Tonight; My First Crash.

[Chris Anthony]

I was at the club field tonight flying my Seagull Jumper 25 around (with FrSKY Taranis X9D TX and FrSKY D8R-II Plus RX) on about my 3rd tank of fuel. I was flying the plane inverted, on this occasion I had been doing so for about 30 secs - 1 minute. The plane was at around 75-100m height, and around 200m away from me (just the other side of the landing patch). My position relative to the plane at that point was "8 o clock high" if "sitting in the cockpit" (as it was inverted). So it the plane was flying away from me at around that kind of angle, if you can picture it. It was flying straight and level, but inverted, with a small amount of down-elevator (25%) on my TX stick, 80% throttle, minimal aileron movement and no rudder.

Suddenly, the plane seems to hike on a load of up-elevator and pitches towards the ground. I had not made any elevator input to cause this. I tried adding down-elevator to correct this, but nothing seemed to happen. The plane dropped to around 25ft of height and also had a slight but constant right roll as it began descending. By the time my first involuntary immelmann was complete, with the fuse parallel with the ground, the right wing was completely dropped. At this point I THINK I remember the engine being down to idle, I don't remember touching the throttle but I may have. With the aircraft being unresponsive to any aileron or elevator input, it continued to move through this orientation in a sickening right-hand, downward spiral which ended seconds later with the plane nose-diving into the ground.

Now, the first problem I have here is that I don't remember exactly what I was doing with the controls in the mere 10 seconds that this all happened in. But most people I'm sure are aware of the feeling you get when your control movements do not result in aircraft response as you would expect, even if you become confused with orientation for a second, which I wasn't. You know when you have lost control in some form, even if you can't quanify the loss. I feel sure that I would admit to any pilot error that I was conscious of. I have experienced the feeling of adding in incorrect elevator when inverted and only realising seconds later. This is not what happened. The pitching that started tonight all came when I did not move the elevator controls at all, and the aircraft did not respond to any further inputs, that I could redognise, from the right TX stick from that point on until crash.

I am also not 100% sure on what the engine was doing. I THINK it was idling as the plane hit the ground, but it may have not changed speed at all. This is important because of what my fail-safe settings are; 0 throttle, and full up elevator (no aileron or rudder). More on this below.

Some more info; my Taranis TX and RX telemetry give 2 audible warnings that I am familiar with and have tested. One tells me when the RX is near it's range limit. This is at around 4 times the distance from me that the plane was at today. The other warning tells me when the RX cannot be found at all ("telemetry lost", which only ever occurs when I switch the RX off after landing the plane. It was only once the plane had crashed that I heard the "telemetry lost" warning from the TX (battery presumably became disconnected upon impact). I was not warned that signal between the RX and TX had been lost when the initial control loss happened in the air.

This flight was different tonight for one - maybe 2 reasons. The second, less credible reason is that tonight was the first time for me flying on my own at the club field. I have just joined as a member, and have flown once before in experienced company. I do not know if there has ever been cases of interference from nearby features or anything.. this is something I am enquiring about with the guys there. The first, and main reason that the flight tonight is different though, is because it was my first flight with fail-safe settings actually set. Essentially, from what I can interpret/remember, from the moment I lost control to the moment the plane crashed, I had full up elevator and, I think, zero throttle - my new failsafe settings (I believe the rolling to the right was just a product of roll speed/inertia I had at the moment that control was lost - and the loss of airspeed just increased this roll effect).

TL;DR - I find it a horrible coincidence that the plane was acting like the fail-safe settings suddenly became active, but I had no warning for loss of signal from the TX, and I was well within normal operating range. Could anyone think of an explanation for this? I have basically written off being able gain any insight from the wreckage because everything (mainly servo positions) changed upon impact.

I appreciate how hard this may be to comment on with no data, no pictures, no video, no audio, no clues from the aftermath and a sketchy description from a biased and forgetful witness. But if anyone could come up with any suggestions as to what might have happened I'd be very grateful.

In tragic coincidence, it was just this morning that I ordered my next plane, after feeling competent enough to sack off the Jumper. Indeed, flying it tonight I felt very confident, almost bored with it, drawing the line at trying to get it to flat-spin. I did not think I would be forced to move on from it so quickly though, and am now worried that I may prepare my next plane in the same way that will cause a similar failure.

No sympathy required! But all thoughts will be appreciated.

