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Why not 'cut and glide'?

I couldn't come up with a concise explanation

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Jon - Laser Engines09/08/2018 12:49:45
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Posted by Toni Reynaud on 09/08/2018 12:03:56:

Also I for one have a great deal of difficulty primarily controlling speed of descent with the throttle - the lag between throttle management and speed of descent change is quite long. Perhaps I don't fly and practice enough.

A great deal of this can be down to powerplant. I find two stroke engines hopeless as they need to wind up a fair bit to give the required power. I find 4 strokes with their bigger blades give a much better and almost instant response. In the case of my big warbirds i am swinging 20+ inch props so even 100rpm more gives a large increase in thrust.

I find electric to be difficult too if the throttle is linear. A throttle curve can really help with this.

Propeller selection (pitch/dia) is also a big factor.

Martin Harris09/08/2018 13:05:44
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Just thinking out loud but perhaps there is also an element of "blown lift" from propwash being deflected by that part of the wing it is flowing round, contributing to the aircraft being able to fly at a lower airspeed.

Another factor is obstacle clearance - the steeper glidepath available with power contributes to a shorter landing distance - the height of any obstacle (hedge, fence etc.) is multiplied many times in terms of distance before touchdown on a typical glide approach.

I find that managing an approach with throttle is very satisfying and there's a real feeling of connection with the model as it's glide path responds to throttle movements - with the right speed (AoA) set, I find that the throttle response feels almost immediate but perhaps it's due to the type of models I tend to fly?

 

Edited By Martin Harris on 09/08/2018 13:10:43

Simon Chaddock09/08/2018 13:17:52
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If you have the right shaped plane, a ridiculously low wing loading, a belly lander and you time it right, "cut and glide" can take on a new perspective.

Videoed this yesterday but to be fair it took six attempts before I got it exactly right.wink 2

Chris Walby09/08/2018 13:18:29
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Are we missing the point?

Is it just about control. if you have options that makes life easier (available thrust) why not use it to your advantage and reduce the risk of something unpleasant happening.

As mentioned in previous posts if you can stay in control with a good margin then your workload is low and any unplanned variables (gusts etc) can be dealt with easily.

Once on the ragged edge then it won't take much for a long walk with a bin liner to be your only course of action.

My point is that some models have a wide flight envelope and others don't, this means that the latter group require a very precise flight path to achieve a successful outcome.

If you want the ultimate cut and glide TACA flight 110 gets the gold star from me. All safe, no damage to the aircraft and only a laundry bill to pay **LINK**

The Wright Stuff09/08/2018 13:35:25
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I don't think we are missing the point, Chris. I like exploring different ways of being able to explain things. The question is not so much why you would choose a particular technique, but more asking why some models have a wide flight envelope and others don't...

Bob Cotsford09/08/2018 13:57:22
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Possibly blown wing effect comes into it but I think the propwash over the tail surfaces has more bearing on how a landing turns out. One of the issues I've encountered in converting from glow to electric on 40-120 size models is the dramatic loss of thrust you get on low throttle with electric compared to glow which has resulted in me making some pretty heavy landings due to lack of elevator sensitivity at the flair combined with a sudden loss of airspeed. I could cope with this on a glow deadstick where I know I need extra airspeed, but I'm still recalibrating my mind to the poor low end throttle response on electric. I need to follow Jon's lead and add some throttle curves!

Piers Bowlan09/08/2018 14:02:17
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'Why do some models have a wide flight envelope and others don't?' Design:- wing loading, aerofoil used, available engine power, high lift devices fitted, configuration, aspect ratio, intended purpose, to name only a few.

Edited By Piers Bowlan on 09/08/2018 14:04:58

Chris Walby09/08/2018 14:32:12
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TWS, I was not implying you or anyone else had missed the point, just a different view.

Is it the model or the thumbs/fingers on the sticks? I am by no way one of them, but there are pilots out there that pull off outstanding landing on a consistent basis seemingly with anything they fly wink

David Mellor09/08/2018 14:44:56
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Posted by Simon Chaddock on 09/08/2018 13:17:52:

If you have the right shaped plane, a ridiculously low wing loading, a belly lander and you time it right, "cut and glide" can take on a new perspective.

