Stalls and leading edge questions

William Pete Hodges

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Petehdgs
I have been using my '64 Moodey Super 21 to travel between Culpeper VA CJR and Dexter MI 2E8 several times last summer. Because 2E8 is 2100 ft grass with displaced thresholds and has a down slope on RWY 6 in the center third of the runway, it has me working on my short field technique quite a lot. I added a lift indicator and train with it to get better at slow flight and stay safe. This training has helped me to identify a glitch that merits further investigation and I decided to ask you all for some advice.

During slow flight practice at safe altitudes I noticed the wings fly well right up to the stall then break quickly and recover quickly causing the airplane to porpoise. At altitude this is not a big deal except the break occurs without warning. When this happens near the ground the results are a hard drop and perhaps a bounce. I would like the stall to be softer with some feel to it, if that is possible without too much difficulty, and without loss of cruise performance and fuel economy.

I reread Harry Ribletts book about GA airfoils and he suggests a leading edge modification (page 42) to the 64-212 (and similar 64 series airfoils) but makes no mention of a similar modification to the 63 series. Since the Mooney uses the 63-215 airfoil, would a similar modification be of any benefit? It occurred to me that someone else may have had the same questions and may have done some research in this area. So how about it? Do any of you have any ideas here?

Before someone comments on my lack of flying skills… I recognize that I can simply adjust my technique to not get so close to the stall during slow landings. And I may be left with just that. But admittedly that will leave some low speed performance on the table, not to mention the added safety margin attainable by softening the stall characteristics of the airplane. So I am investigating to see if it is possible. If you can help or have a suggestion I am all ears.

Thanks in advance
Petehdgs@yahoo.com
 
I'm confused. Why are you stalling the wings on approach? :eek:
 
Is a lift indicator the same as an Angle of attack indicator?? Seems like an AoA would give you more feedback as to how close you are to stalling. @motoadve posts here regularly vids of the benefits of an AoA. Seems he flies perfectly fine with stall horn blaring constantly.
I guess I get what your trying to do but losing lift so suddenly with little room to spare is going to lead to a damaged plane. All it takes is a little gust or loss of gust at a critical moment equaling bent metal. Landing seems like a bad time for thin margins of error.
 
I'm still baffled why you would be surprised that your plane drops when the wing stalls.
 
Are VG's an option?
Maybe, but not my first choice. I prefer not to add VGs just to soften the stall a bit. If I was going for full out STOL, then I might consider that, but I'd be looking for another airplane first.
 
I'm confused. Why are you stalling the wings on approach?
He has giving no indication that he’s stalling on approach. I think he’s practicing slow flight in order to safely achieve the lowest possible airspeed prior to touchdown on a short field.

William. Excellent job practicing detailed slow flight and stalls to learn your aircraft’s tendencies!
Question: Are you trimmed up nicely just prior to the point of stall? If not it can cause a porpoise like tendency during the recovery.
Also: What exactly is this “lift indicator” you mention? Can you provide model please?
As you know, lift is equal to weight, so unless you’re changing weight during flight I’m guessing your lift is relatively unchanged. If you have lift in excess of weight I think you’re experiencing a G force, or load factor which contributes to an accelerated stall condition.
 
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I'm still baffled why you would be surprised that your plane drops when the wing stalls.
I know the airplane drops when the wing stalls. (snarky comment deleted) The question is can the airplane hang at the stall a bit before it drops hard. Or begins the drop softly enough to approach it more slowly. Other models of aircraft have benefited in this area by modify the leading edge with a cuff of some type. I am simply asking if anyone here has some experience in this area. I don't understand your being baffled by my comments. It seemed like a pretty straightforward question to me. You either have information beneficial to this conversation or you don't.

If I have misunderstood your intentions, I apologize.
 
I guess what you are really saying then is that you want more of a buffet or other warning prior to the stall? If you don't get a "break" then you haven't yet stalled the wings. If you stall the wings then it's going to break.

I don't think the Mooney's laminar wing is ideal for what you are trying to do, but that's just my opinion.
 
