Gusts and stalling

[Former Member]

[This posting has been removed]

[BB]

Ok !  I've been in bed all day (N/Worker).  Did this thread turn into another episode of 'Eastenders' ?

 

BB

[Stephen Grigg]

Strangely I know nothing,about the theories,but as my instructors tell me my model must always fly faster than the wind, speed to have control,if a gust popped along as described the model speed against the wind would increase and the model would rise because its in a thermal.The way I land my model is up and down most of the time. so it would be just as usual.

[flytilbroke]

Can`t say I have ever had a "stall" due to sudden changes in wind direction or strength. What I have had is a model out of my control close to the ground, caused by turbulance. Also more often, models recovered from turbulance induced mis-direction.

 

Mention has been made that wind does not appear to change direction almost instantly. I can confirm that given the correct conditions in a specific geography, YES IT DOES. Our Glen Nevis site can and does provide such entertaining conditions. Some might call it turbulance,,, it is still a very rapid change of direction of flow.

 

So, wind/air head on at 9mph to a model traveling at 25mph air speed = ground speed of 16 mph. Airflow direction reverses at and close to the model to local speed of 9mph. Model does not instantly speed up so temporarily loses airspeed, 25mph - 9mph = 16mph in static air

 

 However the wind is now from behind so, 16mph less 9mph from behind = 9mph airspeed. Note the ground speed has not had time to vary it still appears to be 16mph. The model does not now have sufficient lift to follow the required flightpath, it loses altitude, and often appears to have stalled (as most would describe it) due to the rapid unwanted altitude loss. Fixing time ensues.

[BB]

BTW - I like the video Gemma for what it is.  I am fully aware of the misconceptions.

 

BB

[David Turner]

Just caught up with this thread.

 

Can someone explain this to me?...many's the time, when I've been flying light aircraft in very gusty wx, that I've had the stall warner going off, even when the instantaneous ASI reading has been more than twice the one-g stall speed. So, 100kts on the clock...bang...stall warner.

 

Why would that be?

[Martin Harris - Moderator]

Instantaneous change in AoA deflects vane of stall warner but momentum damps effects on aircraft  so very short duration wing stalled condition not noticed?

[Martin Harris - Moderator]

..and inertia/local airflow effects in the pitot and static systems not reflecting the changes on the ASI?

[Dusty]

Yes, if you flying lets say, in the cruise and you hit some turbulance then its not uncommen for the stall waring to "buzz". Immently/during  a stall you should encounter a "buffet", if you can identify this then you know the aircraft is stalling! I presume you are refering to a fullsize? Stall warning systems on older Cessna, Piper type planes are only as good as they are maintained! I have had one sound at 120kt in a 206 due to a insect lodged in the tube and made erratic "BUZZES"!  Yes I did pre flight checks, but older venturi/air tube type systems are not 100% just like if you ever trust fuel gauges in old model type Light A/C or do you trust the trusting wooden di[stick to check fuel tanks then do the math?

[weasel]

Stuart, 

  Look at this Thread caused a lot of grief, and arguments

  So long as the model flys lets not worrie...

  Regards Weasel......

[Stuart Eggerton]

It would be interesting to attach a small video camera looking along the model wing and to place some streamers along the wing chord. This way experiments could be done say on a slope where the camera video's the behaviour of the streamers in normal flight, then fly the model into an area of the ridge where the wind speed decreases suddenly and see if we get the streamers being turbulated by the air breaking away from the surface of the wing (i.e stall) or not?

 

I can not think of a reason other that stall why the streamers would be turbulated (i.e. they should follow the upper surace of the wing contour) except perhaps passing through a layers of turbulence. I would want to calibrate what the stall looked like before hand by stalling the wing in normal flying conditions and seeing the pattern formed by the streamers. This same pattern should be recognisable if it again occured at some other time during the flight. I think the difference between the streamers behaviour when the wings stalls and when passing through turbulent air (i.e the downwind section of ridge) would be evident. 

 

Of course this is all theory, and knowing my luck a load of cobblers! but could theroetically be proved by an experiment. 

 

Stuart

Edited By Stuart Eggerton on 12/08/2011 14:56:13

Edited By Stuart Eggerton on 12/08/2011 14:59:26

[Lazygit]

Good idea Stuart. This could be done on flat sites too, but you wouldn't need a camera. Just stick streamers on to the wing and land slowly in gusty conditions. If the streamers fly up, the air must have come unstuck. If they lift, technically I'd say it had stalled, though ground effect may protect the aircraft from potential nasties.

[John Olsen 1]

I hesitate to join a discussion that so far seems to have generated nearly as much heat as light, but live in hope that maybe we can get things sorted out...