Videoed this yesterday but to be fair it took six attempts before I got it exactly right.wink 2

Nice!

Denis Watkins09/08/2018 15:09:42
3196 forum posts
147 photos
Posted by Bob Cotsford on 09/08/2018 13:57:22:

Possibly blown wing effect comes into it but I think the propwash over the tail surfaces has more bearing on how a landing turns out. One of the issues I've encountered in converting from glow to electric on 40-120 size models is the dramatic loss of thrust you get on low throttle with electric compared to glow which has resulted in me making some pretty heavy landings due to lack of elevator sensitivity at the flair combined with a sudden loss of airspeed. I could cope with this on a glow deadstick where I know I need extra airspeed, but I'm still recalibrating my mind to the poor low end throttle response on electric. I need to follow Jon's lead and add some throttle curves!

Having been caught out with this one Bob, as I have the privilege to test fly and trim most models at my patch, I soon realised that though I/C are usually set with a healthy 2000rpm idle at throttle stick fully down

Their electric counterpart is at zero thrust, at full stick down

I have adapted to this by leaving some stick on as I make an electric arrival

David Mellor09/08/2018 15:10:35
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Posted by Bob Cotsford on 09/08/2018 13:57:22:

Possibly blown wing effect comes into it but I think the propwash over the tail surfaces has more bearing on how a landing turns out.

Bob, I think you're dead right there!!

On a positive note about electric/thrust, I fly some pretty unconventional planes that fly almost entirely on prop wash & thrust, particularly at high angles of attack. They can fly slowly, even fly backwards in a gentle breeze. They can only do that, though, with very large diameter, very low pitch props in order to maintain high current draw at very low (even stationary or reverse) forward speed of the aircraft.

Most observers find it distinctly odd to see a plane landing under high power with little or no forward speed. Its all flown on thrust, with virtually no lift, and the control surfaces therefore function perfectly in the copious prop wash. Having a tiny aspect ratio helps.....

Smaller diameter, higher pitched props are, of course, far more common with conventional electric set ups, but as you've noticed, they just don't draw enough current to generate much thrust (or wash) when the (model) aircraft is slowing conventionally and the throttle stick is eased back.

Dave

Geoff Sleath09/08/2018 15:22:11
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I must be very insensitive because I've never noticed a problem with electric power. My conversions usually have a prop at least as big as the specified glow engine. My Moth has a 13x6.5 prop which is a lot bigger than on the 40 size glow the model was designed for. Moreover I know that when I open the throttle after the 'cut' part of the approach to lose height, which could be 10 or 15 seconds, it will respond perfectly. If you want to keep the prop turning more than the normal windmilling then there's no reason why you shouldn't leave the throttle open a touch.

Geoff

Nigel R09/08/2018 16:01:13
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Good thread gents.
 
For me I find the easiest landings come with a steep approach and enough control to change that angle a bit and get a decent flair, the two big factors for me come down to drag and loading, which sort of goes...
 
Low drag, low loading = power off to try and get the glide as steep as possible, flair with a tickle of throttle, the easiest to land I think
 
Lots of drag and low loading = power on (probably to make headway against wind) and flown down at whatever angle is needed, the flair will be cake with a tiny bit of a blast from the throttle. Foam 3d models, a lightly loaded bipe, a lightly built STOL type, etc. My depron 3d model glides at about 45 degrees because of the drag and cannot flair without power as it simply stops and stalls (although, so light it bounces) I have to physically "fly it at the ground" in any kind of wind.
 
Lots of drag and high loading = power on, no question at all, and still coming down quite steep, with a blast of throttle to get the flair right. Warbird types with good flaps, heavy bipes, that kind of thing, easy enough with practice but not for learners as it can go pear shaped quite easy with the loading being high. My small Bi Fly is like this. Probably a lot like BEB's Kyosho Pitts.
 
Low drag, high loading = crikey.
 
Coincidentally to this thread I just spent the last session doing nothing but shooting touch and goes with my electric trainer which is low(ish) drag (only the U/C really sticking out) and low loading, its a piece of cake to land, and a circuit + touch and go can all be done in the confines of the field (60m square?); it can just about be held at a high AoA balanced on the throttle and elevator (almost like a 3d model in a harrier) - it is one of the easiest models I have ever had to pull off a good landing.
 
