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Model vs Trees

Failure of something!

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Sam Longley18/07/2019 19:33:17
70 forum posts

Well I recently wrote off my Seagull extra because I did the first 90 degree of the rectangular landing approach & as I levelled out I had lost too much speed. I intended going into a glide to loose a bit of height before throttling up & turning the last 90 degrees & approach. Suddenly, without warning, it tipped a wing & spun into the ground.(From Tree top height to stay on thread titlecrook) Normally i do a circular approach & keep the power on as it is always banking. Silly mistake, but probably started in the turn because I had entered it down wind without enough throttle to start with. That being a habit from my learning days. Prompting my thread drift above.

J D 818/07/2019 20:05:38
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Posted by Peter Jenkins on 18/07/2019 19:12:48:
Posted by J D 8 on 18/07/2019 17:55:21:

One feature of model flying I have never been keen on which is often taught is the square circuit / approach, downwind leg, 90 degree turn onto base leg, 90 degree turn onto final approach.

Usually fine with trainer and lightly loaded types but when pilots move on to more advanced types the extra gee in those turns can provoke a stall more so if the downwind turn problem is factored in.

I prefer a more oval descending turn starting at the end of the downwind leg.

JD8, I cannot agree that flying a square circuit with a non trainer is any more likely to provoke a stall. Provided you keep your bank angle to not more than 45 deg and you can also add a whiff of power, as we were taught in full size, you are quite safe. I fly both types and have never experienced a stall in the turn - to date at any rate! Provided you are following a descending path and have a good idea of what your aircraft feels like as she gets slow you should not have any difficulty, If you do, then the answer is to practice some more and not shy away from a standard manoeuvre. Why else would the requirement exist for a rectangular circuit if it were more dangerous. I think this is another myth that needs to be challenged.

          Peter, I do not have any problem flying a square circuit, it is as you say ok if the bank angle is not too steep and the aircraft is descending. The problem comes with newer pilots who over control, bank over to much, level off on the base leg and loose air speed at a low level leading to a crash. Seen it happen or nearly happen many times.   It is a good idea for newcomers to practice the square circuit higher up.  John.

 

 

 

 

 

Edited By J D 8 on 18/07/2019 20:21:43

Peter Jenkins19/07/2019 00:21:57
1217 forum posts
132 photos

Hi John, well I hope those who are training your new pilots impress on them not to make tight turns in the circuit. Provided they are encouraged to maintain a constant descent using the throttle to control the rate of descent and not the elevator then they should avoid the problem you describe. There is a lot going on during the base and final leg of a circuit but that's why it's usually taught at the end of learning how to fly. The other thing that helps is not have too much control movement as that can lead to pilot induced oscillation and eventually a hard arrival.

Peter Miller19/07/2019 08:42:41
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Something that is often forgotten and I have not seen mentioned here. The wind speed drops off as one gets lower, the boundary layer effect. Add to this the variations in wind speed.

I hold up a Dyer windspeed meter and that little ball is up and down the tube like I don't know what. Going from 4 to 10 mph and back constantly

Peter Christy19/07/2019 09:25:19
1487 forum posts
Posted by Peter Miller on 19/07/2019 08:42:41:

Something that is often forgotten and I have not seen mentioned here. The wind speed drops off as one gets lower, the boundary layer effect. Add to this the variations in wind speed.

It can also change direction quite markedly as well! Its not uncommon for the wind at 100ft or so to be in a noticeably different direction than it is at ground level - especially if there are large obstacles - like trees - around!

Trees and high hedges can also produce quite a lot of turbulence at low level. It certainly pays to keep the speed up a bit under such conditions!

--

Pete

Martin Harris19/07/2019 12:18:51
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Quite so. The danger comes from up and down air movement (i.e. turbulence) rather than any sudden stopping or starting of wind - which just can't happen in a fluid system. The defence against these sudden changes in angle of attack or descending air is a margin of airspeed. Put simply, a faster approach speed (i.e. model slightly more nose down) in windy conditions allows you to carry a margin of energy for quick corrections and that faster approach speed is negated by the slower groundspeed meaning that landing distances aren't a concern.