Thanks,

Chris

[Martin Whybrow]

It does sound like the failsafe kicked in, particularly as it pulled up elevator. How quickly do you get a Tx warning when the range limit is reached / when the signal is lost? It may be that it was lost for long enough to cause the incident but not sound the warning.

I have to ask why your failsafe is set to full up elevator, I think that's probably not compliant with the requirement for failsafe setting which is principally intended to get the model closer to the ground to avoid inteference with full size aviation.

Not familiar with the Taranis RX; does it have 2 antennae? If so, how were they positioned, at 90 degrees to one another?

Edited By Martin Whybrow on 28/04/2014 23:35:20

[john stones 1 - Moderator]

Required or not its a shame Chris

Sounds like failsafe if you could not roll out of inverted.

Why ? its usually battery or switch, no point in me commenting on your tx other users are more able to do that.

My failsafe is throttle to idle, that's enough to stop a fly away. Hope you find the cause though, its easier to put behind you then.

John

Edited By john stones 1 on 29/04/2014 00:04:16

[Biggles' Elder Brother - Moderator]

Difficult to diagnose without knowing more. It is possible that you lost signal - but why no warning? It is also possible that you stalled it inverted. The latter would explain some of the behaviour - especially if the wing remained stalled.

The response in a deep stall feels very like loss of control - either the elevator nor the ailerons appear to be effective.

BEB

[stevejet66]

Suddenly, the plane seems to hike on a load of up-elevator and pitches towards the ground. I had not made any elevator input to cause this.

Seen it loads of times, And its happend to me, Not enough elastic bands to hold the wings down, Air gets under the leading edge lifting the wings, Hence the full sudden upward movement without doing anything,

 

Edited By stevejet66 on 29/04/2014 00:28:43

[Chris Anthony]

Thanks John. Indeed a right shame. And as you say, it's something I could come to terms with, accept, and overcome a lot quicker if I knew the cause! If it was pilot error and I knew what I did wrong, I could evaluate and train myself to not repeat, and improve. But as I do not know what exactly I did wrong, I am so very worried that it will be repeated with my next model!

Martin, the "low signal" warning usually occurs when signal is weak but not completely lost. So you still have control over the model when you receive the warning, and enough time to turn the model around and fly it back towards you. What happened tonight was different though, I just lost control without the grey-area warning. If anything, I would have expected the "loss of signal/telemetry lost" warning to have occurred in the air as soon as my fail-safe settings became active, but alas, this did not happen either.

I decided to set the full-up elevator fail-safe so that should I lose control over the aircraft, it would pitch up, stall, pitch down, speed up, pitch up, stall etc all the way to the ground without ever exceeding a certain, nominally low speed during these cycles, to minimise crash damage upon contact with the ground, regardless of altitiude. What you say about straying into dangerous airspace is definitely a valid consideration, but I figure that with zero engine thrust, full elevator would only give 10-20m of futher altitiude gain before descent would occur due to the first stall; and from there, only further descent would be seen.

Of course, I had not considered the case of loss of signal during inverted flight! .. Anyway, after viewing the wreckage of my model from a not-so-great height, you may be pleased to know that I now consider that this particular elevator fail-safe setting may have been a futile effort to save a model, which would probably be a write-off in any case even from that height.

Yes my FrSKY RX does have 2 antenna, and it pains me to admit that they may not have been at 90 degrees to one another when I set the plane up for my flights this evening. I have a horrible feeling that I set them both pointing forward, just parallel to each other, pointing forward down the longitudinal axis of the plane (towards the prop), offset around 80mm from each other (the width of the fuselage). To expand, it pains me to say this most because I found out tonight, before the final flight, the reason that the antenna must be 90 degrees from each other. But, I do not want people to focus completely on this though, and to keep an open mind as to any other possible causes to the failure. If this was the cause, then firstly, why did this only cause a problem during inverted flight? The plane had been at that height, dustance and heading relative to me lots of times at that site before, why would it being inverted make a difference? And also, there is still the fact that there was effectively a loss of signal, and a TX warning should have been present, even in this case, right?

This is again where my inexperience shows, is it possible to lose signal/control without this being registered by a telemetry TX as a loss of signal?

