Flying slow, on the bad side of the L/D curve

[Markdun]

I saw on YouTube a clip about practising flying slow at altitude to build skills for short landings where you land with significant

power with high angle of attack... hanging off the prop so to speak.  It reminded me of a training flight I did in a Blanik as recent solo pilot where the instructor had me land without using the ‘brakes’ but adjusting glider path descent by slowing down below best L/D and minimum sink speed, with a sort of reverse flair a bit above the ground to avoid a hard landing.  Our landing was OK, and it was a good learning experience except for the bit involving the CFI yelling at me afterwards.

I think the article below explains the situation pretty well and why one should be very cautious, no, scared, of flying on the bad side of the lift-drag polar.

 

https://www.aviationsafetymagazine.com/features/behind-the-curve-2/

[facthunter]

It's worth reading and absorbing. In my early days I thought a precautionary approach/landing was done this way but my checkee for the Commercial reckoned it was too slow and not safe. He was probably correct but that was due to a misunderstanding rather than poor flying. Always question something that you may have not got the full  meaning of.  I knew it was not very safe but thought that was what was expected to be done. Carry speed margins appropriate to the situation. Carrier deck approaches seem much the same where there's not enough spare speed to do a proper flare. Once you are near the ground the  "ground effect" helps the situation if you are on the slow side. because it will lower the stall speed but you literally have NO MARGIN for error. Nev

[Markdun]

Yep, but it’s not just ‘error’;  its also chance... that willy willy or an engine hick-up, could deliver the same damaging result.  I know I was thinking in the Blanik those many years ago... i hope there’s no gust because if there is we are going to land heavy. And it was my instructor who got the full wroth of the CFI.  
 

On the opposite of the equation the discussion in the article also clarified why if your approach and flair is too fast why you float, float, float and have to go around, particularly if your wings are long.

[Mike Gearon]

10 hours ago, Markdun said:
 

On the opposite of the equation the discussion in the article also clarified why if your approach and flair is too fast why you float, float, float and have to go around, particularly if your wings are long.

Here’s yesterdays landing. Seems made exactly for topic. My newly purchased Virus SW 121. I fly a tail dragger version in the USA with 15m wingspan. This is tricycle with 10.6m wing span. You’d think this would be way easier and it is unless on my bush runway French Island.  I’m glad my buddy Trev filmed this. In a strong southerly I had it mastered in the Nynja. Don’t fly over the tree line because of turbulence with a 15kn plus southerly like yesterday. Just come in a little faster…say 55kn instead of 50kn and make a final small turn at the 10ft wide smooth part of runway. 
 

Well, that doesn’t look so good when you see the longer Virus wing dipping in right turn. It was about 70kn so a healthy speed  for that part. Not so good for the rest. Long wing didn’t want to drop speed and you can see the air brake fully deployed. It was fully out before literally coming over the fence.
 

Today the northern end gets its fences taken out. I’ll reroute the driveway. I’m laying some nice dam mud in the rough spots to create a new 10ft wide runway some 50ft west for a straight in approach instead of having to turn at the last. I’ll drop 10kn out of approach speed and it’ll be beautiful. Hopefully!

[RFguy]

The classic taught STOL approach (that I practiced dozens of) is certainly not risk free. 

Dragging it in 1.2x stall full flapped, an engine hickup is GOING TO land you short, where-ever that may be.... there just isnt enough energy to do much at all for the situation below 300' in that configuration ...

 

Now, an ag pilot I fly with in his plane from time to time, his technique for short field with obstructed approaches on a short field is very different , and effective. (I have flown in it a couple of times)

1 Come in with good airspeed >= 1.5x  stall probably higher than best glide I suspect.

2 then throttle  idle I think, airspeed decays a bit over the obstructions.....something like 100'-150'

 

* at that point the best glide config he is flying would get him over the obstructions with engine fail *

 

3 and pronounced  nose down once over the obstruction (airspeed increases ) , to get it near the ground..

4 then a strong precision abrupt flare to arrest the high decent rate and plonk onto the ground beautifully.

