Flying a Cessna, there is no doubt when the stall occurs: you get a sinking feeling at the appropriate moment. Technically, the lift coefficient has reached it's max, and is on the way lower, as the angle of attach is increased.

But, for my plane, Savannah, and probably many others, the stall isn't so easy to detect. For example, with power off, as I slow down, the nose gets higher, buffeting may or may not show up, the stick comes all the way back, and airspeed goes down to near zero, the VSI shows maybe 800-1000, and there is no subjective feeling of a break. I can still turn, and both rudder and ailerons are functional.

So, with that kind of response, how would you define the stall speed? The point where you run out of elevator? The point where the sink gets to Xfpm? I have heard of other planes just mushing in, so how do they define stall for them?

Power is another matter. I've been chicken to go all the way back to full elevator with full power. The nose feels like it is pointing up at perhaps 50-60 degrees (like a recliner chair), the airspeed reads zero, and the VSI is still positive. I don't want to get into a tailslide, or something totally weird, so I don't know what would happen with more elevator.

Any ideas?
top

Get some altitude and tailslide that bad boy!!!

I would venture a guess that you are not fully stalling your wing. Since you still have aileron authority you obviously still have airflow over them and I would bet that your propwash is helping in your elevator / rudder authority. The ASI could read 0 if that part of the wing has stalled, but the rest of the wing could still be flying. You are basically in a high alpha maneuver where the inboard section is stalled, but the outboard continues to fly, thus allowing control surface authority. When you lose aileron authority you will be at a full stall on the wing, unless you have full span ailerons in which case propwash will help maintain their authority.

If I remember correctly the Savanah has full span leading edge fixed slats? I would venture to guess that although your wing is not fully stalled, it is not generating enough lift to maintain altitude. Since you have a decent negative vertical speed I would guess that the slats are converting some of that vertical speed into laminar airflow over your wing and preventing a total stall. Same idea as VG's on a wing.

This is simply an educated guess based on how my R/C aircraft behave in high alpha maneuvers and based on real world experience in the Maule. I am sure that there will be others who could give you a definite answer.

A lot of airplanes will not seem do a power-off break into a stall if you ease into it. If you pull up elevator smartly enough, I'm guessing that it will break. Might have to pull more smartly than you like though, not exactly a snap but maybe kind of a whip-stall.
That mush or "controllable stall" is kinda like the old falling leaf descent. Not stalled but not really flying either. Probably pretty useful maneuver for increasing descend rate when way too high on final. Level off or add power at the bottom though, or you will get an unpleasant surprise when you flare -- like no reduction in sink rate. (can you spell slam?)

Eric

Yes, this plane has leading edge slats, full length aileron / flaps, and the ailerons are shifted slightly below the trailing edge. In effect, they fly in their own airstream.
tom

lowflybye wrote:If I remember correctly the Savanah has full span leading edge fixed slats? I would venture to guess that although your wing is not fully stalled, it is not generating enough lift to maintain altitude.

Stalling has nothing to do with speed that is a myth. A wing stalls when it reaches its critical angle of attack period.

Right, so maybe with the slats and separated ailerons, my wing has a very flat Cd / AoA curve?
tom

mr scout wrote:Stalling has nothing to do with speed that is a myth. A wing stalls when it reaches its critical angle of attack period.

Savannah-Tom wrote:Yes, this plane has leading edge slats, full length aileron / flaps, and the ailerons are shifted slightly below the trailing edge. In effect, they fly in their own airstream.
tom

Ah well that explains a lot...you do not have ailerons, you have flaperons. Your flaperons are their own airfoil separate from the wing, and are hung below the wing trailing so they have fresh, undisturbed air which is why you have control authority even if the wing stalls. The flaperons stall independent of the wing so they maintain control.