BEB, as for stalling inverted, I can't think why this wouldn't be possible, but again, I am too inexperienced to recognise this. I actually remember thinking, at some point during the descent, that a very strong wind had caught me out whilst I was inverted. But I did not figure this to be an actual cause of the crash because I could not hear/feel any wind on the ground myself, nor see any effects of it on the hedges less than 50m from the crash site even many seconds after the crash occurred. I did not figure that any kind of stall had occurred because I perceived the speed of the aircraft across the ground beneath it to still be great enough to maintain lift/at least some amount of control force from the control surfaces that I was stimulating.

I am of course open to correction on this, but as you point out, the lack of more knowledge makes only speculation possible

[Chris Anthony]

Steve, thanks for your comment, I've only just seen it. I know exactly what you describe, and I had actually experienced this effect in parts of my flights this evening, and also in a few other flights during the past few weeks. I first encountered this effect (where it scared the pants off me) during the second or third session I ever had with a real, wooden, heavy IC aircraft (this very same Seagull, RIP). On that evening, I got to the field and realised I had forgotten the ~16 or so standard elastic bands that came with the model, so I decided to use the ones from my small Foam-E Ares Gamma trainer (6 weak brown bands only), which is about 1/4 the mass of the Seagull. I then quickly learnt about this effect, and noticed it when applying above 30% elevator rather quickly, at near full speed, usually to instigate a climb; there would be a small delay in pitch response, but then a dramatic overshoot and huge pitch rate gain, where the plane would turn towards vertical much faster than I was used to! This usually resulted in a noticeable loss in forward speed too..

As said, this even happened a few times tonight, which I put down to my elastic bands being a month or so old, and having been damp with nitro fuel for that whole time, which I feel may have caused the effective clamping spring rate of the bands to reduce. But, whilst I think your diagnosis is sound, I don't think this was the actual cause/or only cause responsible for the crash tonight.

I think this because firstly, the pitching occurred without my asking for any up-elevator. The "elastic band effect" only seems to delay and accelerate any pitch motion initially instigated by the pilot, and my downward-elevator control position should have ensured that no air got between the wing and the fuselage. But I also know that this may have easily occurred regardless of what I was doing with the elevator control. Maybe the wind changed, or something like that. Maybe the wing did indeed leave it's mating face with the fuse, and cause an up-elevator pitching effect. But, in my opinion, this was just not enough to explain the sustained 10 seconds of lack of control that followed the initial instigation that I described. Maybe, just maybe, the elastic bands did cause the initial downward pitching of the model, but this must have been followed by some other coincidental failure almost immediately after, causing the final demise of the model. The latter failure being enough to make the former failure seem almost meaningless anyway.

Edited By Chris Anthony on 29/04/2014 01:48:11

[Rich too]

sorry to hear that Chris, its a good excuse for another model though

flying straight and level with 80% throttle it was unlikely to be a stall.

as John said switches (and batteries) are common reasons.

I do wonder though, why do you have to set the antenna before each flight? I fix mine in position - permanently. then I do a range check, and then you can forget about it....

Rich

[Dave Bran]

Bands - Keep them in a Ice Cream Container with some Talcum Powder. After replacing them shake the container to dust the bands and help remove excess oil, etc from the band. Take them out and shake them before use (downwind!!).

You say you can't understand why the plane might get loss of signal when inverted when its been the right way up at that distance OK. It isn't a rigid go or no go process! Perhaps there was something on the plane blocking direct access to the aerials then, perhaps something reduced the signals capability of connection "(interference, etc), or any combination thereof together with things already suggested.

Pay attention to correct setup, ALWAYS make 2.4GHz aerials 90 degrees orientation, ALWAYS position them where at least one will not be masked by battery/rx/servos in line of sight to Tx. If possible bring the aerial part (the end only, not the co-ax) OUT of the Fus and blenderm tape to the outer skin................its only a few mm and you won't see it when its up there!!

My policy with failsafe is to set it to down the plane asap....... allowing a plane to fly on "in case it gets connected again" in the environments I fly would only allow it to exceed safe boundary. I want it to remain inside the safe zone, not end up over roads, people, property, etc, etc.

[Biggles' Elder Brother - Moderator]

First of all - if you take nothing else from this discussion - you must understand two basic ideas in aerodynamics...

1. Stalling has nothing what so ever to do with either speed or your throttle setting!

Failing to understand this is the cause of many crashes and if I had a penny for every time a pilot had simply stalled, failed to recognise the fact, thus failed to institute a proper recovery procedure, became disorientated and panicy because of the apparent failure of the model to respond to control - and final crashed, erroneously blaming radio failure, I'd be much richer than I am now!