Turns in the circuit close to the ground just need basic accuracy and appropriate airspeed/throttle management. In a 60 degree bank, the effective weight of the model doubles so you need more airspeed (adding throttle to avoid excessive height loss) to execute the turn safely - however, you should question the need for such a turn in a well planned circuit. 30 to 45 degree banks should be more than sufficient for your circuit turns and you will only stall off those turns if you are already far too close to the stall. I can honestly state that I don't recall ever stalling off a circuit turn and I almost invariably perform a square landing circuit - often in wind conditions that might be thought of as challenging (I'm not claiming to have any special ability but I have developed an attitude of flying appropriate models in almost any conditions week in week out throughout the year - does wonders for your confidence!)

The key is to learn to judge airspeed by the attitude of the model. Get used to the "sit" in the air and if you find it departing from it, correct with elevator and use throttle to correct changes in height from the approach slope.

Rob Ashley19/07/2019 15:18:59
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Posted by Peter Christy on 18/07/2019 12:28:41:

I was always taught to control speed with the elevator and height with the throttle. There may be exceptions during aerobatics, but it works very well in circuit flying!

--

Pete

Interesting Pete, In the full size word we teach that elevator controls the glidepath angle and throttle the speed. So on final approach we point the nose at the touchdown point from a given distance and height (thus giving the correct angle) and maintain that sight picture position with elevator and then keep your approach speed with throttle/thrust levers. I fly my model approaches like that too, although it is more difficult to judge airspeed of a model when it is flying toward you.

Rob

Martin Harris19/07/2019 15:29:06
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That wasn't the case last century when I was involved with full sized flying but perhaps this is more pertinent to heavy aircraft with higher approach speeds and drag producing devices?

Rob Ashley19/07/2019 16:00:40
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Hello Martin, not really, it is applicable for all. Although I have seen both methods taught, in my experience the power for height method leads to a porpoising style approach and more heavy landings and is therefore not the preferred method. Interestingly, the use of power to adjust height has been a factor in a number of approach accidents and is not recommended. This is because that using power to adjust height takes much longer to see the height change than it will when using the elevator - given that the elevator is there to adjust pitch attitude the response is almost immediate - but as the speed slows then more power is required.

From my own experience flying full-sze for 30 odd years from light prop, through jets to helicopters, they all use the same technique. During my own flying instructional experience of 15 years ranging from ab-initio pupils to experimental test pilots we use the elevator (pitch) to control approach angle technique is used as it is a safer method.

For interest, if we consider an aircraft in its approach configuration (i.e. stable approach angle to the runway touchdown point and lined up on the centreline) maintaining the angle with elevator means that the pilot only has to alter the approach attitude into the landing attitude by a gentle flare then close the throttle to get touchdown - much less to do on the elevator.

Invariably the two controls (pitch and power) are intrinsically linked on the final approach.

Rob

Peter Christy19/07/2019 16:11:36
1487 forum posts

That probably explains the difference between what I was taught 50 years ago, and the air experience flight I had nearly two years ago!

Flying Piper Colts, I was taught to shut the throttle fully and do a glide approach, only opening the throttle if I was undershooting. The theory was, that if the engine stopped, you could probably still make the field. This was particularly important on the Cessna we used for spin training (yes, we had to do that back then, not just incipient spins!). The Cessna had a habit of stopping on the approach quite regularly, to the consternation of all concerned. Never did it to me, but I didn't fly it much! (Didn't like it much, either!)

My air experience ride was also in a Cessna, and I got quite worried when asked to land it using a shallow approach with about 1/3 throttle on. It felt very unnatural! It was also a bit disconcerting, because the approach to the runway was over a steep hill, rising to the end of the runway. If the donkey had quit, we would have had little option but to fly into the hillside!

I would have been much more comfortable starting higher, chopping the power and gliding all the way in! Given the turbulence coming off the hill, it would probably have been smoother, too!

--

Pete

Martin Harris19/07/2019 16:18:30
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I would have difficulty arguing with your credentials and experience Rob, and I certainly agree with the last statement. To be fair, we have been discussing the simplistic effects really and the two factors will always interact.

