Why not 'cut and glide'?

[The Wright Stuff]

I'm sure we should all aspire to improving our landing approaches. Reading this month's mag's reviews, there are many references to landing, where 'this is not a model you can cut and glide', or 'this needs to be flown onto the ground' or 'you need to keep plenty of power on when landing'. My wife, looking over my shoulder, asked why this was.

It's obvious, isn't it? Or is it? The more I thought about it, the less I could come up with an adequate explanation. After all, the whole point of landing is to lose speed and to lose height. Both involve losing energy, so on the face of it, asking why you need to provide energy during the landing phase is not such a silly question.

Or look at it this way, at the moment, the instantaneous moment, that the model is crossing the threshold onto the strip, 6 feet up, all it cares about is (a) having enough airspeed to flare, and (b) appropriate attitude (i.e. wings level, incidence not too high to be close to a stall). It has no 'memory' of how it got to this point.

Put another way, surely any model, whatever the wing load or drag, can be glided to a safe landing - the only variable is the angle of descent required to keep the airspeed constant. I am really talking 'the four fundamental forces' here - i.e. putting considerations like prop wash for surface response to the side for the time being.

Now I totally accept for some models, it may be prohibitively difficult to judge the angle of descent, judge the available airspeed to flare, etc, but is there a fundamental reason why a particular airframe cannot be landed without power on? Or is it more an aesthetic thing?

[Piers Bowlan]

I think you answered your own question there TWS. I think virtually any aircraft, model or full sized can be glided to a landing, power off. However, some aircraft are very draggy, have high wing loadings/stalling speeds. Off a steep approach with a high ROD will require a large round out where the extra drag in the flair will result in a rapid loss of airspeed and further sink (the back side of the drag curve). Still more airspeed will be required to account for this which will make accurately judging the approach more problematical. Far easier to balance thrust with drag on the approach and use the throttle to control the rate of decent while controlling airspeed with the elevator (attitude). A more accurate and controlled landing will ensue. If you want to make an accurate short field landing then you will use maximum flap (if fitted) and lots of power to compensate. Nothing to stop you cutting and gliding but I know which I would do with such a model (or full sized for that matter).

[Max Z]

Posted by The Wright Stuff on 09/08/2018 10:55:06:Put another way, surely any model, whatever the wing load or drag, can be glided to a safe landing

One of the most epic examples in full size aviation being the Gimli Glider

[Peter Miller]

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

[Former Member]

[This posting has been removed]

[Biggles' Elder Brother - Moderator]

I think Piers has pretty well spot on.

If the model has a very light wing loading then it will fly slowly. So the glide approach is fully viable because it can be quite swallow - remember you don't need a lot of speed. In my opinion you still only have partial control here - ie you are trying to control two things (speed and rate of decent) with just one control, the elevator. But because the aircraft is lightly loaded its forgiving and it will let you do that. This is good for beginers because they are not generally good at dealing with more than one control at a time! So we teach them "cut and glide" and this will serve them well, until they start flying aircraft that are much more heavily loaded.

So, moving to the high wing-loading scenarion - warbirds etc. Lift comes from two sources that are under our control: the angle of attack of the wing and the airspeed. Increase either of these and you get more lift. This heavy aeroplane is going need quite a lot of lift for the size of its wings, so either it flies "nose up" (very dangerous and too close to the stall) or you let the nose come down and glide - but you are going to have to glide very fast in a very steep decent to get enough lift to control things at least a little. You will arive over the threshold minus 15 degrees nose down and going like an express train! You will have to get the flair absolutely spot on to save the situation - its an impressive "party piece" if you can pull it off - but its very risky and can cost you a lot of "arrivals"!

What's the solution? Fly it in. Leave the power on. Keep the aircraft basically level (perhaps very slightly nose down) and use the power to control your rate of sink. If you are sinking too fast, open the throttle a little, if you are not decending fast enough close the throttle a bit. Simultaineously use the elevator to control your airspeed, nose up a little puts the brakes on, let the nose fall a little to speed up. You will arrive over the threashold in a much more controlled and dignified way!

Some time ago I had a Kyosho Pitts special - great model, very aerobatic but it was like a brick! Believe me you dreaded dead sticks on that - cut and glide was most certainly not the best way to bring it in. In fact on the odd occassion I had to do it it felt like doing an auto-rotation on a heli, where you use your height to give you speed, then trade it for lift at the last instance! Timing was everything!

BEB

[Biggles' Elder Brother - Moderator]

David types shorter than me and gets in first - but we are basically saying the same thing!

BEB

[Peter Miller]

Found it. Air Transat glided 98 miles to the Azores. Plus circling to line up with runway.

 

**LINK**

This article includexs a long list of airliners that had to glide, many did not make landings on runways

Makes fascinating reading

Edited By Peter Miller on 09/08/2018 12:05:46

[Jon H]

A great deal depends on the drag of the airframe. Many models are very low drag vs full size aircraft and so any sort of power or dive has them accelerating. In this case a powered or steep approach will result in too high an approach speed so that when you flare for landing you are too fast.