 

 

Edited February 11, 2022 by RFguy

[Markdun]

Here is a similar technique. On my strip the trees are a lot closer, and in any breeze you need to punch through the tree turbulence (I use 1.5 x stall speed + 0.5 wind speed for my approach), and then you get wind sheer with little breeze below the trees.  In light winds like in this clip where I took the son’s GF for a circuit, I had a bit too much speed despite the slide slip dropping down over the power lines and trees and landing down hill. No fancy smanchy airbrakes. 10m wing span in a plane with about 420kg TOW.

[kgwilson]

Part of my PPL training umpteen years ago was maximum performance landings and takeoffs. The landing is with some  power on, full flap, and the aircraft stalled but hanging off the prop. This was only the final part. The approach was steep with full flap with little power applied but once over the threshold more power applied and up elevator so the stall horn was blaring. I'd be no more than 10-15 feet above the deck by then. Once planted power off, as much up elevator as possible so as not to get airborne and brakes.

 

If I am at a short strip or I want to make a short landing my approach is normal at 65-70 knots, full flap & 2000 rpm with the aiming point the actual threshold at the start of the strip allowing of course for obstacles. On short final I slow to about 50 knots adjusting power & attitude to maintain the glide slope that would ensure touch down at the threshold. Usually there is a fence so I have to miss that and then ground effect takes over. By that stage power is off, and elevator (attitude) up, directional control with rudder, ailerons level unless there is a cross wind & the ASI is reading <40 knots. Speed degrades quickly & I touch down on the mains. ASI reads 30 knots or less but I rarely look at it as I am looking straight ahead. Then brakes & up elevator till at taxi speed.

[Methusala]

Mark, does the optimum L/D correspond to best climb speed as well as best glide?

[facthunter]

Anytime you can do a quick flare you are NOT carrying any margin at that point.  Gliding at idle with little speed margin  on a long final is dumb, particularly with gusty conditions.  You are better to be using power for those days. Leave your STOL Low wing loading thing in the Hangar. Your aim is to be at the right place at the right speed and height. As you get more experience you will Judge that better unless you operate out of a place with a long runway where you have room to float and float. (and you will see plenty of that going on at most aerodromes that are lengthy).

  IF you are of the belief that ELEVATOR controls airspeed,  don't do any formation flying.  Nev

[Thruster88]

32 minutes ago, Methusala said:

Mark, does the optimum L/D correspond to best climb speed as well as best glide?

Usually yes, best climb and best glide will be same or very close. 

[RFguy]

depends which best climb you are talking of I guess.. Vx or Vy

 

best distance glide in a J230 is 65kts (clean configuration) 

 

Vx (best angle of climb)  = 58 kts, Vy (best rate of climb 78 kts) 

Jabiru PoH also recommends 65 kts for TOSS and also final. 

While it might sound like Jab has TOSS and FINAL at the best glide, in TO you are flying TO flap, and FINAL you are flying LANDING flap.

 

I am not sure how the best destance glide changes with TO flaps (10deg) or FULL flaps (30deg).  Have to think about that.

This would introduce  BEST RATE OF SINK or BEST DISTANCE options...

 

Increasing flap should reduce the (max L/D) speed. Eventually you are right up against the stall, sort of the other coffin corner.

 

Comon you glider pilots, I expect you have something good to input here ?

 

 

 

Edited February 12, 2022 by RFguy

[Thruster88]

The jabiru climb speed may also be about keeping the engine cool.

 

Here is an example of how the back of the drag curve slow approach can go wrong. Lot of risk for no reward. 

 

 

[RFguy]

(thinking (not fact) )

Increasing flap should reduce the (max L/D) speed. Eventually you are right up against the stall, sort of the 'other' coffin corner.

 

Have I got that right ?

