Swept wing

[Microburst2002]

Hello

I need some true experts to confirm-explain (unless wrong) if an idle landing with a swept wing airplane is dangerous when compared to doing the same with a straight wing one.
I remember an article of a 727 accident doing that and it said something about the swept wing AoA-CL curve, which is flatter and with a lower CLmax. However, all airplanes at 1,3 Vs have the same manoeuvre capacity, irrespective of the kind of wing.
So what is the mistery?
Since wings were swept to permit higher and faster flight, what are the disadvantages they have when flying low and slow?

Thanks

[BarbiesBoyfriend]

mistery????:suspect:

MISTERY!???????????????????????:ooh:;)

[Clandestino]

Every landing I've made in A320 was with engines at idle, incidentally A320 is swept wing jet.
Every landing I've made in Q400 was with engines above idle, incidentally Q400 is straight wing turboprop.

As it seems to be type specific ant generally not related to wing planform, what is your definition of "idle landing" anyway?

all airplanes at 1,3 Vs have the same manoeuvre capacity

Maneuver capacity is not limited to capability to pull 1.69G before stalling. E.G. Lockheed F-104 has a bit bigger 1.3Vs compared to Cessna 150 and therefore somewhat larger turn radius at that speed.

Since wings were swept to permit higher and faster flight, what are the disadvantages they have when flying low and slow?

You've already mentioned primary disadvantage:
lower CLmax

[BarbiesBoyfriend]

Clandestino

You don't land a 104 at idle though, do you!

[kenparry]

There can be significant differences between variants of the same type, even.

For example: the early B737-200 models would touch down quite sweetly, provided you got the speed and flare correct, if the power was reduced to idle during the flare. The later 737-200ADV had slats that extended further for landing, and, as I found to my cost, if you used the same technique it would result in hitting the ground like a bucket of bricks. Keeping the power on until just after touchdown avoided the problem.

Why the difference? Not sure of the precise detail, but certainly related to the slat settings. It's a long time ago, but my recollection is that the ADV aircraft had a Vref that was a few knots slower than the basic one for a given landing weight (mass if you prefer), so induced drag would have been higher, making the aircraft more sensitive to power reduction.

[Microburst2002]

Clandestino

When I mean idle landing I mean idle approach fact, actually. I guess you allways touch down ad idle in the A320 but do not retard the throttles until 20 ft or so, right? That means you have idle just before touchdown.
I am referring to an airplane at idle from, say, 400 ft.
In the case of turboprops (or any props) when you reduce power, you loose the slipstream created lift. This can make the airplane touch down less softly than desired, so sometimes it is better to keep power until touchdown.

Good point about the radius of turn. G capabity being the same, maneouverability (or flight path change capability) is actually less for higher speed airplanes. Since a low CLmax airplane will require a higher Vref speed, it follows (I think) that a swept wing airplane has less manoeuverability, all other factors remaining constant.
thanks

Anyway does someone remember that accident, 727 idle approach and landing? I think it was long time ago... And what about the pros and cons of swept wings?
thanks

[Clandestino]

ATR-42 is turboprop but I have always landed it with power levers at flight idle, otherwise it floated. So once again: it's really type/version specific.

Now I see where it's coming from; less than adept mediaperson presenting continous descent approach as if it were flameout pattern. Don't worry about idle approaches down to idle landings, they're not going to happen. Very little saving would be achieved for steep increase in risk.

[barit1]

There were 3, perhaps four such 727 accidents about 1965-66 IIRC. They were at Cincinnati, Salt Lake City, Tokyo Bay, and one over E. Germany (which their regime would not permit a western investigation). One factor was the slow spoolup of early JT8D's out of gnd. idle.

I looked up the accident reports some time ago, maybe I can find them again.

And the F-104 has been landed a few times dead stick. I knew the guy who did the first one.

[Microburst2002]

thanks a lot!

[bjornhall]

Just so this interesting discussion doesn't stop here...

Two relevant factors I have heard are:

- The flatter AoA-CL curve means a swept wing aircraft needs a larger pitching movement to acheive the same load factor.