From a quick Google, it does seem that this subject is an ongoing discussion with differing views from experienced full sized pilots but I do have to challenge your assertion that glidepath corrections are slow - certainly not the case with typical propeller driven models (which after all is what concerns the majority here with the exception of relatively slowly responding model gas turbines). I'm always struck by the direct connection and response of my models to small power adjustments on approach during properly configured powered approaches.

I will readily accept that a high wing loading/momentum full sized aircraft such as an airliner or military jet would respond much slower and it may well be that pitch for aiming point is the better option there.

I could certainly quote from CAA approved literature from the 70s that supports adjusting approach paths with power and my own opinion is that this is still relevant for the majority of "our" models. However, the seeming majority of pilots trained on lightweight foam models and taught to use glide approaches may disagree - at least until they move on to a higher wing loaded warbird and wonder why all the runway is behind them!

Edited By Martin Harris on 19/07/2019 16:19:36

Rob Ashley19/07/2019 16:19:15
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Hi Peter - that's very interesting and a good indicator of how we viewed engine reliability 50 years ago to the way we do now..... I think you are right and probably does explain the difference. It might also be where the cut and glide method transposed to model flying too.

I guess it's like most things - if it works is it wrong?

Rob Ashley19/07/2019 16:43:51
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However, the seeming majority of pilots trained on lightweight foam models and taught to use glide approaches may disagree - at least until they move on to a higher wing loaded warbird and wonder why all the runway is behind them!

Edited By Martin Harris on 19/07/2019 16:19:3

Well said Martin - I guess the argument is then do you try to use the same techniques for all models or do you use 1 method for one (e.g. foamy sport) and a different technique for another (e.g. heavier warbird). For me in the interests of primacy (when things go wrong and panic sets in you revert to the first thing you are taught) I teach the one method..

There are many documents around that have arguements for both methods, although it is the accident rate that drove the recommendations to the method we now teach. Sadly most things in aviation take an accident to change things especially documents or techniques. I often quote to my compatriots/pupils that "the day you stop learning about flying is the day you should stop". Techniques, ideas and understanding evolve all the time especially in the flight test world, howeever it is your students who often 'teach' you a great deal about flying, especially when they get something wrong....laugh

I also agree with you that models react to power changes quicker than full-size due to inertia and as you say with foamies this isnt a realy problem but with higher wing loadings it really is...

Martin Harris19/07/2019 17:02:30
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Quite a thought provoking discussion!

My feeling is that the one method which works for all models is the powered approach and at our field, it is sometimes the only way to get a large model over the fence with sufficient margin for a safe roll out. It's for that reason why I try to teach powered approaches rather than cut and glide - although many will of course end up as such with pupil's natural fear of the ground keeping them too high!

I would have thought that in full size, teaching the powered approach with throttle primarily used to adjust approach path while under supervision of a competent instructor would engender good habits without causing accidents but my instructing experience full size is limited to gliders where the airbrakes were used to control the glide path and speed control was maintained with elevator so perhaps that's why my preference is so ingrained!

 

Edited By Martin Harris on 19/07/2019 17:04:26

Rob Ashley19/07/2019 17:16:34
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243 forum posts
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Certainly could be Martin... I still lead into a banked turn with rudder/yaw pedal due my flying career starting in gliders...

Best

Rob

Peter Christy19/07/2019 18:40:08
1487 forum posts

Rob: I'm not sure the engine reliability has changed that much in the last 50 years! The engines seem to be exactly the same as they were back then!

Our Colts had Lycomings up front. A bit agricultural, but they kept going! The Cessna had a RR Continental - much smoother but prone to unexplained stoppages, nearly always on the approach!

I'm sure it was just that one aircraft that had the issue! If it had been a common fault, someone would have known how ti fix it! As it was, I moved away from the area before they got to the bottom of it - if they ever did!

I still have fond memories of the Colts, though! Just like a full-sized Super 60......! laugh

--

Pete

Peter Jenkins19/07/2019 18:45:01
1217 forum posts
132 photos

Hi Rob and Martin

Well, I was taught elevator for speed and throttle for altitude on full size. Worked well for me on Cessna 150, 172 and Piper Cub. I also flew and instructed on gliders and again the airbrakes were always the go to for adjusting the aiming point with the elevator used for speed control. Clearly, as you closed the airbrakes the speed went up unless you applied aft stick. Having sat next to the pilot in a Hunter on approach it also looked like, once established on approach, height control was from the throttle. Pilots were taught not to allow too high a nose angle on landing as the tail bumper arrived first and put the aircraft into Cat 3 damage territory!

