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Model vs Trees


Chris Walby
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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

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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?

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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...

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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

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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

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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

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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

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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.

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