Stalling - why is it so dangerous?

[Stephen Grigg]

Thats helped Martin Thanks

[Martin Harris - Moderator]

Glad to hear that Stephen.  One thing I meant to emphasise was that regardless of flying upwind, downwind, crosswind or in no wind at all, the pitch attitude for the airspeed remains the same.

[Stephen Grigg]

THe MPX mentor does  not ,seem to stall.Tonight the wind was up a bit and on one landing it seemed to land at a hover.Ive watched another club member with his ans saw how slow he was having to bring it in to land it

[Gemma Jane]

It will stall Stephen.

 

Landing in a  hover was because of the wind. Ground speed zero, but airspeed wasn't.

 

If you want to see the Mentor's stall, take it up a couple of mistakes high, cut the power and hold the nose up with elevator so that you lose no height. Then it will stall eventually.

 

It's a good thing to do if your confident about it, it will give you a good idea of just how slowly it was flying before the stall and allow you to judge your landing approach better. I've only recently started doing this with models regularly, both stalling and low speed flying, I think it has improved my understanding of each model a great deal.

[David Ashby - Moderator]

I'll sneak in here too just to say that Tim's right (WOT4) but funnily enough the WOT4 is so good at simply not stalling and so amazingly easy to fly that it can catch you out when you move on to other models and almost subconsciously expect them to fly the same....ask me how I know   Right I'm gone - back to the experts. 

Edited By David Ashby - RCME moderator on 06/07/2009 08:27:01

[Martin Harris - Moderator]

Posted by Gemma Fairchild on 06/07/2009 00:36:15:

It will stall Stephen. 

 

Not necessarily, Gemma, in the case quoted. If the model has a very forward C of G and/or lack of elevator power, the situation may arise where even full up elevator won't allow the angle of attack to reach the stall during the typical approach although you may be able to force a stall with an accelerated entry.

 

I remember instructing a very subtantial gentleman on a week's gliding course where I was given authorisation to fly with a C of G well beyond the placarded forward limit.  The ASK21 was unstallable in normal flight but needed a considerable increase in approach speed in order to flare successfully!

 

Your observations about airspeed/groundspeed are, of course, perfectly correct and I thoroughly agree with exploring low speed handling with any model at a decent height.

Edited By Martin Harris on 06/07/2009 10:15:03

[Gemma Jane]

Again Martin not really a likely situation with a Multiplex Mentor, but yes with a forward CoG one can lack elevator authority.

 

What Stephen saw was the difference between ground speed and airspeed. He is thinking that the stall is a speed issue, not an aoa issue and that the speed he is observing from the ground is the speed of the aircraft. That is the issue.

 

Clearly if there had been no wind and he had brought it to a standstill over the strip, it would have stalled

 

Of course it is possible to design a model that won't stall, in fact I fly a full-size that takes a considerable effort to stall in ordinary circumstances. Not to be confused with the fact it certainly can stall and would probably do it at the most dangerous time if one didn't keep an eye on things.

 

 

Edited By Gemma Fairchild on 06/07/2009 11:56:09

[John Cole]

Martin: a clarification of your (earlier) posting about looking at the model's attitude: your words sound (to me anyway) as if you meant using the horizon as the reference, which is OK if the model's flying level.  But is it's descending, this would cause the pilot to underestimate the AoA - and could lead to a stall.   So the reference must of course be to the attitude in relation to the flightpath, which I'm sure is what you meant. 

 

Gemma: "is stalling caused by flying too slowly or by excessive elevator?"  Yes, of course they come down to the same thing and I know "too slowly" is the usual full-size wording.  In fullsize you've got a nice little meter that tells you the airspeed.  I used "excessive elevator" simply because a model pilot doesn't really know his model's airspeed (but he can see when he's bending the sticks). 

[Gemma Jane]

Ah John, that is my point. Now I'm self taught with models, not a single lesson from anyone, yet in my view if you can't visualise airspeed when flying models it is very restrictive, one should be visualising airspeed. Stephen's post above shows that he hasn't been taught to do that.. why is that?

 

PS taking the point further, and Martin has already touched on this, full size can live without that funny little meter in the cockpit, because we know the pitch attitude for a given power setting... modellers should too.

