"Attitude plus power equals performance"

[MayorQuimby]

In instrument training, this mantra is drummed into students: If you have a given power and attitude set, you'll get a given performance.

I appreciate this is being somewhat pedantic, but, since it's Friday and I've nothing else to do, I'll plough on: We are also made aware early in our training that a particular power and attitude can result in two airspeeds, depending on 'which side of the drag curve' you're on. Surely this is a contradiction with the first assertion? Shouldn't it be: "Attitude plus power = one of two possible performances"?

Okay, I'll climb back into my hole now.

MQ.

[3 Point]

Mayor,

Only half right. A given power setting will result in two possible stabilised speeds (ie performances), one above Vimd and one below it. The difference however is attitude, the slower speed will require a much higher nose attitude to give a higher angle of attack (aoa) and a greater lift co-eficient (Cl) to produce sufficient lift for level flight while at the higher speed less Cl will be required to produce the lift and this will be achieved at a lower aoa ie a lower nose attitude. The total drag in both cases will be the same (ie equal to the thrust) but in the low speed case it will be made up of a greater proportion of lift induced drag and in the higher speed case there will be more zero lift drag.

So, "Attitude equals either one of two performances"
"Power equals either one of two performances"
BUT, "Attitude plus power equals just one performance"


Now for any real pedant out there I suppose it might be possible with a given power setting to produce a slow speed descending flight path with high nose attitude and high angle of attack and therefore high drag or alternatively to fly a higher speed, level or slightly climbing flight path with the same pitch attitude but less aoa and hence less lift induced drag but a rate of climb (or lower rate of descent) and more zero lift drag to counter the same thrust. That's trick flying at best though and not real world stuff!

How many of us routinely operate on the back side of the drag curve?

3 Point

[mustafagander]

All big jets take off climbs are on the back side of the drag curve, hence improved climb/overspeed.

So, the answer is all big jet pilots.

[FLCH]

Heard a tale of a brand new young First Officer being told by his Captain, to use the yoke to control your speed and power to control your altitude...well next take off the First Officer is furiously yanking the yoke back and forth...the Captain asks: what the hell are you doing?? the F/O replies, I'm trying to get to rotation speed !!

[3 Point]

mustafagander,

Yep, true enough (I'm a big jet pilot (if you call the A320 big!!) so I should have thought of that!). It's probably true to say that most aeroplanes lift off below Vimd however, to be fair, I did say how many of us routinely operate on the back side of the drag curve. Two minutes spent accelerating through it during a six hour sector doesn't count!!

I also seem to recal that in jet aeroplanes best rate of climb is achieved at 1.32 vimd and best angle at Vimd so being below Vimd will do nothing for climb performance. Rotating below Vimd will allow you to leave the ground sooner and so use less runway if you have sufficinent thrust to acellerate against the high drag!

What is Vimd for your big jet in the take off configuration? Is it published anywhere? I know the Vimd for my Airbus clean (Green dot speed) but I don't know what it is with flaps, slats and landing gear hanging in the breeze! Remember, an increase in zero lift drag (eg landing gear) will lower the Vimd!

I read Mayor's question as refering to stady state normal flight manouvres which are taught to a student in IF training; in these circumstances power plus attitude = performance as I said.

FLCH; don't lets get started on the power = glidepath and attitude = speed debate!! I can't stand it!!

3 point

[Capn Bloggs]

For bograt FOs: "Always remember and forever take heed, right hand for glidepath and left hand for speed!".

[Old Smokey]

This thread seems to have drifted into the area of jet handling, whereas I suspect that the original post was related to propeller aircraft. So, in continuation of the current 'Jet' trend, it is true that Attitude plus Power equals performance. The only problem is that jet aircraft engines don't produce power, they directly produce thrust. If we consider the basic formula -

Power = Force (Thrust) X Velocity (Speed),

then it is true for the jet, but as the pilot has no control over the power, he/she must then re-learn the mantra to -

Performance = Attitude + Thrust + Speed.

Now, for a jet, it makes sense. Props are a different matter.

Regards,

Old Smokey

[scrubba]

Smokey,

I suspect that "power" really meant thrust anyway...

[TyroPicard]

Old Smokey...

So what do props produce?

TP

[FLCH]

FLCH; don't lets get started on the power = glidepath and attitude = speed debate!! I can't stand it!!


I humbly apologize sir......

[Old Smokey]

FLCH,

Thanks for your "don't lets get started on the power ...... " comment, so I'll get finished on my comments instead of perpetuating them.

I don't think there's anyone, from a High School physics student through to a NASA aerodynamacist who would argue with the formula -

Power = Force X Velocity, or, in pilot speak -

Power = Thrust X TAS

I've always disliked the "Performance = Attitude + Power" mantra because there's too many flaws in the statement. When I saw the thread, it occurred to me that, at least for a jet aircraft, it was true (at last). Because jet aircraft produce thrust IN ISOLATION to power, Attitude (assuming that Angle of Attack is implied here) + Thrust = Meaningless. If we inject speed into the formula, i.e. AoA + Thrust + Speed = Performance, then, for the jet it DOES make sense, and Thrust and Speed together is power.

I hope, scrubba, that "power" DOESN't really meant thrust anyway, because it's meaningless until we add speed. I'm well aware that many jet pilots continue to use the term "Power" when they really mean "Thrust", so, if they wish to continue to use this mis-noma, OK then, so long as they add the caveat "but make sure that your speed is also appropriate for the performance profile sought" A jet pilot does have power available to his/herself, but must look at 2 sets of instruments to ascertain it, the engine instruments, AND the airspeed.

TyroPicard, props do indeed produce Thrust, as a function of the Engine Power delivered, and an INVERSE function of the forward aircraft speed. The faster that you fly at a constant power setting, the less is the thrust delivered by the propeller. Now we have to consider whether the originator of the Performance = Attitude + Power mantra was referring to engine power, or "Aircraft" power, i.e. Net propeller thrust and forward speed in isolation to the engine power required to produce that thrust. The latter makes sense, the former does not.

Now taking FLCH's advice, I'm outa here.

Regards,

Old Smokey

[Blacksheep]

So what do props produce? As all taildraggers know, slipstream... ;)