 

It is true enough that a horizontal gust cannot of itself change the angle of attack of the wing, it can only change the airspeed. However considering the situation of an aircraft on final approach, if the gust is from behind and reduces the airspeed significantly, there is going to be a loss of lift. This loss of lift will allow gravity to accelerate the aircraft downwards, and the resulting increase in the rate of sink will increase the apparent angle of attack. It is perfectly possible that this increase could be enough to take the angle of attack past the point of stall. So I would argue that a gust can in fact lead to a stall...However note that the stall is a secondary effect rather than a primary effect, eg the aircraft is already in trouble due to the increasing rate of sink, and may in fact hit the ground before an actual stall condition is reached. So whether or not the aircraft actually stalled is pretty academic.

 

If vertical components to the gust are allowed, then of course the gust can directly affect the angle of attack, as per Martins diagram a few posts back, although please note that a downwards gust will give a reduced angle of attack. Back in the days before radio control we used to trim our free flight gliders right on the edge of a stall. This had two advantages...first it gives the maximum duration from a given starting height, which was determined by the permitted towline length. Secondly, should one wingtip enter rising air ( a thermal) then that tip will tend to stall reducing the lift and increasing the drag on that side. This will turn the glider towards that side, and hopefully it will enter the thermal and remain within it. This actually works rather well, I had a glider and a free flight power model fly away due to this, fuse type dethermalisers not being very reliable! Full size gliders are not usually trimmed in that way, they are optimised for maximum distance from height, not maximum duration.

 

Since we know that thunderstorms contain vertical updrafts and downdrafts strong enough to tear the wings off some aircraft, I think it is pretty evident that suddenly entering such an updraft could easily take the wing into a stall condition. Like my first example, whether or not a stall actually occurred is likely to be a pretty academic question in this situation. Another "interesting" situation would be flying through a rotor cloud. These are found in the lee of mountain ranges where arch clouds are forming. The wind passing over the range forms a series of waves in the lee, with characteristic hard edged lenticular arch clouds forming. Sometimes, at ground level under the arch cloud, the wind on the ground will be blowing in the opposite direction to the wind at altitude, and at an intermediate altitude there will be a rotor cloud....a wispy line of cloud at right angles to the wind direction ( eg along the line of the mountain range) rotating furiously. Since the wind at altitude in these conditions might easily be 50 or 60 knots, an aircraft flying into such a rotor is likely to encounter very sudden changes in airspeed, to say the least. Full size gliders do fly in these arch conditions, in pursuit of altitude and distance records, although I beleive they try to avoid the rotor clouds.

 

For our models, where we have no easy way of observing airspeed, it does seem to me that it would actually be quite nice to have some sort of indication, particularly for landing. Telemetry to a transmitter might be one approach, although you don't want to be looking at the display on the transmitter when on finals. Maybe an audible tone proportional to the airspeed could be arranged? Maybe we could even make it an actual stall warning device?

 

regards

John

[Cyclicscooby]

It appears to be at least two separate technical conditions, being broad swept with the term Stall... Easy to see why, as falling out the sky appears to be the end result either way..

Very interesting technical chat tho.. My brain sucks up this type of thing, keep going..

 

 

Luv

Chrisie.. xx

[Wingman]

I've often thought that a tone produced in the transmitter proportional to the position of the throttle stick would be a good idea being as we tend to set our model's airspeed as an amount of throttle stick movement i.e. half throttle = cruise, quarter throttle = approach

(at least that's the way I do it!)

[Martin Harris - Moderator]

Throttle position has very little to do with airspeed I'm afraid and even less to do with stalling.

[Wingman]

Interesting - how do you judge the airspeed of your models.

[Martin Harris - Moderator]

Mainly by their attitude - particularly on the approach where the throttle is controlling the rate of descent and the elevator is controlling the airspeed.

 

Taken to extremes, I often fly at zero airspeed at a high throttle setting i.e. the hover, but a slow entry requires ever increaing elevator to slow it down, combined with throttle adjustment to hold level height.

 

I can stall many of my models at full throttle - just requires enough elevator power...

Edited By Martin Harris on 15/08/2011 18:54:53

[Stuart Eggerton]

I have attached an annenometer to the model, it normally works by the wind causing a fan to turn and thus display the windspeed. I thought it could make an ASI by mounting this device on a model, although don't know how accurate it is when used for that purpose. It is viewed not in real time when flying the model but my attaching a tiny video camera facing the annenometers display, so after flying the model and downloading the video I can see round about what air speed the model was flying at. It does cause quite a bit of drag though!

An FPV model could provide accurate airspeed and video.

Regards

Stuart

[John Olsen 1]

One example of stalling at a high throttle opening is the high speed stall in a tight turn. The aircraft will suddenly flick out of the turn, in an unpredictable direction depending which wing tip stalled first.

