Nose Attitudes

[ryan_vd]

Hey,

Maybe someone can enlighten me to why propeller driven aircraft whether turbo or piston driven have a nose down attitude when on approach to a rwy while jets generally have a nose up attitude as if the aircraft was literally falling out of the sky...

Thx Ryan

[FlyingForFun]

I don't fly jets, so I'll wait to be corrected by one of the jet-drivers out there, but here's my understanding.

There are at least two or three reasons for this. The first is the number of engines. In a single-engined aircraft (doesn't matter whether it's a piston-engine, turbo-prop or jet) you would, in the ideal world, like to approach quite steeply, with almost no power. That way, if the engine fails, you will continue towards the runway threshold and you can land safely. In a multi-engined aircraft, this isn't so important, since you can always add power on the remaining engine(s) and either land safely or go around.

The next reason is also to do with go-arounds. If a jet pilot were to approach with no power, and decided to go around, it would take a considerable amount of time for the engines to respond, which is not acceptable. A piston engine, by contrast, responds much faster. Therefore, jet pilots prefer to approach with lots of power. This means a) they will make shallower approaches than piston engine pilots, and use the power to keep the rate of descent low, and b) they will approach with quite a high angle of attack, which results in the wings producing lots of drag, and they will then use power to overcome this extra drag. Both of these mean that a go-around can be executed almost immediately, because the engines are already producing lots of power.

Finally, the piston engined aircraft you've seen have probably been conducting visual approaches, where the pilot is free to approach at whatever angle he likes. The jet approaches have probably been instrument approaches, where they will follow the instruments to the runway. The instruments will be set up in such a way that any aircraft can use them, but they are usually set to about 3 degrees, which happens to be the type of gradient which jets like. If you watch a piston-engined aircraft doing an instrument approach, you would probably see the nose much closer to where you'd normally see a jet's nose, because it will be following the same 3-degree glidepath.

Hope that helps. I also hope it's (at least mostly) accurate!

FFF
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[scroggs]

FFF is thinking along the right lines, but the reasons are more complex than he suggests. Given a standard 3-degree glidepath (which, incidentally, is or should be taught as standard on all types of aircraft), most prop-driven aircraft will have a lower body angle, or attitude, than most jet aircraft, as Ryan has noticed.

Without getting too complicated, the jet aircraft generally have swept wings, which are optimised for high-speed, high-altitude flight, and the propellor aircraft generally have straight wings, optimised for lower speeds and levels. The jet aircraft's swept wing cannot fly slowly in its clean state, so it requires a large number and area of high-lift devices to reduce the stalling speed to a figure which allows an approach to be made at a speed which gives a reasonable chance of stopping on the average runway. Without these devices, the aircraft would need to approach at speeds in excess of 200kts, and yet would still have a very high angle of attack - giving a much higher body angle than you normally see. The reduced stalling speed allowed by the high-lift devices gives an approach speed of nearer 150 kts, and their effects are tweaked at the design phase to give a body angle which allows a reasonable forward view for the pilots.

The straight-winged propellor aircraft is much closer to its optimum flight regime than the jet when on the approach, and is thus further away from the stall, allowing a lower angle of attack and consequently lower body angle. To fit in with the surrounding traffic, most prop aircraft will fly their approach (at least at major airports) at a higher than normal speed, reducing the angle of attack and body angle still further. As an example, the C130 that I used to fly would be quite happy, at normal weights, with an approach speed of 125kts. At this speed, with full flap, the body angle was fairly neutral. At 150kts and 50% flap, fitting in with jet traffic, the nose would be well below the horizon. The A340 that I now fly, at 160kts and full flap, has an attitude of around 5-7 degrees above the horizon.

The point that FFF makes about power is quite right. Jet engines are somewhat slow to spool up in the event of a go-around, so the drag from all the sticky-out bits on the wing is designed to be enough to require the engines to be producing significant power in a stabilised approach. Obviously, we don't want so much drag as to make a go-round too difficult - especially if we can't retract the gear and flaps/slats, or to make it impossible to regain speed if we get a bit below the ideal, but having the engines significantly above idle power helps a lot when lots of power is needed in a hurry. However, no jet will respond as quickly as a constant-speeding turboprop driving a constant-speed propellor!

I've deliberately not gone into the complexities of swept wing aerodynamics and the explanations of angle of attack for swept wing versus straight winged aircraft, so forgive me if this explanation is too broad-brush. I hope it's clear enough to help!

[bookworm]

I agree with BIK_116.80 but it took me a minute to get there, so may I expand a little?

A typical wing section has a lift coefficient vs AOA curve that rises to a maximum at stall and then falls.

Flaps effectively increase the camber of the wing section, and this shift the curve not only up but also to the left. The maximum lift coefficient tends to occur at a lower AOA than an unflapped wing section.