Scale models tend to have higher drag and so need more power and/or a steep approach.

My Sea Fury is 23lbs and 80'' span. It also has flaps like barn doors and a 58cc engine in the front. I run in and my approach is quite steep with throttle used up/down to guide the decent and very little elevator input. When it comes to the flare i need that at least 1/3 throttle to prevent the model slowing down too fast due to its high drag configuration.

A friend is just getting into 'heavy metal' warbirds and crashed his mustang by trying a long/flat approach instead. As Piers points out, he was the wrong side of the drag curve (see diagram below) and just got too slow.

With the possible exception of my Pulse 125 i do not own a single aircraft that i can 'cut and glide' to land and i always favour a powered approach of some type. Certainly a glide approach with my Sea Fury would be extremely steep. Probably around 30 degrees!

[Ace]

And flight 236 also ran out of fuel over the Atlantic and successfully glided 65 miles with no hydraulics to a rough but safe landing on a tiny Azores island.

**LINK**

Seems I was beaten to it 

Edited By Ace on 09/08/2018 12:02:05

[Toni Reynaud]

Everyone should have done this at least onece - it's part of the A test as a Deadstick Landing. 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.

[Peter Miller]

It is a well known fact. Throttle controls altitude, elevators control speed.

[Former Member]

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[Piers Bowlan]

Posted by Toni Reynaud on 09/08/2018 12:03:56:

Everyone should have done this at least onece - it's part of the A test as a Deadstick Landing. 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.

I think the problem is exacerbated with models that have a large pitch change with power leading to a less stable approach. If you can set the model up to have a small pitch change with power with some mixing or, dare I say 'engine down thrust' (not wishing to be controversial here!) life will become easier. A nice calm day helps too.

[Former Member]

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[The Wright Stuff]

Well, some food for thought (eggs and bacon) here. The 'cut and glide' is not quite as committed as a dead stick, since there is always the option to open up. Can we call it 'black pudding'?

I'd completely forgotten about the Gimli Glider. Seems the commonality between that one and the Air Transat incident is that the Captain was also an experienced glider pilot.

I'm still not completely convinced by the argument of excessive speed, since (in my mind) the airspeed ought to be exactly the same in both cases - the lift balances the weight if there is no vertical component to acceleration (either in level flight, or falling at a constant speed). If there is an excessive build up of speed, then the descent angle is greater than it needs to be. I suspect the main point of difference is that the round out in the flair is greater, as both Piers and BEB alluded to.

Note I am asking whether it is fundamentally possible, not whether it is a desirable practice... I assume that if a full size 767 (or indeed a Space Shuttle) can land with a cut and glide, then pretty much any model could do the same, if that is, we chose to do so...

Edited By The Wright Stuff on 09/08/2018 12:31:13

[Geoff S]

I've done a lot of landings with my DB 58" Gypsy (Cirrus) Moth because a) I need the practice and b) I find a good touch and go satisfying. Being a biplane it has a lot of drag so I can start the approach quite high and I initially 'cut and glide' until the altitude and speed can be controlled with power and elevator. Sometimes the final approach is very low and flat until I reach the point of actual touch down but definitely 'flying it in ' is the way it usually ends.

Of course 'cut and glide' is possible with any aircraft (as Peter mentions the Gimli glider incident applies) but it doesn't offer many options and needs perfect judgement. Attempting to correct any misjudgements by applying elevator often results in heavy stalls.

Geoff

[Former Member]

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[The Wright Stuff]

Posted by David Mellor on 09/08/2018 12:34:52:

It is clear that the word "cut" is being interpreted in two different ways on this thread. The 3 real life examples (4 if you include the shuttle) are totality cuts - there is no prospect of "opening up" if you adopt that particular meaning of cut!

Indeed David. I was really talking about voluntarily closing the throttle, in order to only have to think about one control at a time.

I tend to think of these things quite mathematically. Peter is correct to state "Throttle controls altitude, elevators control speed" as good practice, but I wouldn't go so far as to call it a fact. The physics suggests they are mixed: both elevator and throttle control both altitude and speed. With cut and glide, there is only one input (elevator) and two outputs (altitude and speed). Fewer degrees of freedom means fewer solutions: i.e. there is one glide path and one glide path only. Two inputs gives an infinite number of glide paths, but requires simultaneous management of two channels, as BEB stated. More options, but more decision-making required.

[The Wright Stuff]

Posted by Geoff Sleath on 09/08/2018 12:32:54:

e with any aircraft (as Peter mentions the Gimli glider incident applies) but it doesn't offer many options and needs perfect judgement. Attempting to correct any misjudgements by applying elevator often results in heavy stalls.

Absolutely, that's the key point here. If a voluntary 'cut and glide' is incorrectly judged, the 'bail out' scenario MUST be adding throttle rather than up elevator...

[Jon H]

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 Harris - Moderator]

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 Chaddock]

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.

[Chris Walby]

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 Stuff]

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...