Edited By Gemma Fairchild on 06/07/2009 12:18:55

Edited By Gemma Fairchild on 06/07/2009 12:29:01

[Simon Chaddock]

This very interesting thread brings to mind some of my instructor training in a DH Tigermoth and you quickly learnt why it was such a good trainer.

 

It did exactly what the book says a plane will do, with classic stall & spin characteristics, and perhaps more important, classic recovery as well.

 

As each design (full size or model) behaves differently I agree that exploring (at a safe height) its handling characteristics close or even at the stall is valuable experience, and not just flying straight and level but in a turn as well.

 

Learn how the controls react and how much height is needed to "sort it out" if it fails to respond as you intend.

 

On a model there is no "feel" (or instruments - yet!) so the minimum safe speed for any particular situation has to be judged by eye and the more you practise the better you will get at it.

[Martin Harris - Moderator]

Posted by John Cole on 06/07/2009 11:58:44:

Martin: a clarification of your (earlier) posting about looking at the model's attitude: your words sound (to me anyway) as if you meant using the horizon as the reference, which is OK if the model's flying level.  But is it's descending, this would cause the pilot to underestimate the AoA - and could lead to a stall.   So the reference must of course be to the attitude in relation to the flightpath, which I'm sure is what you meant. 

 

 

 John,  you're quite correct in thinking that - which is why I mentioned "with sufficient elevator power it is perfectly possible to stall a model (or of course, full size) with it pointing directly at the ground" in the earlier post.  A typical trainer won't change its attitude with relation to the horizon significantly with small power changes in the circuit and learning a safe attitude in relation to the horizon will act as a very good guide to maintaining a safe airspeed.  

 

I have to admit to only considering level and descending flight in my posts so far.  As you say, for each phase of flight there will be different attitudes in relation to the horizon and it is these that people need to learn to recognise and judge.

 

Probably as many accidents are preceeded by inappropriate nose high flight on initial climb out, accentuated by sloppy controls with decaying airspeed - I always try to impress on pupils that airspeed gives control and height is of very little use if a model is struggling to get away in the early stages and should be sacrified for adequate airspeed.

 

I think Gemma and I are essentially in full agreement over airspeed/groundspeed and I fully endorse her comments on attitude - in gliding, attitude is the primary reference for setting/maintaining airspeed and the air speed indicator is only used for confirmation/fine tuning.

Edited By Martin Harris on 06/07/2009 15:07:45

Edited By Martin Harris on 06/07/2009 15:13:12

[Klippy]

Hi there, coming in late to this debate, and a good one it is! If I've missed it , my apologies, but no one has mentioned wind sheer! This is one good reason for keeping the speed on, 10kph on the ground can easily be 20kph 10metres up. as you approach on finals you progressively lose speed as you fly down the wind gradient, the attitude of your aircraft remains the same. Ask me how I know! Another thing, try never to side slip an aircraft, unless you know exactly what you're doing. It's a great way to lose height, but, when I was instructing in gliders, so called un-stallable aircraft could be provoked into a viscious stall by first side slipping and then kicking it straight with rudder without putting the nose down. That's my 10c, I'll get my coat!

[Gemma Jane]

Leave your coat Tony, it's a good point,  and also applies in gusty conditions increasing the approach speed increases the safety margin and the approach should be flown that little bit faster in gusts.

 

I tend now to fly a couple of low passes with foamies in windy conditions, it gives me an idea of the sheer I'm likely to experience on approach to land.

 

 

[Tony Jones]

John.

 

You wrote:  But the dihedral means that the angle of incidence of the left wing is in effect increased (that's what makes rudder/elevator models bank when rudder is applied).

 

The angle of incidence of a wing refers to the angular relationship between its chord line and a nominal fore-and-aft decided on by the designer. It is therefore fixed when the aircraft is built and nothing you can do in flight can change it.

Edited By Phil Wood - Moderator on 13/07/2009 19:55:27

[Tony Jones]

To those who find this debate 'interesting' , may I point out that amateur forums like this are breeding grounds for partially informed and badly informed opinions expressed by people who often don't know what they are talking about. Basic aerodynamic theory stabilised in the early part of the last century. There are several very good books on the subject that have been approved by peer review.  If you want to know the FACTS, go read one of those. 