Motto to learn by heart: It is perfectly possible to stall an aircraft going like the bats out hell on full throttle. Stalling is not caused by flying too slow. It is sometimes an indirect consequence of flying slowly - but that is not the cause.

A stall occurs for one reason and one reason only; the wing has exceeded its critical angle of attack. This can happen at any speed - as any experienced pilot will tell you.

2. The second idea you must understand is that only one control can trigger a stall - the elevator. Against what many people seem to believe the elevator is not there to make your aircraft climb and descend - it is there to control the angle of attack of the wing. In having this role it the control that brings about stall behaviour if misused.

There is an old, but very true, maxim in flying - "no one ever stalled an aircraft with the stick forward" (assumes you are right way up!)

You were inverted - therefore the maxim above reads the opposite way round - "no one ever stalled an aircraft with the stick back"!

You where pushing to maintain level flight - there is a possibility therefore (I put it no stronger) that momentarily you pushed too far - exceeded the inverted critical angle of attack - and the aircraft stalled.

Much of what you describe of the plane's subsequent behaviour fits with that hypothesis.

Whether this was the cause or not we cannot say - personally I believe it is much more likely than radio problems in these days of ultra reliable systems. But one thing I would say - whatever the cause - carve on your heart the phrase "stalling is nothing to do with speed or throttle settings"!

BEB

Edited By Biggles' Elder Brother - Moderator on 29/04/2014 10:17:54

[MikeS]

Sorry to hear of you crash Chris.

The amount of times I stalled my Pawnee with a good power setting inverted when I first was flying it. I still do it now but this time planned. Does sound very much like a stall and and yes stalled my trainer many times when first learning to fly in the early days.

Build the next model, go fly, have fun

Mike

[john stones 1 - Moderator]

A lot of crash's are caused by stalling, also a lot of pilot's are unwilling to admit to "pilot error" as BEB says it can happen at any speed, a snap roll for one. Did yours ? I don't know. Failsafe, most people set throttle to idle (i.c.), not as a model saver but because the flight path is then predictable, other inputs could cause unexpected outcomes, its not possible to set an absolute safe setting is it. Throttle to idle will bring a model down safely re nearby people and property at any site I would be willing to use. if it didn't I would not fly there. Examine all your servo's Chris check for stripped gears, check the switch and battery out, have a good think..did I do something wrong when I set up failsafe ? also I always use failsafe when available to me.

John

[Rich too]

Posted by Biggles' Elder Brother - Moderator on 29/04/2014 10:11:59:

Whether this was the cause or not we cannot say - personally I believe it is much more likely than radio problems in these days of ultra reliable systems. But one thing I would say - whatever the cause - carve on your heart the phrase "stalling is nothing to do with speed or throttle settings"!

BEB

Edited By Biggles' Elder Brother - Moderator on 29/04/2014 10:17:54

you only said that because he used a Taranis

Rich

[Rich too]

Posted by Biggles' Elder Brother - Moderator on 29/04/2014 10:11:59:

1. Stalling has nothing what so ever to do with either speed or your throttle setting!

I agree, but he said he had been flying for some time (30 secs) inverted, small amount of elevator, and 80% throttle...

Rich

ps I believe radio failure

[Rich too]

Posted by john stones 1 on 29/04/2014 11:54:41:

A lot of crash's are caused by stalling, also a lot of pilot's are unwilling to admit to "pilot error" as BEB says it can happen at any speed, a snap roll for one. Did yours ? I don't know. Failsafe, most people set throttle to idle (i.c.), not as a model saver but because the flight path is then predictable, other inputs could cause unexpected outcomes, its not possible to set an absolute safe setting is it. Throttle to idle will bring a model down safely re nearby people and property at any site I would be willing to use. if it didn't I would not fly there. Examine all your servo's Chris check for stripped gears, check the switch and battery out, have a good think..did I do something wrong when I set up failsafe ? also I always use failsafe when available to me.

John

I also agree with this, two out of the last three crashes I had were tip stalls (Extras both times) - with the third the plane never even took off!!!