William. Excellent job practicing detailed slow flight and stalls to learn your aircraft’s tendencies!
Question: Are you trimmed up nicely just prior to the point of stall? If not it can cause a porpoise like tendency during the recovery.
Also: What exactly is this “lift indicator” you mention? Can you provide model please.

The Lift Indicator and Lift Reserve Indicator are the same thing different manufacturers since inair instruments closed. The device is an AOA meter that uses dynamic differential pressure at two angles to measure the potential of wing lift above the stall under all conditions. There is a more indepth discussion active on this site now.

Yes I trim for a stable approach and tim back more for the level off prior to the flare. My bird is very stable.

I hope this answers your question.
 
I guess what you are really saying then is that you want more of a buffet or other warning prior to the stall? If you don't get a "break" then you haven't yet stalled the wings. If you stall the wings then it's going to break.

I don't think the Mooney's laminar wing is ideal for what you are trying to do, but that's just my opinion.

You are correct on all points. But it may not have to be that way. I am just trying to figure out if the best of both are possible.
 
I’m sure you’ve considered this, but how’s the CG sitting during your tests?
 
. Since the Mooney uses the 63-215 airfoil, would a similar modification be of any benefit?
It appears that your Mooney transitions between a NACA 63-215 at the root to a NACA 64-412 at the tip. https://m-selig.ae.illinois.edu/ads/aircraft.html
fwiw:
https://en.wikipedia.org/wiki/NACA_airfoil#6-series
"The airfoil is described using six digits in the following sequence:
  1. The number "6" indicating the series.
  2. One digit describing the distance of the minimum pressure area in tenths of the chord.
  3. The subscript digit gives the range of lift coefficient in tenths above and below the design lift coefficient in which favorable pressure gradients exist on both surfaces.
  4. A hyphen.
  5. One digit describing the design lift coefficient in tenths.
  6. Two digits describing the maximum thickness as percent of chord.
  7. "a=" followed by a decimal number describing the fraction of chord over which laminar flow is maintained. a=1 is the default if no value is given."

I don't know exactly what Mr. Ribblets may have done to tweek these foils, and, it may be something of an academic exercise given the type certificated nature of your aircraft. But, in general, to soften the stall, you want a more rounded leading edge - what kind of changes did he make?
 
I’m sure you’ve considered this, but how’s the CG sitting during your tests?
That is a good question, and other than verifying it is within limits I haven't really considered it too much. According to my W&B CG is 44.5-44.9 about 1/3 back from the front of the envelope.
 
Not a Mooney expert here, but if you want the slowest possible speed for the least possible cost (and if you aren't already doing this), the power-on stall speed is slower than power-off, ergo, approach with a touch of power and leave it on 'till touchdown at below Vso. Don't use much, just enough to smooth airflow over the tail like, say, zero thrust. If you use too much thrust and airspeed you will defeat the purpose and land too long.
 
CG can play a big part. Maybe try shifting it a bit and taking note of recovery performance at different positions.
 
I don't know exactly what Mr. Ribblets may have done to tweek these foils, and, it may be something of an academic exercise given the type certificated nature of your aircraft. But, in general, to soften the stall, you want a more rounded leading edge - what kind of changes did he make?
Thanks for the input. I will be studying the references soon. From what I see Harry Riblett looked at the leading edge radius as an enscribed circle with tangent connections to the top and bottom surfaces of the airfoil. Enscribing a larger circle positioned so the location of the upper tangent on the top surface remains constant. The bottom surface is moved down slightly to meet the larger circle and faired to meet the rest of the bottom ordinances in the first 10% or so of cord length. This produces a larger leading edge radius and drops the nose of airfoil slightly producing a gentler stall without affecting cruise speed performance.

I THINK that is what he did to the 64 series of airfoils.
 
Pete - I too own a Super 21, 65 flavor, and find that all of the elements of the stall warning are there although brief. I also have time in RVs and other aircraft that don't have stall strips and find that those aircraft have a more aggressive stall break. Comparatively the Mooney stall is quite tame.

