stall recovery

[Glyn44]

Hi BEB,

Please help my understanding of you sentence saying that you can stall in a dive. I can't understand that. As you stated to recover a stall push the nose down, so are you saying you can stall in a shallow dive, stall and recover by steepening the dive?

Sorry if I'm being dim

Glyn.

[Spice Cat]

There is an interesting video on YouTube called airflow in a Stall.

[Biggles' Elder Brother - Moderator]

No - not your fault Glynn - it me not being clear.

What I mean is suppose you open your throttle wide, then push the nose down - in other words you wind your plane up to the absolute maximum speed you possibly can! Now, yank on that elevator! Induce an AoA above the critical value and you'll stall - for all that speed, you'll still stall. She'll flick and probably spin.

What I was trying to say was that even the most extreme speed your model is capable of (like at full throttle and in a dive) won't protect you. Sorry I wasn't clear.

BEB

PS Old flying expression "no aircraft ever stalled with the stick forward"! In other words its the elevator that stalls aircraft via its control of the AoA.

[Glyn44]

Ok, many thanks

[Martin Harris - Moderator]

Posted by Biggles' Elder Brother - Moderator on 06/07/2016 21:41:52:

PS Old flying expression "no aircraft ever stalled with the stick forward"! In other words its the elevator that stalls aircraft via its control of the AoA.

Even inverted?

[Biggles' Elder Brother - Moderator]

OK - you giot me there!

BEB

[John Privett]

Posted by Spice Cat on 06/07/2016 21:27:01:

Indeed I found it amusing that there are scientists that still believe the earth is flat. The Flat Earth Society is still going strong and I understand you by a tee shirt from them.

It's going off on a bit of a tangent, but I really wouldn't call anyone who believes in a flat earth a scientist.

Scientists gather evidence and propose theories, testing those theories and refining them as more evidence is gathered.

Flat earth believers are either trolls, or people who devise bizarre theories that match a limited set of observations and then deny any observations that don't fit the flat earth idea or ramble about government conspiracies or change the subject to ignore awkward questions. Anyway - as I said, that's a bit of a tangent!

Stalling is - as BEB keeps saying - all about angle of incidence, not speed. For instance, a model I had many years ago was easy to stall and stalled at high speed several times - the final time terminally... From a fast, low pass, I pulled full 'up'. The increased angle of incidence caused one wing tip to stall and the plane entered a spin of sorts whilst still travelling horizontally. That (uncontrolled) phase of the flight did not last long and a black bin bag was required to transport the wreckage back...

[PatMc]

BEB, I think the perception of a "stall speed" is that in normal S&L flight it's often the easiest way to recognize effect of what is in fact being caused by a high AoA. Especialy so if there's a lack of other visual reference.

[Biggles' Elder Brother - Moderator]

Yes - I would agree with that Pat. The problem is it leads to this confusion about actual cause - which wouldn't be a problem except for the fact that you can't undo it until you reverse the true cause!!

BEB

[Spice Cat]

Take the point about speed not being the decisive factor in stalling. I recall that some Stuka pilots managed to stall at 300kph.

As for the flat earth bit; it was a joke OK?? The tee shirts are out of stock.

[The Wright Stuff]

I am interested to hear what you have to say about the c of g as I have moved the battery around a bit and it's a bit of a lump.

Regards.

Edited By Spice Cat on 06/07/2016 21:28:47

At the risk of wading in to something deep here...

In this context, I can sort of understand Denis' point - and I have personally suffered myself a lot in the past with excessively nose-heavy models based on paranoia built up about the dangers of moving back the CoG. My interpretation of what he said is that when flying nose-heavy models, you (the pilot) get used to hanging on the elevator excessively to keep the nose up. In turns, when changing speed, when landing. It just becomes the norm. So pulling back on the stick just becomes the natural thing to do when in trouble. Hence nose heavy models stall more! Not for any direct aerodynamic reason, but because of the interaction with the elevator trim and with the pilot's learned response.

[The Wright Stuff]

Posted by PatMc on 06/07/2016 22:32:04:

BEB, I think the perception of a "stall speed" is that in normal S&L flight it's often the easiest way to recognize effect of what is in fact being caused by a high AoA. Especialy so if there's a lack of other visual reference.

 

I think that's a fair point, Pat.