 

Also an aircraft at its absolute maximum ceiling is simultaneously at maximum speed, stall speed, and full throttle. (Usually the service ceiling is quoted rather than the absolute maximum, the service ceiling is where the rate of climb falls below a specified value. It takes a long (infinite?) time to reach the absolute ceiling, since the rate of climb falls off as you get closer, and the actual ceiling gets higher as you burn off fuel trying to reach it.)

 

Now in those 3D hovers....the outer part of the wing has zero airspeed while the inner part has the propwash passing over it...is either part actually stalled? Maybe not, but if the engine stops they soon will be!

 

I think a hot wire anemometer would be the sort of thing to apply to a model, as they are small and would not contribute significant drag. If the onboard electronics converts this to a voltage, then a telemetry channel intended for a battery could get the information to the ground.

 

regards

John

[Martin Harris - Moderator]

I suspect the conventional anemometer would suffer from significant position error but having been educated on gliders, I learnt to regard instruments as back up for attitude and aerodynamic feedback.

 

I do recall reading that the U2 spyplane had a less than 10 knot operating range at maximum service altitude and turning was a balancing act between stalling the inner wing and mach buffeting setting in on the outer!

Edited By Martin Harris on 15/08/2011 23:28:42

[Lazygit]

Posted by Martin Harris on 15/08/2011 17:52:23:

Throttle position has very little to do with airspeed I'm afraid and even less to do with stalling.

My aircraft is odd then, as If I have the throttle nearly closed, it flies slowly if I try to maintain height with the elevator. It flies so slowly it is nearly stalled. If it suddenly loses lift and drops I have to options to recover and have used either one successfully, one is to open the throttle and the other is to lower the nose using the elevator.

 

If I open the throttle fully it flies much faster, and is difficult to stall. In fact, I haven't been able to stall it in this mode unless I put it into a near vertical climb.

[Tom Wright 2]

Lower the nose .....then apply power .

[Martin Harris - Moderator]

Not as odd as any glider which (full size) may have a speed range of between 35 and 150 knots - all controlled by the elevator and search as you may, you won't find a throttle!

 

With a low energy level from the engine, as you quite rightly state, the elevator is used to maintain a sufficiently high angle of attack to provide sufficient lift to fly level. The angle of attack is what controls your airspeed - further throttle at that angle of attack will cause the model to climb with little increase of airspeed. There is a small effect on the angle of attack relating to thrust/propwash but the primary effect of throttle is to provide power to climb or maintain height. I have several models that I can dive vertically and stall at full throttle.

 

To get back to your idea of an audible indication of airspeed from the throttle position, hopefully you can now see that this won't relate directly to airspeed or particularly to stalling which is the point I was making.

 

A further example is how I set the elevator travel on a combat model - I reduce the movement until the model just doesn't stall at full elevator in a turn at maximum speed and full throttle. In a more or less vertically banked turn, the stalling speed is increased dramatically because of the increased G loading so a model that stalls at an airspeed of say 15 knots in level flight will stall at maybe 45 in an almost vertical banked turn.

Edited By Martin Harris on 16/08/2011 01:15:45

[John Olsen 1]

It is true that if we maintain level flight, we will go faster with the throttle open and slower with it closed, to the point where we can no longer maintain level flight. What is not so obvious is that in order to maintain level flight, we have adjusted the elevator setting, either directly with the stick or indirectly by adjusting the trim. (and in some cases the thrustline of the engine may be such as to affect the fore and aft trim anyway.) Ignoring engine thrustline effects, if you are flying level at low speed and you open the throttle without adjusting anything else, you will begin to climb.

 

With some aircraft you can in fact go even slower than the point where you would normally run out of lift, by going beyond the stall point and using engine thrust to overcome the additional drag, which of course means that you are using a higher throttle opening to go slower. This is routine on some delta winged aircraft, where the low aspect ratio gives very controllable behaviour beyond the stall.

 

We generally already have an audible indication of throttle opening, even on the quieter electric models you can usually hear enough to tell you how fast the engine is going, and anyway barring failures it will be closely related to the thottle setting on the TX. That doesn't tell you what you need to know at critical moments like landing. The local model shop sometime shows a video of various model disasters occurring....apart from a few mid air collisions and structural failures, the bulk of disasters appear to be speed related, eg:

* hauling the model up when going too slow, and immediately dropping a wing and going in,.

* landing too slow and losing it, possibly due to a gust or turbulence .

* landing too fast and hence bouncing back up.

 

Even if the ASI is kaputt, a full size pilot is getting an indication of airspeed from the sound effects, and of course is in a position to feel any prestall buffetting.

 

The increase in stall speed can be related to the G loading on the aircraft. The lift is proportional to the square of the speed, so if the aircraft is pulling 4 G, then it will stall at twice the normal stalling speed. With the wing at the maximum (on the point of stall) angle of attack, at twice the normal stall speed it will generate four times the lift.

 

regards

John