- Many (not all!) propeller driven aircraft display a rather significant trim speed reduction with application of power; in plain English, they tend to pitch up when you apply power. A jet with rear mounted engines will usually not do that. Nothing to do with swept vs. straight wings, but almost all props are straight and most jets are swept...

The result would be that applying power and a rather gentle pull would arrest the sink rate of a straight wing prop aircraft, but would only drive a B727 (or a Citation...) faster into the ground.

Another result of the flatter CL vs AoA curve would be that a swept wing jet could develop a high rate of descent with a nose up pitch attitude without being close to a stall, and such a rate of descent could be hard to detect in time if one is only monitoring attitude.

Take these three aspects together: high RoD with nose up pitch, slow spool time of some jet engines, and the need for a more aggressive pull on the yoke, and we have gone some way towards explaining why unspooled approaches can be more dangerous in a swept wing jet...

Now, standing by for someone to explain what I got backwards, and hopefully learn something in the progress... ;)

[john_tullamarine]

they tend to pitch up when you apply power.

Normally more a problem in the landing configuration missed approach ?

Generally due to what is referred to as the normal prop (or nacelle) force and, for some Types, can be a significant long stab destabilising input.

Mainly a concern with larger motors processing lots of air .. while seen often on the piston-to-turboprop conversions where the engine (and, hence, prop) is moved out forward for weight and balance considerations, the underslung turbofan is a contender with the force showing up at the nacelle inlet.

Sometimes the driver for incorporation of an SAS to modify the stick force characteristics.

Another result of the flatter CL vs AoA curve would be that a swept wing jet could develop a high rate of descent with a nose up pitch attitude

.. perhaps the Cd curve is the more relevant consideration ?

[Microburst2002]

That is the theory I have now, if the following question is answered with a yes:
If a larger AoA change is required to obtain a given CL change in a swept wing airplane than in a straight wing one... Does that mean a greater CD change for the swept wing, too?

But I don't recall any curves for CD in swept wings. I still have to research on the accidents Barit1 mentioned and see what I find.

Thanks

[plt_aeroeng]

This thread is really a tech log discussion, not testing. Interesting nevertheless, though.

I think John T didn't emphasize Cd curve enough.

While I didn't fly the 727, my recollection of the events of the time is that the drag could increase very rapidly as speed dropped on the 727. This was partly due to the swept wings, but also due to the very effective (for the time) high lift devices. The drivers had typically come from DC7s, and were not used to such behaviour.

Indeed, I recall hearing that the 727 could fly slow enough that full power (which was spectacular compared to the prop jobs) would not maintain level flight and speed.

Slow engine spool up was also a factor.

Swept wings with low aspect ratios tend to develop high spanwise flow at low speeds, and consequent interesting tip vortices.

Another example would be the F102 delta of the same period. My recollection of some aerodynamic notes circulated by command was a data point that although the F102 could achieve an instantaneous 5g at only 300K, it would then decelerate at 60K per second, producing dramatic results.

With reference to the F104 discussions, that was an aircraft with very high wing loading and short wings, and the gliding characteristic of a brick. Someone pointed out that some very skilled pilots have successfully dead sticked 104s. However, at the OTU the instructors convinced most line pilots not to try it by demonstrating a simulated flameout circling approach. The drill was to ensure arrival overhead at 18000 feet, circle in a rate 1 turn with gear and flap up, and put down the gear at least 3 and not more than 6 seconds back. (Also, on a normal approach middling engine power was required to operate the boundary layer blowing: cut the engine crossing the threshold would result in eye openiing gyrations in the flare.)

By comparison, the T33 could do an flameout circle from overhead at 5500 feet with gear down all the way.

Really, however, the primary reason for powered flares would be the high drag and consequent rapid speed decay at idle. Swept wing a/c are not alone in having these characteristics.

As a final teaser, it would be interesting to speculate whether the new types with large winglets have Cd curves which don't go up as rapidly with speed loss as do their cousins without the winglets.

[Microburst2002]

Yes, it is more a Tech forum thread. I came here looking for the experts
Thank you very much, by the way!