Stall Training

[Lowtimer]

It is easy to land in a distance which one can never takeoff from later. Isn't that true for any aircraft?

Only some. Agreed, in a type with low power-to-weight ratio, the landing might be quite short but the take-off much longer, I often find this to be the case in the 90 hp Super Cub two-up. But with more power-to-weight ratio, the balance moves the other way. It is very rare for me to get the Yak-52 down and stopped icomfortably using 300 metres of runway. In still air I'd think I was doing OK using 400 metres. (Perhaps I'm a rotten pilot, I accept the possibility.) I usually get off the ground in much less. Slope plays a role too. If you're operating downhill, you can take ages to stop but leap almost straight off the ground on departure. Uphill it works the other way.

But I wouldn't choose to fly on the back of the curve deliberately... there isn't any point.

I don't know what type you're flying and probably haven't flown one, but a lot depends on individual types here too. Some aircraft have a very narrow "back of the drag curve" regime. Types with high aspect ratio wings, e.g. gliders and motor gliders, tend to be most efficient pretty close to the stall. They also tend to have effective direct means of steepening the descent, e.g. spoilers or airbrakes. So for those types there's not a lot of scope for exploiting the back of the drag curve. Other things being equal, a lower aspect ratio wing will be more efficient at a higher margin above the stall. So something like a plank-wing Cherokee 180, or indeed the Yak, has a very useful and quite broad flight regime on the back of the drag curve, and exploiting this makes it very much easier to land tidily at a determined touchdown point and stop in a short distance. In the Yak, with gear and flaps down, at 160 kph you are on the front of the drag curve. At you reach 150 you can just feel it settle onto the back, and then you have the ability to make small rapid variations to speed and rate / angle of descent quickly in any desired sense. (Only on a good steep approach though, no "dragging it in"!). It doesn't stall until 100-ish in that configuration so the width of the regime is a lot greater than the few knots you might get in a motor glider.

[M14P]

FFF - I agree, most reasonably powerful aircraft need a little more room to stop than to go but beware sweeping generalisations (eh 540!).

I can usually stop a Bonanza really short mainly due to gear geometry and the way the wing flies on the back side of the drag curve (ok 540 so there is a use). Getting airborne is another matter entirely due to the climb gradient decrement that is unavoidable if one tries a soft field style take off. Not a problem if you have a reasonable climb out area in terms of obstacles. This technique is very useful in cases of rough or very wet fields (not just short places). I find that the 114 is very similar.

Single Comanche or SF260 are the other way around since the geometry and drag characteristics makes this sort of approach impracticable.

In either case proficiency in that regime is needed.

540 can't see the point of any of the above.

540 - I have other ideas about why people give up. Nearly all of it is down to cost and motivation. I think managed groups and a scheme of mentoring would help greatly (I have developed some plans for both) but I agree that the 45hr PPL course casts the average pilot adrift at the end of it.

Bear in mind, however, that not everybodies idea of Valhalla is to be adjusting the V/S control on a mode control panel.

[Bluebeard777]

Maybe some of the stall pundits here can shed light on the following.
My aircraft stalls at 31 mph IAS at max weight, the book figure is 35 mph "in level flight". The ASI has recently been checked and found accurate.

Recently on a near-windless day I thought I would check this against the GPS, out of curiosity. So I went up to altitude (2900 ft density altitude), near max weight and middling CofG, slowed gently and stalled on a downwind heading, then upwind, then 90 deg each side. To my surprise and puzzlement the GPS speed in each case was 54-55 mph with little variation.

I do not know whether the GPS figure will include the vertical speed component, which I reckoned was about 500 fpm (= 6 mph) at the stall.

Has anyone else here tried this?

[Genghis the Engineer]

(1) You were at altitude, and therefore CAS<TAS. The aircraft stalls at a fixed CAS, not at a fixed TAS.

(2) GPS lag - it's not an instantaneous speed device and needs to stabilise for some seconds - something the aeroplane is unlikely to do.

(3) ASIs are calibrated down to 1.3Vs and up to Vne, it's common to get greater than the permitted errors outside that range.