BEB, I will politely both agree and disagree with you. I think it's a bit too strong to simply disregard the stall speed as a myth. What we are trying to do on this forum, surely, is, amongst other things, help our fellow pilots achieve their ambitions in this hobby. Just because something is technically correct doesn't necessarily mean it's the most helpful explanation. I could equally argue that gravity isn't a force, it's a curvature of space time, but does that help a modeller land their new plane?

Flippancy aside, surely all that is needed is a more precise and robust definition of stall speed, and then all is good again! For a model in straight and level flight, or very close to it (e.g. very gentle landing descent) then to all intents and purposes, for any given model there is an airspeed at which the wing needs to reach its maximum angle of attack in order to generate enough lift. We define that to be the stall speed. In the context of talking through a landing approach, is this not a good compromise?

 

Edited By The Wright Stuff on 07/07/2016 12:29:28

[Jack Banner]

But why talk about stall 'speed' at all when landing? Speed isnt the deciding factor, AoA is. Whenever I land a model plane I can hear my dear old dad yelling, "keep the nose down!". So I do.

[cymaz]

I was taught, land with the throttle and not elevator.

[Biggles' Elder Brother - Moderator]

I see your point TWS - but my fear is that the myth, that low airspeed causes stalling, is so strong and ingrained in flying folklore that any compromise on this is potentially dangerous! We must understand that we can't stall recover by adding speed because it wasn't the cause and that, similarly, stalls can occur at any speed.

I personally think as far as I would be happy going is the statement I make above - to the effect that the lack of speed was a 'provoking or influencing factor' - noting it was not the cause.

For this reason I dislike the term "stall speed" - its misleading in my view. It implies cause and effect when in reality all it is is that speed at which, in order to maintain AoA-induced-lift in the absence of speed-induced-lift, the AoA must be taken to an unsustainable value that provokes a stall.

BEB

Edited By Biggles' Elder Brother - Moderator on 07/07/2016 13:43:42

Edited By Biggles' Elder Brother - Moderator on 07/07/2016 15:24:03

[The Wright Stuff]

Posted by Jack Banner on 07/07/2016 13:33:20:
But why talk about stall 'speed' at all when landing? Speed isnt the deciding factor, AoA is. Whenever I land a model plane I can hear my dear old dad yelling, "keep the nose down!". So I do.

But how far down? A couple of degrees? Five? Six? That's just as subjective. I'm not saying that AoA arguments are wrong, merely that in practice, the speed isn't irrelevant, either. How do you know if the nose is too far down? Excessive increase in speed?

[The Wright Stuff]

For this reason I dislike the term "stall speed" - its misleading in my view. It implies cause and effect when in reality all it is is that speed at which, in order to maintain AoA-induced-lift in the absence of speed-induced-lift, the AoA must be taken to an unsustainable value that provokes a stall.

BEB

I hear you, BEB - perhaps we are arguing subtly different points. I agree 100% that the wing stalls because, and only because, the AoA is too high.

I disagree slightly that this automatically implies that the concept of a 'stall speed' is misleading. Under the carefully described scenario I described in straight and level flight, the airspeed and the AoA are related in a one-to-one manner. It may not be the cause of the stall, but I don't see why the correlation has to be causation in order to be useful feedback to the pilot. If I choose my definition of 'stall speed' according to this constraint, I don't think there is any implication that the speed causes the stall. I might look at a thermometer before going outside to see if I need a coat. The thermometer isn't causing the weather to be cold!

I think we can agree that the way to recover from a stall (or better, to avoid a stall before it happens) is to decrease the AoA (allowing the airflow to reattach). Again, under the constraint of straight and level flight, this will necessarily and intuitively result in an increase in speed.

I only have experience of flying relatively lightly loaded models, but I tried high speed stalls out of curiosity and failed - presumably because the elevator movement was insufficient to rotate the pitch fast enough that the critical AoA could be reached before airspeed was lost.

So maybe I'm overthinking this, but my point is that we should use all of the information that is available to us. At some orientations, it is easier to see the AoA, at others, the speed (bearing in mind airspeed versus groundspeed). We learn to make judgements based on both of these observations.

[Biggles' Elder Brother - Moderator]

I would agree with a lot of what you say there TWS. I'd certainly see low speed a "danger factor" where stalling is concerned - due of course to the fact that it draws you into increasing the AoA - possibly without even being aware that you are! - in order to maintain height.

