Brain Teaser: Stalling by PUSHING the Stick

[Guest pelorus32]

Here's one for Yenn,

 

In another thread in here I made the blanket statement that the aircraft will always stall with the stick in the same (aft) position. With some minor caveats that is a good working statement that will do more good than harm.

 

Yenn then posed the question about stalling the aircraft with the stick forward and I answered that you would be in inverted flight.

 

There is however a circumstance where the aircraft is oriented in an upright position but you can stall the aircraft inverted by pushing the stick. What is this circumstance? Please explain how and why it would happen.

 

CAVEAT: Please do NOT try this (when we find out what "this" is) - our aircraft are generally not up to it!

 

There's a Lolipop at the next flyin for the person with the best and most complete answer :big_grin:

 

Regards

 

Mike

 

 

[motzartmerv]

Can you restate the question?? "where the aircraft is oriented in an upright position but you can stall the aircraft inverted by pushing the stick.".. could just be my day light saving brain not firing yet but how can the plane be in upright position and inverted at the same time??

 

 

[Barefootpilot]

I think what pelorus is trying to get at is a Negative G stall. So instead of pulling back on the stick and increasing the AoA to roughly 16% you push forward and reach an AoA of -16%. Thats as good as my brain gets at this time on a Sunday morning I hope I helped.

 

 

[motzartmerv]

I don't think it works like that, -16 deg stall i mean... have a look at this airfoil simulator. http://www.grc.nasa.gov/WWW/K-12/freesoftware_page.htm

 

Change the output to the lift meter and then change the angle.. set the speed to just above a typical stall say 70 mph. .. slowly change the angle and watch the lift meter.. its a cool little program.. ive been useing it in my stalling brieff..

 

have fun...

 

 

[Ultralights]

My vampire has a Symmetrical aerofoil profile... so is it possible to stall at -16 deg AOA?

 

 

[Guest pelorus32]

Can you restate the question?? "where the aircraft is oriented in an upright position but you can stall the aircraft inverted by pushing the stick.".. could just be my day light saving brain not firing yet but how can the plane be in upright position and inverted at the same time??

Ah Merv,

 

that's why it's a Brain Teaser:big_grin: I think that the way the question is stated really accurately portrays the exact situation. Think of your wing - and no the simulator probably won't help you.

 

Adam is getting close but we need a more detailed explanation and description for a lolipop.

 

Regards

 

Mike

 

 

[Guest airsick]

When you push the stick forward you are effectively increasing the AoA on the horizontal stabiliser thereby increasing the lift it produces. The extra lift produced by the tail causes the aircraft to pitch nose down (assuming you started from a straight and level attitude).

 

If you increase the angle of attack of this surface such that it stalls then it will fail to produce lift and the tail will drop causing the aircraft to pitch upwards. At this point the main wing AoA may exceed the critical stalling angle and then stall itself.

 

Do I get a lollipop?

 

 

[Guest pelorus32]

When you push the stick forward you are effectively increasing the AoA on the horizontal stabiliser thereby increasing the lift it produces. The extra lift produced by the tail causes the aircraft to pitch nose down (assuming you started from a straight and level attitude).If you increase the angle of attack of this surface such that it stalls then it will fail to produce lift and the tail will drop causing the aircraft to pitch upwards. At this point the main wing AoA may exceed the critical stalling angle and then stall itself.

 

Do I get a lollipop?

No lollipop! We are talking about a main wing stall. And whilst on the subject can others please avoid a meander into canards - not relevant to this either. This is your common or garden 3 axis aircraft.

 

Just on the subject of canards I like this definition:

 

An unfounded or false, deliberately misleading story

 

Regards

 

Mike

 

 

[Guest airsick]

I was on about a conventional aircraft! What's more my explanation leads to a stalling of the main wing. Who mentioned canards?

 

Push stick forward -> stall the tail of the aircraft -> nose pitches up -> main wing AoA increases -> main wing stalls.

 

I want my lollipop.

