Model vs Trees

[Denis Watkins]

Trees effect full-size too

[Peter Jenkins]

Posted by Sam Longley on 16/07/2019 18:50:43:

Posted by Levanter on 25/01/2019 07:38:08:

Taken to extreme and the windspeed is greater than the airspeed and the aircraft can appear to stand still or even go backwards! But we all know that is "impossible" in aerodynamic terms zo we readily accept the concept of a headwind. . It also explains the common and infamous stall on the downwind turn.

I suppose it would be too much of a thread drift to ask for you to elucidate a bit more on that one

The infamous stall on the downwind leg is caused by pilots thinking that a high downwind ground speed can be reduced by reducing airspeed! The result, is the infamous stall on the downwind leg.

It is quire surprising how many experienced pilots, and even an examiner on one occasion, do not understand the vital difference between air speed and ground speed.

An earlier comment by Simon Cragg falls into that category.

Where things do get confused is when you are descending through a wind gradient (that is wind speed reducing as you approach the ground - exactly like the boundary layer on the wing). In this case, you need to increase your approach speed when landing otherwise as you descend your airspeed will drop away as the speed of the block of air reduces. That is unless you have a very light foamy that immediately reacts to the change in airspeed.

Aerodynamics is not something most model pilots have studied. Where they have it tends to be at a very basic level. Some of us who post on here actually have studied aerodynamics to degree level, have done flight test courses and flew full size power and gliders. It's up to you if you wish to ignore the advice but don't complain when you end up with a broken aeroplane because you ignored the advice.

[J D 8]

When I was learning to fly gliders [ full size ] in the 70's one of the instructors had a very annoying habit. You would be on finals about to turn in to land when if the aircraft felt the bump of a thermal [ coming off a large area tarmac at the edge of the old airfield ] He would snatch the controls catch the lift and up you would go to start again. It was hard to learn anything with him.

One day though he did not catch the lift and had to drag it in over the fence as he had done before. However a new large road sign had been erected on the road around the field. As he passed over the sign about 20 foot up the old T21 Quit and dropped to the ground with a hell bang ripping the tail off on the boundary fence. I was not the pupil on this occasion I am glad to say.

.

[Levanter]

Posted by Peter Jenkins on 16/07/2019 23:53:09:

Posted by Sam Longley on 16/07/2019 18:50:43:

Posted by Levanter on 25/01/2019 07:38:08:

Taken to extreme and the windspeed is greater than the airspeed and the aircraft can appear to stand still or even go backwards! But we all know that is "impossible" in aerodynamic terms zo we readily accept the concept of a headwind. . It also explains the common and infamous stall on the downwind turn.

I suppose it would be too much of a thread drift to ask for you to elucidate a bit more on that one

The infamous stall on the downwind leg is caused by pilots thinking that a high downwind ground speed can be reduced by reducing airspeed! The result, is the infamous stall on the downwind leg.

It is quire surprising how many experienced pilots, and even an examiner on one occasion, do not understand the vital difference between air speed and ground speed.

An earlier comment by Simon Cragg falls into that category.

Where things do get confused is when you are descending through a wind gradient (that is wind speed reducing as you approach the ground - exactly like the boundary layer on the wing). In this case, you need to increase your approach speed when landing otherwise as you descend your airspeed will drop away as the speed of the block of air reduces. That is unless you have a very light foamy that immediately reacts to the change in airspeed.

Aerodynamics is not something most model pilots have studied. Where they have it tends to be at a very basic level. Some of us who post on here actually have studied aerodynamics to degree level, have done flight test courses and flew full size power and gliders. It's up to you if you wish to ignore the advice but don't complain when you end up with a broken aeroplane because you ignored the advice.

Hi Sam

Peter has answered your question for me and almost certainly made a better job of it too.

Thanks Peter

Levanter

[OZ e flyer]

Stall on downwind leg. Grumman Mallard. January 2017.

unfortunately 2 people tragically lost their lives. This happened in front of thousands of people setting up for our “Australia Day” celebrations.

**LINK**

[Martin Harris - Moderator]

Too slow and dropped a wing off the stall - look at the nose high attitude during the turn and the result would have been the same whichever direction he was travelling in. (Technically speed is not the whole story - a wing stalls at a critical angle of attack and this can happen at very high airspeeds, but in normal flight regimes is a good guide in most circumstances). If there was a strong wind it's possible that the proximity of the surface during the low turn may have deceived the pilot by making the apparent speed higher than his airspeed.

I wonder if the pilot realised he was too slow and opened the throttles - the sound heard could have been that of only one engine responding and could have exacerbated the situation. Difficult to assess when the throttles were opened at that range though.

Edited By Martin Harris on 17/07/2019 14:15:06

[Sam Longley]

Posted by Peter Jenkins on 16/07/2019 23:53:09:

Posted by Sam Longley on 16/07/2019 18:50:43:

Posted by Levanter on 25/01/2019 07:38:08:

Taken to extreme and the windspeed is greater than the airspeed and the aircraft can appear to stand still or even go backwards! But we all know that is "impossible" in aerodynamic terms zo we readily accept the concept of a headwind. . It also explains the common and infamous stall on the downwind turn.

I suppose it would be too much of a thread drift to ask for you to elucidate a bit more on that one

The infamous stall on the downwind leg is caused by pilots thinking that a high downwind ground speed can be reduced by reducing airspeed! The result, is the infamous stall on the downwind leg.

