flush and round headed screws.

Poor mans vortex generators. :dunno:

 

[tgrayson]

What he said.

Still, I disagree with your conclusion that just because we have washout, we don't need other means of ensuring that the tip doesn't stall. There is a limit to how much washout you can put into a wing before you start incurring higher induced drag penalties. If washout solved all the problems, we wouldn't need the great variety of wing flow control devices such as stall fences, vortilons, stall strips, etc. Therefore, your reasoning that the stall strips on your airplane cannot be there to prevent wingtip stalls because washout exists doesn't really work.

 

[bLizZuE]

Agreed. However I said the combination of plan form and wash out create the stall progression patterns. I fully agree that some wings require the stall strips to ensure certain areas of the wing stall first. However in the specific application that was in use at the school(and the whole point of me bringing this up) the stall strip is not placed in a location that would benefit either wing root or wing tip stall priority.

It's installed on the mid section of the wing. If it were installed at the root, there would be no discussion from me. The point of this application on this aircraft, is merely to provide earlier than normal indication of a stall. It enhances the buffet, and provides a longer buffet before actual stall.

 

[Mike H]

To clarify, Piper used two different wings on many of their planes, from the Warrior to the PA32. I have flown both wings on most of those planes. The last digit of the model number denotes the wing shape. On the PA28-150, the "0" denotes the straight wing. The "1" in PA28-151, has a tapered outboard section.

Interesting. I never knew that before.