 

 

[Guest pelorus32]

I was on about a conventional aircraft! What's more my explanation leads to a stalling of the main wing. Who mentioned canards?Push stick forward -> stall the tail of the aircraft -> nose pitches up -> main wing AoA increases -> main wing stalls.

 

I want my lollipop.

Now airsick, there's no lollipop:sad:

 

Sorry about the confusion - I wasn't referring to you WRT canards, rather just trying to forestall others from wandering down that particular blind alley.

 

I think you'll find that if you stall the tail the nose pitches down in most aircraft. Despite that it's not the mechanism that's in play here.

 

Oooh isn't this fun?

 

Regards

 

Mike

 

 

[antzx6r]

Not sure if I have this right, but are you refering to a negative G stall push over such as the when you enter one of those forward tumbling aerobatic manouvers? (Sorry, don't know the terminology)

 

 

[Guest pelorus32]

Getting there ant,

 

but let's have some detail and an explanation.....

 

Regards

 

Mike

 

 

[antzx6r]

Detail...? Um, A stall, being the point at which relative airflow over the wing fails to produce lift due to an increase of AOA irrespective of speed, is also attainable in positive G, negative G, inverted or staight and level. So straight and level or in a slight climb, push on the stick untill the wing stalls. Prey.

 

Not recomended unless in an Edge 540 (or similar) with the proper training.

 

 

[Guest palexxxx]

Are you talking about something like a lomcevak?

 

 

[antzx6r]

I think so yes, but on googling to find out what that is, there's a lot more to it than just push the stick and prey. (I suppose I guessed that already)

 

 

[Guest pelorus32]

I came across this teaser when browsing some stuff written by Neil Williams - a well known British aerobatic pilot of the 60s, 70s and maybe 80s.

 

I was momentarily confused by his description of being in erect (upright) flight but stalling inverted. It kind of twists reality a little.

 

Firstly some basics: in a "normal" stall we stall when the a/c reaches the critical angle of attack (AOA or alpha). At 1 g this occurs at a particular airspeed and at greater loadings the airspeed increases as a function of the square root of the load factor. We all know that.

 

So what happens at zero g? The a/c has "no" mass. That means that the wings have to provide no lift to keep the aircraft there.

 

Now translate this to an aircraft in negative g but in erect flight. The example I came across was that of an outside loop. You roll inverted and then after attaining your entry speed you push. As the aircraft approaches the top of the loop it is in erect flight at very low airspeed. The temptation is to push hard to get the aircraft across the top and to get the speed up as it completes the second half of the loop.

 

Pushing hard generates negative g so the wing is loaded "inverted". The airspeed is low so there is every chance of stalling the wing inverted as you increase the negative g.

 

The angle of attack is measured as an angle above the wing just as it would be in inverted flight. Hence the notion of stalling the aircraft inverted whilst in erect flight. You of course do this by pushing the stick forward rather than pulling it back, just as you would induce a stall in inverted flight.

 

Adam was on the money with this one.

 

Regards

 

Mike

 

 

[Yenn]

Pelorus. Somehow I missed this until now and I see several have had a go so here is my interpretation.

 

You are in vertical flight as at a stall turn, which used to be part of the flying training years ago.

 

Aircraft heading upwards and speed falling off rapidly, unless you are in an Extra or something similar.

 

There is no lift from the wings and you push the stick forward elevator goes down. The plane starts sliding backwards and the down elevator causes the tail to go towards what appears to the pilot to be down. The front of the plane falls in what appears to be upwards and you are in an inverted stall.

 

I know what I am trying to say but hope it is understandable to you all.

 

 

[Guest Redair]

What about straight and level flight, then slam the stick forward as fast and as hard as possible? The tail flys up, forcing the aircraft into a nose down attitude, (i.e. pointing at the floor) and inertia makes the aircraft try to maintain its original flight path. So in effect, the aircraft is trying to fly straight up, that is to say, horizontal to the ground, but up, in relation to the top of the aircraft. Relative airflow hits the top surface of the wing, no lift is produced, and I re-write the laws of physics!

 

Simple really. Redair.