(4) It's common at high AoA for the ASI to significantly underread due to poor flow into the pitot.

(5) You were lighter than MTOW, which is the weight at which Vs is quoted.

(6) GPS only gives groundspeed. Descending at a high RoD you are actually flying rather faster than GPS will give.

(7) You are mixing up knots and mph somewhere along the line.

(#) Any combination of the above.

G

[noisy]

Thanks Genghis, this is all good Gen, particularly (3) & (5)

[M14P]

I agree with all of the above and possibly there may be some excess 'G' involved if you were trying to maintain level flight. A very slow deceleration in a slight descent should ensure that you creep up to Vs under (near as dammit) 1G conditions

[Genghis the Engineer]

If it's (5), the stalling speed you see should be roughly the declared stalling speed x the square root of (Actual weight divided by MTOW).

M14P - increased g should increase the stalling speed, not decrease it. On the other hand, less than 1g during a slightly greater than necessary descent should decrease it.

Another factor I forgot to mention is deceleration rate. The book figure should be for bang-on 1kn/s, a greater deceleration rate should give a lower stall speed, and a higher deceleration rate *may* give a higher stall speed.

G

[M14P]

Genghis

Isn't that what this chap is expressing concern at - an increase in stall speed (31-34 scheduled versus 50-odd observed)?

I always need to descend to achieve 1kt/sec at idle for schedule speed checking.

[Genghis the Engineer]

Ah, but a good test pilot gets lots of data from one test. What I saw in there was:-

- Low apparent stall compared to book figure.
- Large disparity between GPS ground and ASI indicated speeds.
- Apparent lack of difference between GPS groundspeed into and downwind.
- Difference between TAS and GPS groundspeed due to RoD
- Use of non-aeronautical units.

And was trying to cover all of those without getting too complicated and including power and CG.

G

[M14P]

I'm not sure that an IAS was quoted for the GPS based test conditions!

Nice post re stalling - Stintons book is a great source for that sort of stuff. I can commend it to anyone with an interest in testing...

[Bluebeard777]

These are interesting comments.

Ghengis:

(1) TAS v. CAS: Yes, but hardly so as to have such a large difference as I observed?

(2) GPS lag: I did "creep up" slowly on the stall each time. Definitely the stalls were approached at less than 1 kt/sec decrease. GPS speed was observed constantly (by the passenger) and stabilised. I am convinced GPS lag cannot have been a factor, and even if I were wrong, its effect could perhaps be 1 or 2 mph, not 15 or 20.

(3) and (4) Yes the ASI is in an position at stalling AoA which is likely to give inaccurate readings. If one starts from the premise that the GPS does not lie in respect of speed over the ground, then the inaccuracy must be in the ASI. This particular GPS has been tested for accuracy of its speed readings, on the ground.

(5) I did say I was near max weight, probably about 98% of MTOW. By your own formula this would make an unrecognisable difference to the stalling speed.

(6) I covered the point about the vertical speed component in my previous post. For a rate of descent of say 500 fpm or even 1000 fpm this cannot account for such a large difference, as a simple calculation will show.

(7) No, there is no confusion of units here. The ASI reads mph as also does the GPS, and the book figures are given in mph also.

And your further comments:
Wind condition: This test was deliberately done on a day with almost calm conditions. There was a small difference (2 mph or less) in the up/downwind stalling ground speed.
Power: Power was slowly brought back to idle as the airspeed decayed.
CofG: I mentioned that this was near the mid position.
Use of non-aeronautical units: ???

[Genghis the Engineer]

My experience of GPS is that it takes considerable time to give a stable airspeed reading once you have a constant speed and height. By this I mean 20-30 seconds very often and I've never seen it give meaningful data during a deceleration to stall.

If doing an ASI calibration using GPS, I'd normally fly into wind and downwind (into wind being defined at altitude as that heading which gives the lowest GPS groundspeed) at a range of IAS values. I'd then take mean of each pair of groundspeeds to give TAS, adjust for density altitude to give CAS, then plot an IAS .v. CAS graph. I'd then take limited extrapolation of the curve down to my IAS stalling speed.

