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

[ClintonB]

On 12/02/2022 at 11:42 AM, Thruster88 said:

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. 

 

 

There was no discernible sink rate with this slow approach, lots of stol videos I have seen show a high rate of sink to come in over obstacles like trees, high terrain, that they seem to be able to arrest quickly at the flare (maybe with a quick burst of power.

as a kid I watched an ultralight above a row of trees near cressy in Tasmania, not moving forward in the wind, thought it was cool at the time. Later on when I first started flying I realised how potentially dangerous it was.

 

 

[onetrack]

I'm afraid I fail to understand what the bloke with the Super STOL in the video was actually trying to do? If you're going to fly right at stall speed, you'd better be almost on the ground, or at 1000 feet. That rapid wing drop has killed a lot of people when it happens at low altitude.

[Old Koreelah]

He was showing off the slow-speed capabilities of the aircraft, which he is Au agent for.

[RFguy]

Maybe , for a second, he got the aircraft a little out of balance. 

 

I have found in stall work, half a ball out is enough to provoke the wing drop when right on the hairy edge of a power-on stall. 

 

In the 230, if I am just holding it in the mushing zone power-on, concentrating, (it isnt obvious until you look at the VSI falling at like 500 fpm ) and I just for a split second lose a bit of ball co-ordination,  boom ! the wing drops. 

Edited February 14, 2022 by RFguy

[Methusala]

1 hour ago, RFguy said:

and I just for a split second lose a bit of ball co-ordination,  boom ! the wing drops.

Or a gust! Watch out!

[Markdun]

And pick the wing up with rudder.

Do Jabirus have washout so the wing stalls near the wing root first?  That could be why you are mushing  down at 500fpm... maybe half your wing is stalled.

[APenNameAndThatA]

On 12/2/2022 at 10:42 AM, Thruster88 said:

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. 

 

 

I read somewhere where that if you are coming in to land very slowly, you should control direction with long rudder inputs and control roll with brief stabs on the rudder. (That makes sense as an aircraft will have much more inertia in yaw than roll.)(I have no intention of ever needing to try that out.) 

 

Anyway, the pilot was controlling roll with ailerons the whole time and when the wing dropped he tried to pick it up with the ailerons. That makes me suspect that the pilot did not intend to fly so slowly but was distracted because they were at an airshow. 

[kgwilson]

He was attempting to demonstrate the slow flight capabilities of the aircraft to potential customers and fu#@ed it up. Pretty dumb that close to the ground.

[APenNameAndThatA]

3 minutes ago, kgwilson said:

He was attempting to demonstrate the slow flight capabilities of the aircraft to potential customers and fu#@ed it up. Pretty dumb that close to the ground.

Im not saying you’re wrong. But if he was a self-declared low and slow hyperlegend, why wouldn’t he use good technique? 

[Markdun]

I’ve always thought flapping the controls (quick inputs) just adds drag (which can be a thing you want sometimes). Flapping the rudder definitely does add drag on a sail boat.  The secondary effects of the rudder are pretty much immediate in my experience. I’ve flown many a model with just rudder (including aerobatics) or rudder and elevator; and I’ve practiced circuits with just rudder and elevator trim tab in the Cygnet (ie. hands off the stick).  The issue there, for landing particularly is that when you move the elevator trim you get a cyclic pitch response... about 3 ups and downs before it settles down again.... so you would need a very long approach.

[facthunter]

Don't fly on trim. It's there to relieve control loads. Control inputs add drag. Picking up a dropped wing with rudder is overemphasised and less effective when you get too slow. By that time you should be doing something to take the load off the wings and/or raise the speed.. Don't forget staling is an angle of attack issue . As soon as the wing drops the angle of attack on that one goes way up  and the rising one unloads slightly. You are asking the rudder to do a big job at that stage.. One main point of emphasis. Don't try to pick up the wing with Aileron at that stage. Washout on wings, like Cessna's Highwing allows aileron control right up to the stall, but there's a limit to  that. Nev

[skippydiesel]

Lift (Dump flap) as you flair (within .5m of ground or lower) works a treat. Pretty well overcomes ground effect/float. Doesn't work so well with electric (slow actuation) flaps.

[APenNameAndThatA]

2 hours ago, facthunter said:

Don't fly on trim. It's there to relieve control loads. Control inputs add drag. Picking up a dropped wing with rudder is overemphasised and less effective when you get too slow. By that time you should be doing something to take the load off the wings and/or raise the speed.. Don't forget staling is an angle of attack issue . As soon as the wing drops the angle of attack on that one goes way up  and the rising one unloads slightly. You are asking the rudder to do a big job at that stage.. One main point of emphasis. Don't try to pick up the wing with Aileron at that stage. Washout on wings, like Cessna's Highwing allows aileron control right up to the stall, but there's a limit to  that. Nev

That is true, of course. The elevator is more important. 

