Tractor vs. Pusher Stability Question

[Dave Hederich]

In a discussion of low-speed, high-alpha flying by military jet fighters at airshows, the argument was put forward that it's a lot easier to do this with a tractor prop aircraft because the prop is up front and the aircraft is hanging on the prop like a helicopter, whereas with a jet you are trying to balance it on its tail with thrust from the rear.

Aside from the obvious advantage of prop wash over the control surfaces, are there any technical reasons why in a high-alpha pass there would be a more inherent stability making it less difficult to balance a tractor prop aircraft with the prop in the nose than a jet with the engine(s) in the rear of the fuselage (or a pusher with prop at the rear), or does it really not matter at which end of the aircraft the thrust is being generated?

[Ollie]

With the thrust applied ahead of the CG, as in a tractor, any misalighment of the thrust vector with the resultant of the weight plus drag vectors is self correcting and inherently stable.


With the thrust applied behind the CG any misalignment of the thrust with the resultant of the weight plus drag vectors leads to yaw and as the yaw angle increases the moment causing the yaw also increases. This is inherently unstable. If the aerodynamic stability exceeds the thrust instability there can be net stability with a pusher. To achirve the same net stability the static margin (CG farther forward) of the configuration or the active stabilization by gyro or pilot input has to be greater than in the tractor case.

[steve lewin]

A bit like it's a lot easier pulling a piece of string in a straight line than pushing it ;) ?

Steve
(sorry, couldn't resist :()

[Dave Hederich]

Ollie, that is what I was thinking -- in a much less technical way than you describe it. Someone was arguing that because the engine and airframe are rigidly locked together that it didn't matter at which end the thrust was being applied. The fellow advancing the argument did so in many words, most of them highly technical. But it just did not make as much sense as your explanation. Thanks.

[Ollie]

If the center of drag is not on the thrust line then a moment producing couple is formed between the drag vector and the thrust vector. The location of the center of drag changes with pitch attitude because the induced component of drag changes with pitch attitude. Therefore, the thrust vector and drag vector can only be aligned at one pitch attitude.

[Peter D Rieden]

<In a discussion of low-speed, high-alpha flying by military jet fighters at airshows, the argument was put forward that it's a lot easier to do this with a tractor prop aircraft because the prop is up front and the aircraft is hanging on the prop like a helicopter, whereas with a jet you are trying to balance it on its tail with thrust from the rear.>

Actually the situation is the exact opposite! The thrust "output" site isn't really relevent; it's the thrust *intake" point that's the issue, and in this respect a tractor prop (or jet intake mounted well forward) is destabilising and a pusher prop (or aft-mounted jet intake) is stabilising. This happens because the engine (prop or jet) doesn't just accelerate air backwards (aka "thrust") but it usually bends it upwards, downwards and/or sideways as well in any flight condition where the thrust line isn't exactly parallel to the aircraft's "flightpath" (aka "velocoity vector").

It's that chap Newton who's at the root of it - when he said that every action has an equal and opposite reaction. In the examples show in the attached diagram the air is bent downwards so the aeroplane experiences an equal and opposite upward force acting at the centre of the prop disk (or the intake guide vanes/ducting of a jet engine). If this is at the front then a nose-up attitude causes an upward force on the nose, which is destabilising, whereas with the prop on the back a nose-up attitude causes an upward force on the tail, which is stabilising.

Note that this applies just as much in yaw as in pitch, and it can be a major contributor to taildragger "swing" problems during cross-wing take-offs.

Note that in the case of a jet it doesn't matter where the engine is or where the exhaust is; it's where the intake is that matters - to be precise it's the point in the intake system that pulls the air around a corner. Thus even if the engine and exhaust are at the back this effect can be observed if the intake is right at the front. It is one of the reasons why nose-intakes on jet fighters went out of fashion.

Now the only jets which simultaneously fly at very high power settings AND low airspeeds (where control effectiveness us smaller and aerodynamic stability is minimal) are VSTOL ones. If you read up on the history of the Harrier programme you'll come across a problem they called "intake inertia drag" which made the aircraft very sensitive to side-slips and crosswinds in the hover. Intake inertia drag is this exact problem - the effect of bending the intake flow around. A certain US company whose recent VSTOL fighter contender had the air intake mounted well forwards elected to ignore warnings from a certain British company (which had experience in this field) that this would lead to handling problems at low speeds and during hovering & transitions. When the demonstrator was flown the US company professed to being surprised that said aircraft's hover handling "would need significant development work to optimise". Go figure!

<Aside from the obvious advantage of prop wash over the control surfaces, are there any technical reasons why in a high-alpha pass there would be a more inherent stability making it less difficult to balance a tractor prop aircraft with the prop in the nose than a jet with the engine(s) in the rear of the fuselage (or a pusher with prop at the rear), or does it really not matter at which end of the aircraft the thrust is being generated?>

It really doesn't matter - the factors governing handling during high-alpha manoeuvers are essentially the same for both.

PDR

[vintage1]

I think that apart from the fact that a tractor flows the prop stream over the flying surfaces, it makes no difference as long as the thrustlines are in exactly the same in the two cases.

The chap talking about a rigid airframe is precisely right.