[Tony Jones]

Firstly, my sincere apologies to Gemma Fairchild for misquoting her completely in my post regarding angle of incidence.  I should have attributed it to John Cole. (Mr Moderator  - if you can delete this post , I would be most grateful).

 

That said, my comments about angle of incidence being fixed in relationship to the aeroplane and not variable in flight still apply. John?

[Former Member]

[This posting has been removed]

[Tony Jones]

On a more positive note, may I recommend  A C Kermode's classic 'Flight  Without Formulae' published by Pearson Education? Particularly good for the mathematically challenged such as myself.

[John Cole]

Tony: I wrote that the angle of incidence is IN EFFECT increased, and the "in effect" was meant to imply that I was using the term loosely. 

What I was trying to convey is as follows: Picture the plane horizontal in both pitch and roll. If the plane is slipping left and the wing has dihedral then draw a new datum which is horizontal  but skewed left in the direction the plane is slipping.  The angle between the left-wing chord line and this new datum is increased (whereas for the right wing it is decreased).  That's just geometry, not aerodynamics.  For any point on either wing the AoA is therefore also changed: for the left wing it is increased, and for the right wing decreased.  So if the plane is close to the stall, is slipping left and has dihedral, the left wing will stall as it is at the higher AoA.

 

 

 

 

[Gemma Jane]

Tony it's a bit rich to come on here going on about amateurs and then recommend a book like Kermode's 'Flight without Formulea'

 

 

[Stephen Grigg]

Gemma, you recommended I fly high slowly to get the feel of sloppy controls.I was doing that last night with an electric Pioneer trainer and it was very interesting.When the model started to get unstable I was able to give it height and practise some slow dead stick touch and goes.Tonight it was a bit windy but I flew the Scorpion and lovedit As it takes so long to react to any control is this good practise?

[Gemma Jane]

Stephen, how many posts do you read where is says something like 'ARTF undercarriages are not strong enough', 'that model is very floaty'.... hmmm all because of flying the approach way too fast because of fear of the stall.

Learning slow flight will give you much greater confidence with landings and low passes. Trust me it adds to the skill bank to be able to explore the flight envelope fully. I've seen myself going from flying at almost WOT all the time to having a far better feel for my models by exploring slow flight.

 

 

 

[Stephen Grigg]

Thanks for that Gemma,Im getting there,promise

[Tony Jones]

Gemma

I have clearly upset you - even though I have apologised for misquoting you. However, I make absolutely no apologies for recommending Kermode's Flight Without Formulae. It is a highly regarded book written by an acknowledged expert that does a great job in explaining the mechnanics of flight without resort to too much maths. It has been used by military and civilian flying schools for decades.  We can't all be mathmetical geniuses like you. And actually, as with many highly qualified experts you appear to have a problem in explaining things in plain English.

I have now updated MY profile. Go check it out. 

[Gemma Jane]

Don't kid yourself Tony, it would take someone far more clever than you to upset me.

 

 

Perhaps you upset yourself because you came on here ranting about amateurs when on the subject matter you are one yourself.

 

 

An 8000 hour pilot who thinks a gust can stall a wing? Hmmm, perhaps you better keep your nonsence on the forum and not try to argue with me by PM.

 

 

At least then I correct you in public. You know I know some 30,000 hr pilots who have misconceptions regarding flight. It doesn't matter how much time you spend at the controls it doesn't increase your technical knowledge does it. It certainly isn't on my mind when I'm flying.

 

 

How can you explain a complex subject in plain English then Tony? The language of aerodynamics is mathematics, it can't be helped. I didn't actually find maths easy myself but through hard work I overcame my difficulties to achieve what I wanted to in life, I'm not about to appologise for the fact.

 

PS the truth about Kermode is it is full of nonsense and half truths... it has to be dumbed down for those who can't grasp the truth of the subject. Aerodynamics for Dummies... hmmm it was never going to be a highly respected book was it

Edited By Gemma Fairchild on 14/07/2009 09:55:50