OTOH I have retired (for now) my heavy (well, not especially heavy by old style glow model standards), slick 2 stroke glow aerobat, until I can fit flaps, as our field is too simply small for its glide path (unless I am on right down at near stall speed and then I have no directional control options).
Paul Marsh09/08/2018 17:37:29
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Posted by Peter Miller on 09/08/2018 11:44:37:

There was a similar event over thye Atlantic when an airliner coming across from North America glided for hundreds of miles to land on one of the islands in the Atlantic.

I watced the program may years ago but cannot remember the details.

I know the problem was caused by thye confusion between pound, and gallons.

I must see if I can google that one

Air Transat Flight 236 think, landed on the Azores.

J D 809/08/2018 20:07:50
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In the early day's of flight with the Royal Flying Corps cut and glide was the accepted landing method due mainly to the possible unreliable pickup of the rotary engines.

Any pilot seen motoring in [ rumbling in it was then called ] would have to stand a round of drinks at the mess that night.

Geoff Sleath09/08/2018 22:26:16
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3014 forum posts
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I think the only way of controlling the engines was by cutting the magneto. The carburretors were very crude and didn't incorporate a throttle. The aircraft were also prone to nosing over. Flying was a dangerous business in those days.

Geoff

J D 809/08/2018 23:20:40
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And painful to,that darn engine blip button could deliver the pilot some nasty electric shocks!surprise

Martin Harris10/08/2018 00:20:08
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8015 forum posts
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Posted by Geoff Sleath on 09/08/2018 15:22:11:

I must be very insensitive because I've never noticed a problem with electric power. My conversions usually have a prop at least as big as the specified glow engine. My Moth has a 13x6.5 prop which is a lot bigger than on the 40 size glow the model was designed for. Moreover I know that when I open the throttle after the 'cut' part of the approach to lose height, which could be 10 or 15 seconds, it will respond perfectly. If you want to keep the prop turning more than the normal windmilling then there's no reason why you shouldn't leave the throttle open a touch.

Geoff

More than no reason why you shouldn't leave the throttle open a touch - more a case of why don't we set a "flight idle" on our electric models? I hate to see electric models (especially scale ones) taxi out, stop their props while completing the turn into wind and restart after a second or two for the take off run. Maybe a bit "anal" but I always make the effort to keep the prop turning and some of the comments in this thread are prompting me to doing a little programming along these lines (although in general, my scalish models tend to be IC powered). It will be interesting to see if there is a noticeable difference in fine control of the flare...

Chris Jones 710/08/2018 08:41:01
280 forum posts

Posted by Martin Harris on 10/08/2018 00:20:08:

More than no reason why you shouldn't leave the throttle open a touch - more a case of why don't we set a "flight idle" on our electric models? I hate to see electric models (especially scale ones) taxi out, stop their props while completing the turn into wind and restart after a second or two for the take off run. Maybe a bit "anal" but I always make the effort to keep the prop turning and some of the comments in this thread are prompting me to doing a little programming along these lines (although in general, my scalish models tend to be IC powered). It will be interesting to see if there is a noticeable difference in fine control of the flare...

Interesting - I’ve always set up my electric models so the prop is always spinning at zero throttle and requires the arming switch to be flicked to stop it. I do this even on belly landers simply flicking the switch at 1ft above landing. I seem to remember reading somewhere that a spinning propexhibits a greater braking effect that a stopped prop? Which may be why I fly with power on when landing on most models?

The Wright Stuff10/08/2018 08:53:17
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1362 forum posts
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Intriguing point, Chris. I seem to recall that about spinning props, too, but I didn't see any explanation.

I think it might be the ability to spin (i.e. windmilling - the prop itself is not braked) that gives a braking effect. This is logical, because the prop is taking energy from the air to produce the rotation. I can't see a simple explanation as to why using motor power to provide a spinning prop should have any additional braking effect over windmilling. Presumably, the braking effect is zero when the propeller speed is exactly matched to the forward motion of the air.

Looks like the scientific answer is: it depends...

Edited By The Wright Stuff on 10/08/2018 09:07:20

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