So you can say speed "plays a role" - and so I'd agree that an awareness of the impact it might have is very useful. Just as long as we can help folks to avoid seeing it as the physical cause.

BEB

[Peter Miller]

As I see it, as the pilot slows the model he increases that A ofA to keep it flying. So the pilot is increasing angle of attack.If he flew the approach a bit faster he would not have to pull the nose up to generate the extra lift to keep the model in the air and therefore exceed the A of A.

[Spice Cat]

I have just finished watching a program on Sky about Aircrash Investigation. Cheerful stuff with the holiday season upon us but it concerned a China Airways 747 that went into a 6,000 metre dive from 41,000 feet (sorry about the mixed units) in 1985.

The sequence began when the No. 4 engine stalled at a low thrust setting and flamed out. That engine had failed twice during previous flights and in each of those cases, the engine was restarted after descending to a lower altitude. The maintenance response to the logbook entries that noted the problems included engine inspection, fuel filter drainage and replacement, vane controller inspection and replacement, water drainage from Mach probes, insufficient modules, and other filter replacements. None of those acts fixed the recurrent stalling and flameout problem of the No. 4 engine.

After the flameout, the captain instructed the flight engineer to attempt to restart the engine, while the plane remained at 41,000 feet with the autopilot still engaged and the bleed air on. This was contrary to the flight manual procedure, which required the plane to be below 30,000 feet (9,100 m) before any attempt to restart a flamed-out engine. The attempt failed.

The airspeed continued to decrease, while the autopilot rolled the control wheel to the maximum left limit of 23 degrees. As the speed decreased even further, the plane began to roll to the right, even though the autopilot was maintaining the maximum left roll limit. By the time the captain disconnected the autopilot, the plane had rolled over 60 degrees to the right and the nose had begun to drop. Ailerons and flight spoilers were the only means available to the autopilot to keep the wings level as the autopilot did not connect to the rudder during normal flight.

The crew managed to regain control at 9,000 feet. A restart attempt brought No. 4 back into use. They began climbing and reported to air traffic control, and this bit leaves me speechless; "condition normal now" and continuing on to Los Angeles! They then noticed that the inboard main landing gear was down and one of the plane's hydraulic systems was empty. Because they did not have sufficient fuel to reach Los Angeles with the drag added by the landing gear, they diverted to San Francisco. An emergency was then declared (!) and they flew straight in to the SFO airport.

On final approach, the elevators didn't respond to the captain's control inputs, forcing him to control the plane's pitch angle with differential engine power. The plane landed without further incident. I have left out a fair amount, but this is the crux of it.

An NTSB investigation concluded that the 'mishap' (as the programme quaintly termed it) was mostly the fault of the crew in that initially they had failed to descend below 30,000 feet to attempt to restart the engine. The engineer had failed to shut an air bleed valve on the engine worsening the restart situation, they disregarded their instruments and the pilot let the AP in control for too long, didn't, and this is the bit that really interests me, prevent the stall by using the rudder.

It also looked at the crew working pattern and in particular to Circadian Rhythm. At the time this was relatively new research and the investigation found the pilot had flown through something like sixteen different time zones in the last six days. He had slept on the flight as part of a rest regime but admitted that on a flight the captain rarely sleeps well.

Bit of a ramble but for me the bit about the use of rudder was very interesting and although the programme did not say it, the AP using ailerons had probably worsened the situation.

There are photos of the damage, parts of the elevators and undercarriage doors torn off, that are quite spectacular. The programme concludes that the crew got the aircraft into the situation then got them out of it but probably the true hero was the 747. This aircraft had been pushed far outside its design parameters and survived to save the day. Apparently the wings were permanently bent up two inches after! Well done Boeing.

[Martin Harris - Moderator]

Posted by Spice Cat on 12/07/2016 12:35:59:

An NTSB investigation concluded that the 'mishap' (as the programme quaintly termed it) was mostly the fault of the crew in that initially they had failed to descend below 30,000 feet to attempt to restart the engine. The engineer had failed to shut an air bleed valve on the engine worsening the restart situation, they disregarded their instruments and the pilot let the AP in control for too long, didn't, and this is the bit that really interests me, prevent the stall by using the rudder.was very interesting and although the programme did not say it, the AP using ailerons had probably worsened the situation.