Rich

[Chris Anthony]

Thanks Rich, yes it is new model time now for sure! At the moment I am positive that there was power to the RX, because I got no "power loss" warning until after the aircraft was in the ground, verifying that the power warning system was functioning at least! I am interested though, how is it possible for an RX switch to switch itself during flight? Is this a common problem for inexperienced pilots/builders? Again, I don't consider this a possible cause in my case, I just ask out of curiosity, and can't imagine how a switch facing outside of the aircraft can be moved by airflow/vibration alone.

I had only reset the antenna in this case because I had to take the RX out of the plane the afternoon before to reach the fail-safe button, as I set the fail-safes for the first time just before the fateful flight.

Dave, thanks for the practical band tips and antenna orientation advice. I still haven't ruled out incorrect antenna orientation as a cause, there is just the lack of a "signal loss" warning needs to be explained. I still feel that this warning should have been present if this was the cause of the crash. I will remember your idea on sticking an antenna outside of the fuse though. I have also decided to remove any elevator input for my future fail-safe settings, based on this experience and subsequent advice.

BEB, thanks for that very strong clarification. I have heard this clarification made a few times before from varying sources, but had never focussed too much on the details. I used to feel happy and satisfied that I could recognise a stall in progress, and knew how to correct for it, and that that was enough, without having to disseminate what caused the stall; a lack of speed, or too great an angle of attack, etc. But I started getting a horrible feeling in my stomach reading what you have said and how it seems to correlate with what happened the more and more I think about it. I would like to make it clear now that, especially after your last description, I'm starting to consider a stall very possible. Especially when I received no "loss of signal" warning from my TX. (Please see more below).

Edited By Chris Anthony on 29/04/2014 13:01:35

[Chris Anthony]

Indeed, the plane's behaviour after the initial perceived "loss of control", now that I think about it more, was very similar to when I was deliberately inducing stalls and trying to induce spins in the previous few flights (I am probably using the terms "stall" and "spin" loosely here, maybe inaccurately, but my point is, I agree now that the plane may not have been operating in it's "aerodynamic safe zone".

What still has me puzzled though, is how the stall actually started. I suppose this is something that you will have to take my word on, but the initial pitch acceleration I witnessed just seemed too great to have been caused by gravity (due to loss of lift) alone. It really did seem like the pitching was induced by an involuntary control input. The aircraft still maintained its forward speed throughout the whole ordeal, with no noticeable velocity vector in any other direction - i.e. it wasn't "slipping" through the air. The aircraft seemed to follow a very standard path curvature, and maintained a very normal slip angle during the initial downwards pitching and descent. This is contrary to the behaviour of aircraft I have seen that are stalled, where aircraft may pitch or they may not, but also, the new speed will not be just forwards any more, the total velocity vector will be at some angle to the longitudinal axis of the plane (this is the definition of angle of attack, right?), and as you say, this angle is likely to be greater than the wings can cope with, hence the reason they are stalled.

Yes I had elevator input at the time, but it was minimal and my inputs were as smooth as ever. Even if a large enough angle of attack (which as I say, wasn't visibly present) had been created, I can not see, think or understand how. As I'm sure you can imagine, this trainer would float along at what looked like a good 10-15 degrees AoA more than comfortably at the right speed, and that was very noticeable! Unless, and this is something I need to confirm, airflow in that patch of the sky suddenly changed direction, due to turbulence or thermals or something else? My experience again limits me here, and I'll have to ask club members about our particular field regarding this.

Or maybe the cause of the initial pitching was different, perhaps it was elastic band stretch as previously mentioned, but then bad corrective input from me lead to a stall, which caused the further described loss of control. As already said, this is the horrible problem of having to rely on a poor memory with no objective data to hand! Even better would have been having an experienced hand and eye next to me at the time. Back to speculation..

By the way, BEB the way you described the elevator as an angle of attack controller is fantastic, I will forever fly my planes differently with this thought in mind. I do want to talk more about stalling, speed and angle of attack a bit though if you don't mind, to clarify a few things. I know you say that speed, or lack of, is not the direct cause of a stall, it is angle of attack. I understand this principle, now a lot greater than before. I would like to test some ideas with you though. These are just thoughts I have come up with, so if there is common aerodynamic knowledge that I am either ignoring or patronisingly repeating in my thoughts then please let me know:

There is a reason that most pilots attribute air speed to causes of stall, it is because for each individual aircraft, air speed and achievable angle of attack are directly linked, coupled, if you will. As you say, the cause of a stall is excess angle of attack. But there is also certainly an observable correlation between stalling and air speed; usually, it is fair to say, most planes/pilots seem to stall at low airspeeds. This therefore implies that there is also a coupling between airspeed and angle of attack. If I were to say that at lower airspeeds for most aircraft, a higher angle of attack is achievable than at higher air speeds, would you agree? As I see it, this simply comes down to horizontal stabiliser design and elevator control surface design, and perhaps their distance from the CoG/CoL in each individual aircraft's case. Consider my high-wing trainer, for example; the stabilising moment created by the H-S is going to be much greater than the maximum pitching moment that can be created by the elevator control surface at a high air speed, and so will prevent a large angle of attack from being achievable, even at full elevator. And as an example of the opposite, I would comment on the planes you mentioned that experienced pilots have managed to stall at high air speeds. I presume, that these had large elevator control surfaces relative to H-S size, perhaps with "shorter tail lengths" too, which means that large AoA attitudes are actually achievable even if full elevator is applied at much higher air speeds, right?

Point being, given that stalling potential is based on aircraft architecture, and given my model type, don't you think it unlikely that I could have stalled so long as I had maintained a normal air speed, (unless something exceptional occurred, like I flying into "bad air", because the aircraft design ensures that a large AoA can never result from this?

[The Wright Stuff]

A short answer to a long question is that the higher the angle-of-attack, the more lift is produced for any given speed. Thus a slow flying aircraft has to fly at a higher angle-of-attack in order to generate enough lift to balance the weight. So yes, slow flying might not be the direct cause of a stall, but is associated with stalling for this reason.

[john stones 1 - Moderator]

The tiniest part can bring the whole lot down Chris, a plastic clevise can fail you., cutout for switch could be too tight causing it to vibrate off. Old timers have seen it all, if it can happen, it already has to someone. To be honest I hope you stalled it.. lesson learnt, we have all done it. Better that than unexplained radio issues

Mistakes will be made, the trick is not to repeat them.

John

[Rich too]

Chris,

I've been flying over 30 years and my knowledge is a fraction of some experienced flyers!

I only learned in the past year (from a very experienced rc'er and supplier) that when landing use the elevator for controlling your speed and throttle for height....who knew?? I always thought it was the other way around

I still cant do it properly!

Rich

[John Muir]

Chris,

You mentioned the engine was at idle when the plane came down. Could that have been the cause rather than a side effect? If the engine coughed, spluttered or faded that might easily cause a stall. Normally if your engine dies or falters the first thing to do is get the nose down. When right way up with a stable trainer you get plenty of time to do this. When inverted a trainer becomes an unstable handful with anhedral, a high CoG and a highly inefficient upside-down wing section and may well have a pretty vicious stall, nothing like it would have right way up. Time to react to an engine problem would be next to nothing and if the plane was some distance away you might not have noticed the engine note change.

Just a thought. To be honest though, you are never going to know exactly what caused this crash as there are a number of possibilities and you don't have all the info needed to pin the true cause down. Does your Taranis log its telemetry data? That would be a good thing to have a look at. Beyond that, all you can do is check and double check your radio gear ready for the next plane.

John.

[Biggles' Elder Brother - Moderator]

Chris you say you had a small amount of elevator in - in the OP you quantify it as about 25% - that's not a small amount of elevator!

This is a trainer - I assume with a flat bottomed aerofoil section? Such aerofoils produce a lot of lift and are very lift effective (at the cost of quite high drag) - right way up. So effective in fact that they will still produce upward lift at a negative AoA. In fact if you look at the CL curve for a Clark-Y section you'll see it actually produces upward lift right down to a AoA of about -4 degrees.

Now invert this - this means that until you get the inverted AoA to at least +4 degrees (the equivalent of -4 degrees right way up) you are still producing "downthrust" - so to fly inverted with this wing you actually need a pretty high AoA - of maybe 10-12 degrees. This is why inverted trainers often appear to be flying "nose up". Add in the anhedral effect and the fact that this aerofoil is very twitchy inverted and you can see the potential for easily getting it wrong.

In fact its much harder to fly a clean inverted circuit with a trainer (equipped with flat bottomed wing) than it is to do that with a highly aerobatic model - say like an Edge, an Extra or an Sbach. These latter models have symmetrical aerofoils that work equally well either way up. They also have no dihedral - so have the same stability characteristics either way up. This means they can fly inverted at the same AoA as the can right way up, and that the stall characteristics are same both ways up!