 

 

[Flyer]

What about straight and level flight, then slam the stick forward as fast and as hard as possible? The tail flys up, forcing the aircraft into a nose down attitude, (i.e. pointing at the floor) and inertia makes the aircraft try to maintain its original flight path. So in effect, the aircraft is trying to fly straight up, that is to say, horizontal to the ground, but up, in relation to the top of the aircraft. Relative airflow hits the top surface of the wing, no lift is produced, and I re-write the laws of physics!Simple really. Redair.

Just add full right aileron full right rudder and full throttle and you have a lomcevak which is a tumbling manoeuvre in a stalled condition. The torque of the engine keeps the tumbling motion going...well it works that way with model planes.

 

May not have explained it well though.

 

Regards

 

Phil

 

 

[facthunter]

Correct answer.

 

In my view, airsick had a "correct" answer to the question posed. What's more it has some relevance to our flying environment, insomuch that if an aircraft is loaded tail heavy, ( by a fair amount) at slow speed, the horizontal stabiliser & elevator combined may be incapable of providing the lift required (even with full forward stick, which may exacerbate the situation) to prevent the tail falling, and inducing an uncontrolled pitch-up, and an almost immediate stall on the mainplanes. This is an unrecoverable situation worthy of your attention. Nev..

 

 

[Guest airsick]

Thanks Nev. I want my lollipop now Mike.

 

 

[Guest pelorus32]

In my view, airsick had a "correct" answer to the question posed. What's more it has some relevance to our flying environment, insomuch that if an aircraft is loaded tail heavy, ( by a fair amount) at slow speed, the horizontal stabiliser & elevator combined may be incapable of providing the lift required (even with full forward stick, which may exacerbate the situation) to prevent the tail falling, and inducing an uncontrolled pitch-up, and an almost immediate stall on the mainplanes. This is an unrecoverable situation worthy of your attention. Nev..

G'day Nev,

 

This is good, I suspect I'm about to learn something!

 

My understanding of the effect of the tailplane is that, unless you load yourself outside of aft C of G limits, the tailplane is providing downforce and the result of it failing will be a pitch down, rather than a pitch up. Haven't got access to Kermode ATM to check this.

 

Could we explore that bit please?

 

After that we can talk about whether airsick gets a lollipop and if so what sort:cool:.

 

Regards

 

Mike

 

 

[Guest airsick]

The centre of gravity is forward of the centre of pressure so yes, it would nose down in the absence of any forces from the tail. This is a feature that adds to the longitudinal stability of an aircraft and is desirable. In practice I think there would be little difference in straight and level flight.

 

Despite this I still would very much like a lollipop.

 

 

[facthunter]

Pitch stability.

 

Pelorus, the NORMAL set-up design-wise is to have a bit of downforce on the tailplane assembly, for stability in pitch, (and safety). From an efficiency point of view this is not a good idea as the aerodynamic download might as well be actual weight, in fact the OPTIMUM situation for efficiency is to have the tail assy doing some of the lifting. This is used in some large jet transports to gain extra range and fuel is transferred fore & aft to achieve this, and a more rearward C of G is permitted at higher indicated airspeeds, not in take-of or landing. Flying boats are preferably loaded close to the rearward limit...

 

Note...Rules applying to weight and balance as applies to us do not permit USEABLE fuel to be used to bring the C of G into the design range.

 

The rear-heavy loading error MUST occurr to produce the tailplane stall ( resulting in the rear of the aircraft descending) and causing loss of pitch control. As the speed reduces this effect will show up. SO you need to

 

(a) be tail-heavy (load shift or error)

 

(b) get fairly slow. Nev...

 

Lollypops are not good for you anyhow.

 

 

[Yenn]

To be able to stall with forward stick when you have extreme rearward C of G is really a normal stall, but how do you get to be flying in that situation. You must have been using full forward stick at take off. Of course it could happen if your C of G was moved rearwards by a large amount in flight. Fuel usage would not be enough. You would have to have something large move rearwards. I have heard of test flying being done with a weight on a screw thread, which could be moved a long way.

 

It is not something we would be able to do in normal flight.