Your original issue was short-field grass performance. I'd recommend you look for the MAPA PPP or Don Kaye handout that recommends a short-field approach speed of ~73 MIAS, even less when lighter. I generally fly power off approaches and come across the threshold at ~78 MIAS and comfortably operate out of less than 1800 ft grass strips.

TLDR: Fly by the numbers and don't worry about changing airfoils, only understand how they react.
 
This produces a larger leading edge radius and drops the nose of airfoil slightly producing a gentler stall without affecting cruise speed performance.
I ain't no expert, but it seems that with a rounder nose and a bit more camber it may be a bit optimistic to assume no affect on cruise. Though, the effect may be small.

63 vs 64 series - seems like you could do pretty much the same thing. YMMV
 
isnt this really all academic anyway? without an STC or the financial backing to get one, there is nothing you can do to the the airfoil of a certificated aircraft anyway.
 
It appears that your Mooney transitions between a NACA 63-215 at the root to a NACA 64-412 at the tip.

I don't know exactly what Mr. Ribblets may have done to tweek these foils, and, it may be something of an academic exercise given the type certificated nature of your aircraft. But, in general, to soften the stall, you want a more rounded leading edge - what kind of changes did he make?

A picture is worth a thousand words. Here is a picture that should address your question. I am looking at analyzing a mod like this in one of the airfoil programs to see if a mod like this would provide enough of a benefit to warrant further pursuit.

One Miracle at a time.

Screenshot_20201129-111650_Drive.jpg
 
Pete - I too own a Super 21, 65 flavor, and find that all of the elements of the stall warning are there although brief. I also have time in RVs and other aircraft that don't have stall strips and find that those aircraft have a more aggressive stall break. Comparatively the Mooney stall is quite tame.

Your original issue was short-field grass performance. I'd recommend you look for the MAPA PPP or Don Kaye handout that recommends a short-field approach speed of ~73 MIAS, even less when lighter. I generally fly power off approaches and come across the threshold at ~78 MIAS and comfortably operate out of less than 1800 ft grass strips.

TLDR: Fly by the numbers and don't worry about changing airfoils, only understand how they react.

I looked for the pamphlet but didn't find it, so I joined MAPA and asked for copy of it. Thanks for your advice.
 
As much as I love Mooneys (and lusted after an Echo) they aren't good for every situation. If you are really in danger of hitting a stall in routine operations you are flying too slow. If you have to do that to make it into your chosen strip, you have the wrong airplane. There are airplanes that are really good at that. What the Mooney is good at is going fast.
 
As much as I love Mooneys (and lusted after an Echo) they aren't good for every situation. If you are really in danger of hitting a stall in routine operations you are flying too slow. If you have to do that to make it into your chosen strip, you have the wrong airplane. There are airplanes that are really good at that. What the Mooney is good at is going fast.

Since the DA corrected takeoff and landing distance is less than half of the size of the airstrip I am using, I respectfully disagree in this circumstance. I do concede that at times I am pushing the edge of the envelope harder than some pilots might feel comfortable with. It helps me that the builder of the airstrip owned a 68 Mooney Super 21 and built the strip to fit that airplane. I figure if he could do it then I can also.

One Miracle at a time.
 
Since the DA corrected takeoff and landing distance is less than half of the size of the airstrip I am using, I respectfully disagree in this circumstance. I do concede that at times I am pushing the edge of the envelope harder than some pilots might feel comfortable with. It helps me that the builder of the airstrip owned a 68 Mooney Super 21 and built the strip to fit that airplane. I figure if he could do it then I can also.

One Miracle at a time.
Pete, I'll let Clint Eastwood handle this one:
Pete, you shouldn't be in danger of a stall on every approach, because sooner or later you'll have one when you're high enough to prang the ship or possibly your pax.