I agree that when checked you are only looking at a couple of percent error between GPS and TAS due to a 500 fpm descent rate.


Ultimately, my money is on an unstable GPS groundspeed reading due to a changing velocity vector. I suggest checking that by trimming the aircraft to a few knots above the stall into wind, confirming 5+ satellites and waiting for GS to stabilise over 30 seconds+. Then do the same downwind and see what you get.

But I'm often wrong !

G

[M14P]

Bluebeard777

Can I ask if your book values are IAS (I assume they are) and if so do you have a chart for position error correction?

It would be interesting to find out the CAS just to check if it is significantly higher than the IAS. This might be a good starting point.

[Bluebeard777]

I doubt that it would have taken up to 30 secs to get a stabilised GPS speed. Experiments with this unit on the ground, where a weaker signal is more likely, suggest a stabilised speed can be achieved in a few seconds. Also the test procedure was for the pilot approaching Vs to continuously call out the IAS to the observer, who compared this to the GPS and took notes. If the GPS speed was dropping when the IAS was stable just above Vs, the test conditions would not have stabilised and the readings would obviously not be a valid comparison (i.e. obvious to the observer). Having said that, I cannot prove now that there was no GPS lag and the test is worth repeating.

If the stalling speed (TAS or CAS) were really 55 mph, then the approach speed should be over 70 mph, which does not seem credible for this aircraft.

No graph of IAS v. CAS is available, creation of this would be a useful exercise. I need a day of stable air, with low wind speed, not so easy to get these days!

[M14P]

What sort of aircraft are we talking about here?

The TAS difference at 2900' density is less than 5 knots - i.e. you would expect to see nil wind gps speeds (or directional averages) about 5kt higher than observed.

As for gps giving too high a speed would not the horizontal component of velocity be less given any descent (or climb) since you can safely assume WGS84 to be 'flat' over such a small distance?

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Anyway - has anybody had any really good, positive training experiences pertaining to stalling and slow flight? That should bring us a little closer to the original thread, too...

[hoey5o]

There is an awful lot of test pilots in here for a private flying forum.

I am surprised no one has mentioned incipient spin stage yet which, as I was taught , is another description of the wing drop. If I can recall it was defined as the first 180 degrees of uncommanded roll and the recovery required simply centralising controls and then standard UP recovery. When comparing this with the other technique of a bit of opposite rudder to prevent further roll, there was no difference in height loss, it was a lot simpler to do and no chance of flicking it the other way with the rudder as mentioned before.

[M14P]

Incipient can be used to describe anything leading up to a stable or oscillatory spin. In other words, there is auto-rotation but not all of the classic defining features of a spin.

As I mentioned before - one only needs to 'prevent further yaw' for as long as one is 'unstalling' the wing. The yaw/roll is present only whilst one wing is beyond the L/D peak that defines the stall (i.e. one wing is fully stalled and thus creating large amounts of drag). No stall = no wing drop.

It might also be worth mentioning secondary stalling (I've managed it in a P210 as well as YAKs etc!) which can be very 'energy state' related. This is why 540s slightly befuddled stall recovery description some pages back can be rather dangerous. Simply put (and without hogging the whole thread) some aircraft need rather more than just a quick shove on the stick and throttle before resuming normal service.

Well - anybody got any positive ideas about banishing stall/slow flight terrors? L-Plate, how's it going now?

[hoey5o]

M14P, with a name like that I guess you do a bit of Yak52 flying. Is there any chance you could make a comment or two on 52 flat spinning. Its a current thread in the tech log section. cheers.

[M14P]

I'm reluctant to start talking about individual cases here (especially the 52) mainly because I don't consider myself fully proficient with the 52 flat spin.

Genna at Skytrace or Ian Austin will help you out. Personally, I feel that the risk is there BUT it is smaller than you might have been led to understand. It's far more likely with very aft CofG.

Get ACTUAL training with Genna or Ian if you feel that you are regulary exposing yourself to the dangers. I feel that the 52 is pretty naff at stall turning well...

[strafer]

I think Cessna L Plate's been bamboozled away from his own thread...