I suspect that the issue of where the aircraft sits on the drag curve on approach also has a bearing on the way you handle airspeed and altitude adjustment. I know that for delta wings it is possible to get into a position where even full power will still not accelerate the aircraft and you must lower the AoA to get your speed up since deltas do not exhibit the normal behaviour of straight wings. Indeed, I once had a delta wing at around 80 deg in a smoke tunnel with both vortices functioning perfectly. The slightest yaw destroyed the vortex on the less swept leading edge. The power needed to maintain an 80 deg AoA would have been phenomenal and the pilot would have just been looking at blue sky!

I have always used the elevator controls speed, throttle controls height on approach with models. My experience has been that you get into a lot of porpoising with the opposite technique and lots of bent aircraft.

I have zero experience of flying heavy aircraft and so Rob your experience speaks for itself. However, I watch lots of pilots flying F3A (competition aerobatics) and by listening to the motor usage on approach, they all use the technique that I have mentioned above. F3A undercarriages are not the most robust and yet you hardly ever see any landing accidents or incidents with these 2 mtr aerobatic birds. Landing in high cross winds, i.e. 20 kt 90 deg cross wind, is another story however!

Perhaps this is an issue that needs to be addressed when flying high wing loaded aircraft that are operating on the wrong side of the drag curve.

Peter

Levanter19/07/2019 20:16:04
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878 forum posts
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I do a lot of flying for work but as a passenger. I like to observe things and this is what I find.

At the end of the cruise all the aircraft pitch down and reduce thrust to keep within Vne. The pitch down is clearly a function of the elevators.

Most aircraft maintain a pitch down for most of the descent with the engines on idle. Air brakes extended if the pitch down is steep to keep within Vne

At the end of the descent the slats and flaps are extended (amount depending) and the aircraft pitches up and engine thrust increased as the aircraft goes on the back of the drag curve.

Gear down and sometimes a bit more thrust.

I have not experienced / noticed significant changes in pitch angle at the late stages of flight except on the Canadair Regional Jets and Turboprops. I do regularly experience big changes in thrust setting and the sink rate is instantly reduced (lift quadruples with doubling of speed) without changes in trim. The aircraft keeps the trim into ground effect and dropping the thrust reduces the flare until touchdown.

In the Canadair series the aircraft seem to fly right down to the deck at a constant pitch down angle only to pull into a flare just before touchdown (hopefully). Same with the tuboprops which have massive prop breaking.

Last week I was on a 20 minute flight from Mallorca to Menorca and the pilot made a very steep approach only settling into a nose up attitude at low height. It was quite disconcerting.

Different aircraft are clearly flown on different flight profiles but I did find Rob Ashley's explanations surprising and turned my understanding completely on its head.

Levanter

Peter Jenkins19/07/2019 23:57:14
1217 forum posts
132 photos

Hi Levanter

I think you'll find that the pitch down is due to the thrust being pulled back to idle which is what you want. Once the required rate of descent is reached that's when the elevator is used to nail the descent speed which will not be Vne - velocity never to be exceeded. That is the edge of the flight envelope and airlines don't normally like to have their aircraft at that speed since the slightest turbulence will most likely push you into an overspeed or overstress situation. The acronym is PAT, power, attitude and trim, in order, to move from any trimmed condition to another.

If an automatic approach is flown it is always much smoother than a manually flown one where the auto throttle is disengaged as well. We looked at engine usage in auto throttle and manual throttle cases and found that there were many more throttle movements and larger ones at that, when the aircrew were flying the aircraft fully in manual. This is a function of human time constant not being able to keep up with the machine! That's why you will find that most airlines encourage the use of the autopilot to fly the aircraft down to almost touchdown since the amount of airframe and engine life used is minimised.

terry mckenna 120/07/2019 22:07:22
3 forum posts

Any Trimmed condition? PAT is the method for levelling out from a decent Peter but it's certainly APT levelling out from a climb.

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