It is quire surprising how many experienced pilots, and even an examiner on one occasion, do not understand the vital difference between air speed and ground speed.

An earlier comment by Simon Cragg falls into that category.

Where things do get confused is when you are descending through a wind gradient (that is wind speed reducing as you approach the ground - exactly like the boundary layer on the wing). In this case, you need to increase your approach speed when landing otherwise as you descend your airspeed will drop away as the speed of the block of air reduces. That is unless you have a very light foamy that immediately reacts to the change in airspeed.

Aerodynamics is not something most model pilots have studied. Where they have it tends to be at a very basic level. Some of us who post on here actually have studied aerodynamics to degree level, have done flight test courses and flew full size power and gliders. It's up to you if you wish to ignore the advice but don't complain when you end up with a broken aeroplane because you ignored the advice.

Thanks for posting that & I asked because as a yachtsman (My main hobby so I tend to get out of practice each year) I am aware of speed over ground & speed through the water caused by tidal drift. I expected the same for an RC plane in the wind

In training for my A test the instructor spent a lot of time teaching me throttle control & would get me to reduce throttle on down wind legs. I could never understand why, because if a model needs 30 mph to fly then if the wind was 10 mph it would do 20 mph up wind & need 40 mph down wind. Hence dropping back to 30 mph down wind always seemed wrong.

We had a senior examiner from the BMFA come to the club to talk about work of the BMFA & flying demonstrations were given to show various aspects of testing. He also mentioned reducing throttle on down wind legs. I was too nervous to challenge his comment at the time, as he was a senior examiner & I had not yet passed the A test. (I have now)

However. I have had several stalls & the inevitable spins just as I have turned at the downwind leg (did not happen with trainers, so problem never arose) & you really have shown me that I must try to forget the habit that I got into in my learning & make sure I open the throttle a lot quicker before starting the turn.

Thank you & sorry for the thread drift

 

Edited By Sam Longley on 18/07/2019 09:11:10

[Bob Cotsford]

It's fine to reduce the throttle on the downwind leg, just don't try to maintain the same rate of climb or maintain altitude (depending how much you reduce throttle). When learning to fly it's very easy to get too far downwind and then have to fight to get back to the field with the extra difficulty of flying towards yourself while panic levels are starting to ramp up. Hence reduce throttle to slow downwind progress and give yourself more time to plan your turn at the end of the downwind leg.

[Martin Harris - Moderator]

Personally, I would avoid even the suggestion of throttling back in the circuit unless you wanted to reduce the rate of climb or altitude.

If the wind is strong and you want more time in the downwind leg, extend the into wind portion and correctly offset the headings in any crosswind legs to avoid drift.

Bad habits are easy to learn but persist with getting the basics right and it will reward pilots over many years for their efforts. Correct use of throttle and elevator on the approach is a skill that often goes unlearnt resulting in close calls and model damage time after time.

I'm a little shocked (but not totally surprised) that your senior examiner should have advocated the practice of throttling back downwind - hopefully something did not come across as intended but there are many experienced model flyers who could usefully learn some more basic aerodynamics.

[Peter Jenkins]

I heartily concur with Martin Harris. All I would add is that if you need to commence your descent on the downwind leg then reducing the throttle in order to commence the descent is fine. Many pilots tend to close the throttle completely and just use the elevator until they land. Unless you are high, this is poor practice as the elevator controls speed and throttle controls height. Too high an approach speed will mean the aircraft will float a long way and may still have flying speed at the end of the strip. Too low a flying speed and you run the danger of stalling. Practice the right speed at a safe height - generally speaking a shallow descent with 1/4 throttle is good. If you are in danger of undershooting add sufficient power to flatten the flight path so you can reach tge strip and then return to 1/4 throttle or less. Maintain the attitude of the aircraft with the elevator so speed remains constant. As you begin the round out close the throttle. It is worth spending time on getting landing in the right place at the right speed right as it helps to reduce damage. Always overshoot if things start to go wrong and have another attempt.

[Peter Christy]

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

[Martin Harris - Moderator]

Thanks Peter for correcting my laziness and expanding on my points to perfection! There is little more satisfying in model flying than setting up a scale approach and watching the immediate glideslope corrections resulting from throttle movement. The only elevator movements needed are fine compensatory ones for the throttle changes or more robust inputs for gust effects until the flare commences.

Pete - I suspect your full size experience contributes to your understanding?

[J D 8]

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.

[Peter Christy]

Posted by Martin Harris on 18/07/2019 12:48:41:

Pete - I suspect your full size experience contributes to your understanding?

Well, my full size experience is well out of date! I did go up for an hour long air experience flight at Dunkeswell in 2017, to celebrate the 50th anniversary of my first solo! I was doing quite well until we got on to steep turns, which is when I realised that my stomach isn't as strong as it was 50 years ago!

JD8: The oval descending turn - otherwise known as the "fighter break", favoured by Spitfire and Hurricane pilots during the war as they couldn't see the runway over the engines during a straight approach!

And yes, it can make judging the approach a little easier, as it can be stretched or shortened by adjusting the turn.

--

Pete

[Peter Jenkins]

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.

[Sam Longley]

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 title) 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 8]

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 Jenkins]

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 Miller]

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 Christy]

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 Harris - Moderator]

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 Ashley]

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 Harris - Moderator]

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 Ashley]

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 Christy]

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