I suspect they should have said prevent the wing drop rather than the stall, but a very interesting account!

To return to the airspeed discussion, surely for 99.9% of model pilots, the measurement of airspeed is non-existent (although I do have airspeed telemetry installed in a couple of models). Therefore, how can you even begin to assess airspeed in order to apply any relevance in model flying? The only indication for the majority of model flying is the aircraft's longitudinal attitude - and even then it really only applies in unaccelerated (i.e. steady with no elevator inputs) flight.

As suggested a couple of posts back, control of AoA (thereby airspeed) is primarily (almost entirely) by use of the elevator which means not letting the nose rise excessively during the approach to maintain a margin for inattention and vertical gusts. Learning to regulate the descent path by use of the throttle while maintaining an appropriate AoA (equated/approximated as airspeed in many full size aircraft) is the key to safe and accurate approaches.

[Shaunie]

Part of the problem for us modellers is that we cannot see or measure AoA directly. As a result it is common for modellers to estimate AoA with respect to the horizon and not the path of the plane, if the aircraft is sinking as on finals then AoA is actually much higher than the pilot perceives it. Would that be right BEB?

Like someone already posted (sorry forgot your name, it was a couple of pages ago) I liked fast passes and sharp pulls into the vertical with a Seagull Edge540 68", overdid it one day and practically got a snap roll, luckily it wasn't a ground level job and some nifty stick twiddling got me out of trouble, no bin bag required thankfully. Definite high speed stall.

Unfortunately I tend to fly fun fliers a lot and they develop bad habits, as finding a defined stall can be extremely difficult on occasion.

Shaunie..

[PatMc]

Posted by Spice Cat on 12/07/2016 12:35:59:

Bit of a ramble but for me the bit about the use of rudder was very interesting and although the programme did not say it, the AP using ailerons had probably worsened the situation.

Very different from what's been discussed earlier in this thread. You have a multi engined aircraft in an asymmetric power situation therefore it would be yawing towards the dead engine. The rudder is the control to correct a yaw.
The yaw would cause a roll in the same direction. If the AP can't use rudder it can only deal with the effect, not the cause, therefore the problem will repeat & the AP will repeat it's action ...until it's disengaged.

Edited By PatMc on 13/07/2016 00:18:15

[The Wright Stuff]

Posted by Shaunie on 12/07/2016 23:16:07:

Part of the problem for us modellers is that we cannot see or measure AoA directly. As a result it is common for modellers to estimate AoA with respect to the horizon and not the path of the plane, if the aircraft is sinking as on finals then AoA is actually much higher than the pilot perceives it.

This is my take on it. We can't measure either airspeed or AoA with any great precision. All we can do is observe the overall behaviour as much as we can: apparent speed, apparent AoA, responsivity of the controls, throttle position, noise, and more, and mentally 'crunch' all that data to give a best summary of what is happening. This complexity is part of what makes model flying harder than it might appear. For example, I think it is possible to get a reasonable idea of the airspeed from observing the apparent ground speed of the model on the crosswind leg before turning in, and that gives a good idea of the sort of throttle setting to be used for the descent. A good reason for getting into the habit of flying formal rectangular landing circuits.

It's technically correct to state that the AoA should be measured relative to the direction of travel, but unless you are descending at a particularly steep angle, I doubt most pilots would be able to accurately perceive the few degrees of difference. If the model is descending so quickly that this effect is important, I imagine that the fact that the wing was completely unstalled when the model hit the ground will be of little consolation...

 

 

Edited By The Wright Stuff on 13/07/2016 01:48:43

[Denis Watkins]

Spice, I am back and apologise for having a strop with the Mod too.

Spice, A recipe for a "wing over" out of control...............Forward C of G

Recipe for a spin, go to a safe height, reduce throttle to idle, hold up elevator......ok

Recipe for landing, Reduce Throttle, Hold Up Elevator.......ring any bells........some similarities....ok

Forward C of G.........hold up elevator even longer keeping the nose up, due to the forward weight............. result bang!...........ok

Put another way, the elevator is not only an elevator at low speed......air brake? crow brake? at the back of the model

So at Low speed, nose up, forward C of G making things worse...we then apply more airbrake? and more airbrake

Please please tell me that I have a point BEB