 

[Markdun]

 Don, it all depends on what you mean by ‘best’.  If you are punching into a headwind and ‘best’ is travelling the most distance into the wind across the ground for a given loss in altitude you  need to go faster than best L/D, & going with the wind you’d want to go at minimum sink rate which is usually a bit slower than best L/D.  With climb there’s a few other factors.  For example you know my Cygnet with the 1835VW would climb best (fpm up) at a bit under 65kts, but it’s best L/D was 57kts. My guess is this is because the engine developed a bit more power and the prop a bit more efficient at the faster speed.  With all the extra horses off the Jab2200 i haven’t really looked as climb is always more than necessary and why cook the engine going slow?&

 And what about the beauty of side slips and air brakes; you stop it or put them away and presto, the plane is in takeoff configuration.... unlike full 40 degree flaps.

 Not sure what went wrong when I inserted the video clip of the Cygnet landing.

 Mark

[facthunter]

Any flap will add lift but at a drag penalty which makes the "new" L/D worse. IF you are driving into a headwind increase airspeed to have a better chance of getting there. Extreme case, the airspeed equals the headwind . You will never get there. . If you have a tailwind, reduce airspeed and you have the tailwind assistance for longer and will save fuel.. IF you surf the lift along a rising slope, you can save fuel or cruise faster. Nev

[Methusala]

Primary interest is catastrophic power out so making it to the best crash site. Interesting point that into wind, higher airspeed to make more distance. Another point in favor of not turning back if insufficient altitude to make it back to the strip. Who wants to land downwind off airport?

[facthunter]

Losing the advantage of the headwind and adding the downwind, the "difference is" double the windspeed, Plus as you aren't used to it, you may easily muck it up some way or other.  The LOW stall speed we have is a real factor here plus any slowing up you manage to achieve makes it more survivable Losing CONTROL of the plane is common if you turn, but very unlikely iF you continue basically ahead and avoid major solid obstacles. Nev

[Markdun]

Don, in NZ (Paraparaumu) gliding a guy bunged off at about 100’  because he took off with his brakes out (he misinterpreted the tugs rudder waggle as a wave off). We lost sight of him behind the tower... he was over some market gardens (not good landing). He did do the 180 degree turn, but wholly shit, despite being low, and landing on the first bit of airport, he was fast with the downwind component. He rocketed through all of us ; caravan & gaggle of gliders) at the downwind end of the grass, crossed the GA bitumen runway and only just pulled up before some hangers.  You have to be pretty desperate to land downwind off field.

[Markdun]

 Forgot to mention..... aide memoir for outlanding:  DLOSSS.  Direction into wind first, then length, then obstructions (fences powerlines gullies), then slope, stock and sun.

 

 But remember if you have that catastrophic money to noise  converter stop at altitude you will cover more ground to get to that good paddock going downwind. 

[Methusala]

Tumut (home airport these days) is pretty good in both directions. Not so with some ( Wambalong ?).

[Markdun]

In the Cygnet, Wombalong is a fiddle either uphill or downhill. The Corby is much more demanding landing downhill..... its short coupled so is very twitchy, and tends (with my unskilled hand) to gain too much speed when you slip down through the trees landing downhill. Have to admit I’ve had to do several go-arounds and once actually used the brakes... and this is a 700m long strip!  Despite the low 30kt stall, 50kt approaches seem awfully slow, particularly in the very thermic conditions we’ve been having recently.

[pmccarthy]

46 minutes ago, Methusala said:

Tumut (home airport these days) is pretty good in both directions. Not so with some ( Wambalong ?).

I can still remember the Tiger Moth aerobatic displays at the opening of the Tumut Aero Club in 1957. Must be a bit of a Methuselah myself!

[Methusala]

1 hour ago, pmccarthy said:

I can still remember the Tiger Moth aerobatic displays at the opening of the Tumut Aero Club in 1957

1957 I was 6 and living in Cooma East.

[Yenn]

The Corby and also the RV4 that I had would seem to float far too far landing downhill. They both seemed to want to keep flying in the flare and the strip is out of sight.

[facthunter]

Any plane that's not a real brick will do that. It's very difficult to predict the touch down point accurately on a downhill runway even if your speed is  right on. . Nev