The reason a wing stalls is related to the AOA but is not because of the AOA. The reason it stalls is due to the release of the laminar airflow across the top of the wing. The AOA is the culprit that causes the separation and that can happen at any airspeed. As suspected, your leading edge slats help prevent the stall by re-directing a lot of air from below the AOA. Since the slot is large at the leading edge and tapers to a much smaller exit slot across the top of the wing it creates the venturi effect. This venturi effect blows the faster air across the upper wing surface and essentially pulls the air around the leading edge down across the top of the wing which maintains the laminar flow at a higher AOA. This is what I was referring to in my first post with regards to your negative vertical speed aiding in preventing the wing from completely stalling.

lowflybye wrote:Get some altitude and tailslide that bad boy!!!

I would venture a guess that you are not fully stalling your wing. Since you still have aileron authority you obviously still have airflow over them and I would bet that your propwash is helping in your elevator / rudder authority. The ASI could read 0 if that part of the wing has stalled, but the rest of the wing could still be flying. You are basically in a high alpha maneuver where the inboard section is stalled, but the outboard continues to fly, thus allowing control surface authority. When you lose aileron authority you will be at a full stall on the wing,

.

Actually loosing aileron authority doesnt mean the wing is fully stalled it only means your ailerons are stalled. for most light aircraft you could just add wash-in to your wing you will loose your ailerons long before you stall. That's why wash-out is desirable in most certified light planes.

My wing has no washout, but the ailerons do. They are split halfway out and one section is rotated slightly relative to the other. The outer sections are slightly higher than the inner sections, so the inner has a higher AoA.
tom

mr scout wrote:Actually loosing aileron authority doesnt mean the wing is fully stalled it only means your ailerons are stalled. for most light aircraft you could just add wash-in to your wing you will loose your ailerons long before you stall. That's why wash-out is desirable in most certified light planes.

I'm scared to.
tom

lowflybye wrote:Get some altitude and tailslide that bad boy!!!

Let's see, you bought an experimental aircraft, and you wonder about the stall characteristics.

There is a reason for all that certification mumbo jumbo.

When you build or buy something that has not been certificated it may be really difficult to determine precisely what the heck is going on with those wings.

Good luck.

MTV

mr scout wrote:Stalling has nothing to do with speed that is a myth. A wing stalls when it reaches its critical angle of attack period.

Yes and no. Like flybye sez, it isn't AOA either but separation of airflow over the wing that causes a stall. Easiest way to quantify or identify it is by speed- how many POH's list the stall AOA? Airspeed as applicable to a particular part of the flight envelope is the easiest for me to deal with. I have a little tick of white paint on my ASI at my reference approach speed, about 1.3 VSO-- just a visual cue to keep me from getting too slow (aka too high an AOA) maneuvering. I adjust my speed from there depending on what I'm doing- aka "keeping a little airspeed in the bank".
Saying slow speed causes a stall is like saying a fall will kill you-- no, it's the sudden stop at the bottom that does that. But it's just easier to say "don't fall". Same as saying "keep your airspeed up".

Eric

My opinion is, Your both right and both wrong. Airflow separation is what a stall is, what causes it is when the wing reaches its critical angle of attack, as I stated above.
I was responding to Toms question above of (how do you define stall speed)

Doesn't the Vso speed of your aircraft change with weight? I am quite sure mine does. I would need a sliding tic mark I guess.

Like I said, that tick is just a reference, I adjust my actual target speed up or down according to what I'm doing. That tick is actually about 1.4 Vso now that I think about it- that's Vso at 1G, & at my normal flying weight (mostly full fuel and me). So would be 1.3 or maybe 1.25 or so loaded.

Eric

Well, I got out the airforce flight testing manual and read what it says about defining the stall speed. Here's what it said:

If you get a definite break by slowing down at a moderate rate, say 1 knot per second, and nothing else of note happens, then that break point will be your stall speed.

If other things happen first, they may be the defining action. Examples could be buffeting, loss of aileron control, a wing dropping, end of elevator authority, or an excessive sink rate. In this case it is up to the tester to quantify the action and define the stall testing around that.

In my case, the manufacturer of the kit claimed 35 mph clean. That is what I use for my stall speed, but being a curious sort, I like to verify stuff like that by my own testing. As I mentioned earlier, the end of elevator authority seems to be the defining result.