LEDs tend to delay the onset of the stall and extend the rise in the curve to a higher maximum lift coefficient, but don't much shift the part of the curve below that. Thus the maximum lift coefficient tends to occur at a higher AOA than the wing section without the LED.

Abbott and von Doenhoff gives some exemplary numbers for a Clark Y wing section. The plain aerofoil has a max lift coefficient of 1.29 at 15 degrees, the flapped has a max of 1.95 at 12 degrees, and the slotted has 1.77 at 24 degrees. The combination gets 2.18 at 19 degrees.

Thus aircraft that achieve their high-lift low-speed configurations with LEDs tend to have much higher nose-attitudes in that configuration.

[ryan_vd]

Thx guys, you all have more than satisfactorially answered my question!! its great...Ryan

[paulo]

I'm not sure leading edge devices are a primary factor - the ultimate nose up approach is Concorde - no LEDs as far as I know.

As a interested amatuer, I'll take Scroggs answer.

[bookworm]

I'm not sure leading edge devices are a primary factor - the ultimate nose up approach is Concorde - no LEDs as far as I know.

Two factors at work there. Concorde has neither LEDs nor trailing-edge flaps. Thus it has to do it all with angle of attack.

Perhaps more significantly, the lift vs AOA relationship of a delta wing has a much shallower slope than a conventional wing but much higher stalling AOA, hence the very high AOAs observed at low speeds.

[shuttlebus]

Concorde's high AoA is necessary to produce vortex lift over the wing.

Briefly, lift is generated by vortices travelling travelling over the wing rather than the the normal flow that one gets :-)

Very simplified answer. I recommend Brian Calvert's book "Flying Concorde" for a fuller "simple" explanation or "Fundamentals of Aerodynamics" by Anderson for a couple of full chapters on Delta wings.

Regards,

Shuttlebus

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The dominant aspect of the subsonic flow over a delta wing are the two vortex patterns that occur in the vicinity of the highly swept leading edges. These vortex patterns are created by the following mechanism. The pressure on the bottom of the surface of the wing at a high angle of attack is higher than the pressure on the top surface. Thus the flow on the surface in the vicinity of the leading edge tries to curl around the leading edge from the bottom to the top. If the leading edge is sharp, the flow will separate along its entire length . This separated flow then curls into a primary vortex which exists above the wing just inboard of the leading edge.The stream surface which has separated at the leading edge loops above the wing and then reattaches along the primary attachment line.

Inboard of the leading edge vortices, the surface streamlines are attached, and flow downstream virtually undisturbed along a series of straight line rays emanating from the vertex of the delta (A bit like a normal wing).

The leading edge vortices are strong and stable. Being a source of high energy, relatively high vorticity flow, the local static pressure in the vicinity of the vortices is small. Hence the surface pressure of the top wing surface is reduced near the leading edge and is higher and reasonably constant over the middle of the wing. The spanwise variation of pressure over the bottom of the wing is essentially constant and higher than the freestream pressure (+Cp) Over the top surface, the spanwise variation in the midsection is essentially constant and lower than the freestream pressure (-Cp) However, near the leading edge the static pressure drops considerably (the values of Cp become more negative).

The leading edge vortices are literally creating a strong "suction" on the top surface near the leading edge. The suction effect enhances the lift; for this reason the lift co-efficient for a delta wing exhibits an increase in Cl for values of alpha at which conventional wings would be stalled. For a 60° delta, the stalling angle of attack is in the region of 35°.

[Lowtimer]

Hardly like to question a moderator, but...
Given a standard 3-degree glidepath (which, incidentally, is or should be taught as standard on all types of aircraft),
Surely not. Within the airline / transport categories, maybe, or when operating out of major airports (though didn't the last generation propliners, Super Connie, DC-7 etc. prefer a flatter 2.5 degree approach to keep the engines running at a decent power level?), but _all_ types? Tiger Moth? Pitts? Super Cub? Yak-52? Any glider?

[DubTrub]

I agree with Lowtimer, I think Scroggs is wrong to wish all aircraft to do a 3-degree slope.

Helicopters would sure find this hard.

Fixed wing singles should do glide approaches as standard.

[paulo]

Fixed wing singles doing glide approaches as standard? Suits me. I'm always too high. I can now think of it as a safety manoeuvre. ;)

[scroggs]

I didn't get into the details of the effects of leading and trailing edge high-lift devices, nor unusual cases like Concorde, because I wanted to keep my answer fairly simple and reasonably generalised; however bookworm's addenda are quite correct.

The 3 degree glidepath is the standard for IFR operations. There are plenty of exceptions, but they are just that - exceptions. VFR circuit training in light aircraft should equip the pilot to operate to those standard procedures except where the aircraft or airfield can't accept them. If you are flying an aircraft that cannot accept a 3 degree glidepath, or you wish to fly some other non-standard glidepath (such as a glide approach), then you should inform ATC and/or the other users of the field you are operating at.

[willbav8r]

Great thread. Thanks all.