[APenNameAndThatA]

On 15/2/2022 at 7:55 AM, RFguy said:

Maybe , for a second, he got the aircraft a little out of balance. 

 

I have found in stall work, half a ball out is enough to provoke the wing drop when right on the hairy edge of a power-on stall. 

 

In the 230, if I am just holding it in the mushing zone power-on, concentrating, (it isnt obvious until you look at the VSI falling at like 500 fpm ) and I just for a split second lose a bit of ball co-ordination,  boom ! the wing drops. 

I will have to experiment with that. I'm not usually that precise! 

[facthunter]

Nice to have redundancy, but check out how hard it is to  do one day. Pretend the strip is at 1000 feet for safety.   How an aircraft is rigged can make a lot if difference.  One Auster I flew would roll to the right from a leftwing down, left rudder applied situation.  That's not normal though..Nev

Edited February 16, 2022 by facthunter

[Markdun]

OK, I’ll bite; why not fly on trim?  On my Cygnet it’s a servo tab on the elevator with a Bowden cable control.  And as Nev said, it’s also there for redundancy say for a broken elevator control wire (but I can’t see that happening) or the elevator control horn falling off from too much rust (it is down there in the tail which gets dragged through the creek and mud occasionally and more frequently the wet grass).  As I said I’ve tried to fly a circuit on trim, but It’s bloody hard giving the pitch oscillations.

[facthunter]

It's a lot harder to land with trim only than it is to do the circuit where a few oscillations are only an annoyance.. An Airbus was landed in Iraq with only the engines to control pitch as being under the wing, have a pitch up effect then thrust is increased.. Nev

[Methusala]

Go Airbus!

[Markdun]

Airbus obviously  need more washers on their upper engine mounts for more down thrust..... shouldn’t be big pitch change  with power change.

[facthunter]

Every underslung motor does it to a greater or lesser extent. Pure Physics and unavoidable If the thrust is displaced from the centre of drag vertically. you get a pitch change with power change. The reverse happens with high thrust line sea planes. More  effect when you are slow and more marked when flaps extended. Pitch is the one control you must have  or you can't survive. Nev

[Old Koreelah]

Those of us with an engine out front have it easy: all have to do is follow the prop.

[facthunter]

That's there to keep the pilot cool. When it stops, the Pilot sweats. Nev

[Markdun]

Maybe the pitch forward needed to offset the pitch up from full thrust is built in for the Airbus so in case of EFATO, there’s no rush to push the stick fwd to avoid a stall like you have to do in a Thruster?

Having built several engine out the front aircraft, I know that thrust line adjustments of the engine are usually needed in most aircraft builds to minimise throttle pitch effects, and this is true for low, mid and high wing aircraft.  It all depends on the displacement of thrust from drag vertically.  And you can make small adjustments with washers under the engine mounts (or failing that modify the engine mounting).  It also depends on what the pilot finds acceptable. On my mid-wing Minimax I adjusted ad-nausea to the point where power changed pretty much affected rate of climb/decent but not AoA or speed. The Corby (low wing) tends to pitch up with the throttle over 100kts with its zero downthrust, as does the Cygnet (shoulder wing) despite its (from memory) 3 degree s of downthrust.

[skippydiesel]

On 20/02/2022 at 7:46 PM, Markdun said:

Maybe the pitch forward needed to offset the pitch up from full thrust is built in for the Airbus so in case of EFATO, there’s no rush to push the stick fwd to avoid a stall like you have to do in a Thruster?

Having built several engine out the front aircraft, I know that thrust line adjustments of the engine are usually needed in most aircraft builds to minimise throttle pitch effects, and this is true for low, mid and high wing aircraft.  It all depends on the displacement of thrust from drag vertically.  And you can make small adjustments with washers under the engine mounts (or failing that modify the engine mounting).  It also depends on what the pilot finds acceptable. On my mid-wing Minimax I adjusted ad-nausea to the point where power changed pretty much affected rate of climb/decent but not AoA or speed. The Corby (low wing) tends to pitch up with the throttle over 100kts with its zero downthrust, as does the Cygnet (shoulder wing) despite its (from memory) 3 degree s of downthrust.

Are you suggesting that engine alignment is the primary cause of increasing pitch up with increasing air speed??

[Markdun]

No.... increased air speed, lift force on wing moves fwd etc. But in an aircraft with a 30hp motor, throttle position or power setting has v. little to do with speed, except on the ground.