With a rigd body, it doesn't matter where the force is applied, what matters is the line along which its applied.

I.e. force and thrust is a vector. Not a point.

[Ollie]

Peter & Vintage 1,

You are right and I was wrong.

I'm sorry for posting my misinformed opinion on this subject.

[Dave Hederich]

It figures that the explanation that made more sense to me would be the incorrect one. :confused:

So the following statement would be incorrect:

It's a lot easier for a prop aircraft to fly high alpha without a computer because the aircraft is hanging on the prop like a helicopter. Balancing a jet on its tail is a whole different ballgame.

[Peter D Rieden]

<So the following statement would be incorrect:

It's a lot easier for a prop aircraft to fly high alpha without a computer because the aircraft is hanging on the prop like a helicopter. Balancing a jet on its tail is a whole different ballgame.>

Correct - the statement is balderdash, although not necessarily for the reasons discussed in these posts! Remember that these turbine guys have mortgaged their houses to fund the toys and they have to try to justify their extravagance somehow - claiming some macho "you guys have it easy but we're the REAL pilots" would be part of this process.

In this context "balancing on a jet" is often likened to trying to balance a broomhandle on your finger. The analogy is false because whilst your finger only pushes upwards (regardless of which way the broom moves) the force from a jet is firmly nailed to the airframe - so if the airframe tilts the force tilts with it (use the force, Luke!). In the case of the jet ite depends where the intakes are rather than the position of the ouitlet, but to explain that one full would need a digression into a few pages of maths.

The result with the jet isn't necessarily stable, but it's no less stable than a prop at the front. The mechanism I described earlier gives a degree of static stability provided the intakes are behind the CG, otherwise from a thrust perspective the two are equally stable and it's down to detailed design.

PDR

[Dave Hederich]

Originally posted by Peter D Rieden
otherwise from a thrust perspective the two are equally stable and it's down to detailed design.

From the detailed design point of view it sounds as if it would it be fair to assume that a tractor prop aircraft that has been properly designed from the ground up specifically to perform high-alpha maneuvers in airshows would probably be easier to control in that regime than a military jet fighter without fly-by-wire controls that was designed primarily for high-speed air-to-air combat.

[Peter D Rieden]

<From the detailed design point of view it sounds as if it would it be fair to assume that a tractor prop aircraft that has been properly designed from the ground up specifically to perform high-alpha maneuvers in airshows would probably be easier to control in that regime than a military jet fighter without fly-by-wire controls that was designed primarily for high-speed air-to-air combat.>

Well yes and no!

If you're talking about *models* of military fighters then the "design features" are irrelevent - even if they've been modelled accurately they wouldn't translate to model sizes and speeds anyway.

If you're talking about the full size then the high-alpha handling IS actually a design objective. There are two fundamental reasons for this:

1. The high-alpha handling determines the stability & control characteristics on the landing approach, and military pilots (being essentially lazy types who don't like to work too hard) insist on aeroplanes which at least try not to tent-peg themselves on the approach. Personally I don't understand this, because I would have thought the ensuing accident investigation, funeral etc would need less effort if the smoking holes were conveniently sited close to the airfield, but apparently they view it differently.

2. In the case of air-combat platforms (a relatively small proportion of fast jets) and close-air-support platforms (which indulge in tight manoeuvering close to the ground) the high alpha handling determines how hard they can pull without worrying about flicking out ("departing from controlled flight" is the official term). The holy grails of designing such aircraft are "carefreee handling", "Departure resistance" and "manoeuver beyond the stall". "Carefree handling" gives you the ability to pull right into, or even through, the buffet without any unexpected "nasties". "Departure resistance" is an autostabiliser function which limits control deflection when close to a departure point, and "manoeuver beyond the stall" gives the aircraft the ability to retain control over attitude and heading even when it is developing insufficient lift to remain floating in the air in precisely the same way bricks don't. Design features for the latter are always subtle - minor variations in airfoil camber, the odd vortex generator or six, outward or inward leanings on the fins etc etc.

Thinking about it, it occurs to me that few modellers actually realise how much subtlety and detailed engineering goes into apparently simple wing designs. For example I have a 3,000 word summary of a lecture that was given on the wing design of the F104 Starfighter.Most modellers just assume that this is a small slab of unswept wing but, as the extract shows, it waws the result of thousands of man-hours of highly sophisticated design effort. If you're interested, and it doesn't break any rules, I'll post it here (it's a 17k text file).

PDR

[Ollie]

One tool for single taper, swept back wing design, available to scale modelers, which avoids one or two of the pitfalls Peter mentions is the Culver twist formula. See:
http://www.b2streamlines.com/Culver.html

This relatively simple approach to wing design has limitations compared to the more powerful tools used by professionals.

[Sparky Paul]

Irv Culver worked as an aerodynamicist at the Skunk Works.

[Peter D Rieden]

<Irv Culver worked as an aerodynamicist at the Skunk Works.>

That's supposed to be a recommendation??

The Skunk Works produced the F-117!!!

:D

PDR

[feathermerchant]

All this explains why my Wattage F-22 flys so well at high alpha. It's really stable. In fact it wants to stay right side up and never yaws which is good because it has no rudder function. I can fly at walking speed then ram it to WOT and it just accelerates smoothly upward.