Since back country type folks fly J3s and some other planes that sort of mush in instead of stalling with a distinctive break, I was hoping someone would have the definition for some of these "mushers". I understand the physics of a stall, and how the various high lift devices contribute to lift or control at low speeds. It was the definition of stall speed as given in the POH that I am curious about.

thanks,
tom

i have a champ, and after installing the stol kit the only way to get it to power off stall was to slow up to about 50mph and yank the stick into lap.if i went into it real slow it would just porpus along plus or minus 50 feet, power on stalls still work as do spins

Tom,
I Think as far as FAR 23 goes the speed could be 1 of two things, a stall in which we all think of one as being or the point at which the limit of authority has taken place. In your case if the stick all the way back, it's all the way back you cant move it anymore so you have reached that point, just my interpretation though I could be totally wrong.
I have VGs on the Scout and it just mushes if I pull the stick back slowly, as many planes will do, however instead of pulling slowly if you fly it level at 65 say, then abruptly pull the stick back it will break every time. Depending on the weight that point changes but it will definitely show you a stall. You don't want to try that at a much higher speed than your normal stall or you can generate a whip stall which is not good.

mr scout wrote:however instead of pulling slowly if you fly it level at 65 say, then abruptly pull the stick back it will break every time.

Ever done a snap roll?

In my brief experience with aerobatics, in a course I took in Reno, one of the maneuvers I learned was a snap roll, which is a violent and extremely fun maneuver that is basically a spin done along a horizontal line. There's really no gain in altitude because the wing never has a chance to increase lift from the increased AOA. It goes from flying to beyond critical in an instant.

Slow to 65 or 70...jerk the stick back all the way quickly, and stomp full rudder deflection. You basically do a hard accelerated stall, and combine hard yaw with it. As far as the recovery...that part was a blur. I guess the fact that you're moving forward with enough airspeed allows you to effect the recovery once the offending control inputs are relaxed and right side up is blue again.

This same aerobatic instructor taught me how to do somewhat level attitude approaches in a Super Cub by dialing in the trim and power just right, so that my site picture seemed relatively level, but I was losing altitude. Flying, but the net lift is such that the aircraft is sinking at a controllable rate, with power being the variable.

The bottom line is that during my training, to understand AOA, airspeed, and lift relationships, I considered the direction of the relative wind to the wing. Sounds obvious, but the things that cause the AOA to change are sometimes not so obvious: Centripital force from turning increases AOA, hence the increased load factor on the wing. Hauling ass, the relative wind is pretty much parallel to the wing surface. Slowed down, with gravity pulling me downward and not so much lift being generated, that relative wind is hitting my wing at much more of an angle, due to the fact that the aircraft is now moving downward, not necessarily pitched downward, but moving that direction at a higher rate.

Perhaps the wings on the Zenith/Savannah aircraft handle this high AOA condition better by maintaining more cohesive airflow even at high AOA, by letting it slip thru the flaperon gap. Does it make much sense to pay attention to speeds when the lower end performance of your wing is so nebulous? It would be cool to find an instructor who had experience with that particular aircraft, or least talk to someone who's put it thru its paces, and perhaps even spun it...

Growing up, my Dad had a friend that had a Taylor Cub. The guy had installed a rather large AOA indicator attached to the wing strut. I don't know how well it worked, but that's been over 30 years ago. I'm surprised they aren't standard on airplanes today.

An aquaintance of mine has an electronic AOA indicator mounted on the panel is his Avid Catalina. ** Which is for sale BTW if anyone's lookng for a flying boat. Email me... **
There's a couple different AOA gizmo's in the Spruce wish book and advertised in Kitplanes/Sport Aviation as I recall.
Regarding that old Taylor Cub, are you sure it was an AOA indicator? Back in the old days, it was not uncommon to see strut mounted airspeed indicators, kind of a pointer deal with a wind-paddle mounted on the other end. A friend of mine has one from her old 1930-ish Student Prince biplane.