That's not to say the stall horn shouldn't be blaring the whole approach. But you shouldn't be hitting the stall at any point until your wheels hit the ground. There's a guy who lands teeny tiny airstrips in his red Mooney.
Notice the stall horn doesn't squawk until he's on short final. I'm not him, and I'm not going to start trying to be him. Like I said, if you're stalling out on the way into the strip the problem isn't with the airplane, it's with the nut flying it. Vortex generators will make the aircraft more controllable in the stall, and they might even lower the stall speed a bit, but they won't make your Mooney into a Super cub. Yes, you can do 2100 feet even if occluded. A friend of my did 1500 in his Ranger. I think I actually did it at 6Y9. Your landing distance is far less, if you do everything just right. The fact that we have this thread is evidence that you aren't doing everything just right.
 
Pete, I'll let Clint Eastwood handle this one:
Pete, you shouldn't be in danger of a stall on every approach, because sooner or later you'll have one when you're high enough to prang the ship or possibly your pax.

That's not to say the stall horn shouldn't be blaring the whole approach. But you shouldn't be hitting the stall at any point until your wheels hit the ground. There's a guy who lands teeny tiny airstrips in his red Mooney.
Notice the stall horn doesn't squawk until he's on short final. I'm not him, and I'm not going to start trying to be him. Like I said, if you're stalling out on the way into the strip the problem isn't with the airplane, it's with the nut flying it. Vortex generators will make the aircraft more controllable in the stall, and they might even lower the stall speed a bit, but they won't make your Mooney into a Super cub. Yes, you can do 2100 feet even if occluded. A friend of my did 1500 in his Ranger. I think I actually did it at 6Y9. Your landing distance is far less, if you do everything just right. The fact that we have this thread is evidence that you aren't doing everything just right.

To hear you tell it, you must think I am porpoising down final with the stall horn blowing the whole way down! I assure you nothing could be further from the truth! For me the stall horn blows sometime during the flare or on the level off above the runway. In fact I have been working on getting slow enough to get the stall horn on reliably on the level off. That's what you are supposed to do right? My problem seems to be properly planting the wheels just as the wing stops flying. There is no feel in my wing that the stall is beginning, it just let's go fully. If I happen to be a little high above the runway I plunk down abruptly. My airplane doesn't appear to have any settling or mushing as the wing stops flying. It just let's go of the air.

This is less pronounced with less flaps. So I can always reduce flaps some on short final and it puts me right where I want to be just before beginning the flare, and I can usually make a smooth landing every time. But I am not getting clean landings reliably with full flaps.

Now, at the start of this thread I noted that there are other airplanes that are getting improvements in the softness of the stall by adding a cuff under the leading edge of the airfoil. These are aftermarket STC add-ons if available, but I also noted that I may be stuck with what I have and have asked if any of you have more experience in this area. Many of you have come back with really good points and suggestions. Some of you have not. An M20E should be an ideal airplane for a 2100 ft grass strip and suggestions that I buy a different airplane aren't very helpful at all.
 
I've just skimmed this thread so forgive me if I've got it wrong, but it sounds like you want to modify your airplane because you flare too high and the airplane hits the ground hard when the wing stops flying? Doesn't that sound ridiculous? Level out closer to the ground. Get up with a good instructor and figure out what is going wrong.
 
I've just skimmed this thread so forgive me if I've got it wrong, but it sounds like you want to modify your airplane because you flare too high and the airplane hits the ground hard when the wing stops flying? Doesn't that sound ridiculous? Level out closer to the ground. Get up with a good instructor and figure out what is going wrong.
You might be right. When my annual is finished I intend to do just that.
 
Hi Pete. We used to own a C-172 that had a STOL modification before we bought it. It had the cuffs on the leading edge you indicated along with stall strips on the top of the wings and slightly drooped wingtips. It would controllably fly into the stall and stalled at a very low indicated airspeed. The mod was already done when we bought the airplane, and I do not remember what the logs said about when it was installed, etc. A similar mod could likely be done to your Mooney, but the cost might be more than reasonable for the lower speed capability you would gain and it might lower your cruise speed. Probably better to spend the money on perfecting your technique which sounds like the way you are headed. :) I just wanted to let you know what you are talking about is indeed possible and has been done.
 