As has been said we will never know for sure - but I am suggesting that a possible scenario that you should be aware of is that you were flying inverted - running a pretty large AoA for all the reasons given above. Then, who knows, may be there was a bit of gust, you subconsciously pick this up, reflex with a tad more push and that's enough to push the wing into a stall.

When a stall hits you inverted expectantly - unlike when you do it intentionally - it often does seem to happen very fast!

BEB

[Rob Jones 2]

It is possible that if you were using Futaba rather than Teranis, you would be able to rule radio problems out completely.

[Peter Jenkins]

Posted by Chris Anthony on 29/04/2014 13:02:11:

There is a reason that most pilots attribute air speed to causes of stall, it is because for each individual aircraft, air speed and achievable angle of attack are directly linked, coupled, if you will. As you say, the cause of a stall is excess angle of attack. But there is also certainly an observable correlation between stalling and air speed; usually, it is fair to say, most planes/pilots seem to stall at low airspeeds. This therefore implies that there is also a coupling between airspeed and angle of attack. If I were to say that at lower airspeeds for most aircraft, a higher angle of attack is achievable than at higher air speeds, would you agree? As I see it, this simply comes down to horizontal stabiliser design and elevator control surface design, and perhaps their distance from the CoG/CoL in each individual aircraft's case. Consider my high-wing trainer, for example; the stabilising moment created by the H-S is going to be much greater than the maximum pitching moment that can be created by the elevator control surface at a high air speed, and so will prevent a large angle of attack from being achievable, even at full elevator. And as an example of the opposite, I would comment on the planes you mentioned that experienced pilots have managed to stall at high air speeds. I presume, that these had large elevator control surfaces relative to H-S size, perhaps with "shorter tail lengths" too, which means that large AoA attitudes are actually achievable even if full elevator is applied at much higher air speeds, right?

Point being, given that stalling potential is based on aircraft architecture, and given my model type, don't you think it unlikely that I could have stalled so long as I had maintained a normal air speed, (unless something exceptional occurred, like I flying into "bad air", because the aircraft design ensures that a large AoA can never result from this?

Chris, I don't know what your background is in aerodynamics but your discussion above is only true when the aircraft is being flown at a constant speed and not being manoeuvred. If you are pulling more than 1 G, the stalling speed rises. BEB is quite right in what he says.

On a full size aircraft where you get speed indication you can get a range of stalling speeds dependent upon the rate at which you are reducing your speed, the load factor i.e. how much 'g' you are pulling or pushing, and the air density or altitude. For example, in a jet fighter manoeuvring in a dog fight at, say, 420 knots, it is quite easy to stall the aircraft by pulling too tight a turn, say 8G or 9G - this does not apply to modern fly by wire jets where the computer flies the aircraft and has it nibbling at the stall which in this case is a high speed stall. With non FBW aircraft, approaching the high speed stall usually manifests itself as a judder through the airframe or controls and the pilot then eases off fractionally to come out of the stalled condition - indeed, this is what the certifying authority wishes to see as it gives the average pilot a chance to fly the aircraft to its limit without exceeding it.. If the pilot continued to pull despite this warning, the aircraft can violently snap out of the turn - in aerobatic speak that is a flick roll. At this high speed the AoA achieved to reach the stall is exactly the same as the one that you approached at the slow end of the scale. Critical AoA is not a function of speed. When we fly model aircraft we do not know the airspeed nor the amount of G we are subjecting our models to so we need to stick to some basic rules to keep our aircraft in one piece. Break those rules and you break the aircraft.

At this stage in your RC flying career, try and keep things simple as aerodynamics is anything but and trying to understand the aerodynamics of what happened to cause your crash may make the situation more confusing than it needs to be. Stick to a few simple rules as you don't have time when you are flying for complication. Most of the important issues have been flagged up above. Another point you may wish to reflect upon is the position of the CG. The further aft the CG the more responsive the aircraft controls become until it drifts through a neutral zone into an unstable zone. If something heavy were to come loose in an aircraft and go aft, it could cause mayhem with the aircraft's stability and lead to an unstable, and virtually unflyable, aircraft.

Hope that helps.

[john stones 1 - Moderator]

Not making a point its just info, semi symmetrical wing on Jumper