To hear you tell it, you must think I am porpoising down final with the stall horn blowing the whole way down! I assure you nothing could be further from the truth! For me the stall horn blows sometime during the flare or on the level off above the runway. In fact I have been working on getting slow enough to get the stall horn on reliably on the level off. That's what you are supposed to do right? My problem seems to be properly planting the wheels just as the wing stops flying. There is no feel in my wing that the stall is beginning, it just let's go fully. If I happen to be a little high above the runway I plunk down abruptly. My airplane doesn't appear to have any settling or mushing as the wing stops flying. It just let's go of the air.

This is less pronounced with less flaps. So I can always reduce flaps some on short final and it puts me right where I want to be just before beginning the flare, and I can usually make a smooth landing every time. But I am not getting clean landings reliably with full flaps.

Now, at the start of this thread I noted that there are other airplanes that are getting improvements in the softness of the stall by adding a cuff under the leading edge of the airfoil. These are aftermarket STC add-ons if available, but I also noted that I may be stuck with what I have and have asked if any of you have more experience in this area. Many of you have come back with really good points and suggestions. Some of you have not. An M20E should be an ideal airplane for a 2100 ft grass strip and suggestions that I buy a different airplane aren't very helpful at all.
I will repeat. You are flying the airplane wrong. Get onto Mooneyspace and explain your problem. You'll get really good advice from CFIs who've taught in Mooneys for decades. You might even get an offer of instruction, which I honestly think you need far more than airframe modifications.
 
Other leading edge cuffs like the sportsman's, horton, owl, and robertson droop the leading edge/ camber it down. I'd look to copy that approach.

You should do vgs (microaerodynamics for example) before you spend any time on anything else though... They seem to keep the flow attached, which should give you more time/margin.

You will also find that coming in steep, with a bit of power, and flaring hard, with a burst of power, should keep you in the safety zone during low approach, and the hard flare should dissipate your energy/ speed enough to get you stopped fast.

Be careful: playing on the edge has consequences!
 
As much as I love Mooneys (and lusted after an Echo) they aren't good for every situation. If you are really in danger of hitting a stall in routine operations you are flying too slow. If you have to do that to make it into your chosen strip, you have the wrong airplane. There are airplanes that are really good at that. What the Mooney is good at is going fast.
I don't get the feeling that he is trying to make the Mooney do something it can't. He is trying to squeeze the most efficiency out of it that he can. Absolutely nothing wrong with that.

I don't understand why everyone is so quick to criticize his question rather than try to actually offer advice regarding the question he asked.

I wonder if an AoA indicator would give @William Pete Hodges the information and warning he is looking for?
 
I don't get the feeling that he is trying to make the Mooney do something it can't. He is trying to squeeze the most efficiency out of it that he can. Absolutely nothing wrong with that.

I don't understand why everyone is so quick to criticize his question rather than try to actually offer advice regarding the question he asked.

I wonder if an AoA indicator would give @William Pete Hodges the information and warning he is looking for?

My only issue with the post is he describes flaring too high and dropping when the wing stalls. That's not an airplane problem, that's a pilot problem, one we've all probably had to deal with at some point in our flying.

I think he recognizes that now and will seek out an instructor. Issues like that are usually a quick fix unless you are just staring to fly, then it might take a little longer.
 
There is no feel in my wing that the stall is beginning, it just let's go fully.
Isn't this what the AOA indicator is intended to solve? I'm not an instructor, not an expert, and have zero time in a Mooney. But if I was going to make a wild-ass guess, it sounds to me like you are a couple of knots too slow coming over the fence and there's nothing left to flare with. But that's just a WAG and agree with others that say maybe an hour with an instructor will fix it.
 
I did most of my training in a 172M with a Horton STOL modification to the wing. STOL fences and leading edge cuff. When I bought by 172M, it doesn't have a STOL mod. I did notice a difference, a couple of knots in when the wing would stall.
 
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