Dense People!!!!

[Dubya]

I just got a headache, dave


I agree with you in principle (or i think i do ).... with the exception of this:


CL being the Coefficent of lift?

Why would CL be reduced if you push the nose down with a constant velocity? Assuming no abrupt control inputs, it should stay the same. Remember, The relative wind also changes since your direction of travel changes by pushing the nose down. Thus Lift would still = Weight.

Once weight EXCEEDS Lift, the airplane would stall, wouldn't it? When approaching stall in slow flight, you continouesly increase AoA to obtain enough Lift to counter weight as you slow down. Same in a high speed stall. When you pull to many G's where the Weight factor exceeds the maximum lift produced by the maximum AoA for steady flight, you're wings stall.

Again, as far as i know, Climbs/Descents are due to acceleration in either direction. Either produced by the engine or by gravity. Of course this is for normal flight. You could, afterall, stall a airplane and let it 'drop like a rock' and 'descend' that way



Man.. this is getting confusing.

(please don't smack me to hard if i got myself mixed up )

Okay...but can I smack your avatar's arse instead?

W

 

[Mr. Cole]

Matt,

You have some good points my friend. To answer your first point, if you don't create any abrupt accelerations, a change of pitch will cause the plane to rotate about the lateral axis, but the direction of the relative wind will not change instantly. Remember that a change in the relative wind, which is a vector, requires an acceleration. If you pull a plane up into a high G maneuver by pulling back on the stick, the plane will rotate about the lateral axis, but the high acceleration will also change the relative wind fairly quickly. If the relative winds changes quickly enough the AOA should either remain the same or decrease. In most cases, under normal accelerations, when we pull back on the stick the pitch changes more quickly than the relative wind, so AOA increases.

CL is a function of AOA, and as you also mentioned we frequently control AOA by either pulling back on the stick such as in slow flight, or by pushing forward on the stick. Therefore we also control CL by taking those same actions. You are also correct that weight greater than lift equals a stall in theory. But we usually define a stall as a condition where we exceed the critical AOA and the CL drops rapidly. But this is a drastic case of when weight exceeds lift. You can also have a less dramatic case when you're below the critical AOA and in S&L flight, but something occurs that causes the velocity to drop. At that point the velocity is just slightly less than that required for level flight. There is a net acceleration downward and a descent occurs. But assume for a second the nose didn't drop and the pitch attitude remained the same, the intial drop would cause an increase in AOA and until L = W. If the plane is in a descent it will now do so at a constant rate.

In reality what happens is that the nose drops until speed is regained and lift once again equals weight. Thanks for keeping me honest.


Dave

 

[enigma]

oh one other question...if you're in a constant airspeed decent (sticking to the basic 4 forces, lift, weight, thrust, drag) what causes you to decend? an excess of weight? or a negitive angle of attack? thanks

Jmmmccccutcc,

Someone else probably got this correct, (if so, I apologize. I must have missed it in all of the obscuring physics) but the answer is GRAVITY.

Some of you guys need to step back and check out the forest. It's a pretty cool sight ya know, besides all of the trees tend to look the same.

Seriously, Sticking to your request, of your four choices, weight would be the best answer. Gravity, acting upon the weight of your airplane, pulls the airplane towards the earth.

Now, go ask your CFI buddy what keeps a earth orbit satellite from shooting out into space.



enigma

 

[ventus]

.
Once weight EXCEEDS Lift, the airplane would stall, wouldn't it?
.

If weight is greater than lift, the airplane will descend.

 

[uwochris]

weight exceeds lift

It will not only descend, but it will acclerate downward (IAS will not be constant).

 

[Jmmccutc]

this is true hence the argument that my roomate made...that a nose down pitch is the only factor that causes you to decend...

oh about the dog and the pail of water, it's about 22hours of constant humping to make the water even slightly warmer...

 

[Mr. Cole]

Jmmccutc,

But what if you have a plane that's in mushing flight with a nose level attitude or higher? Does it not descend without having a nose down attitude?

Dave

 

[Lead Sled]

...if you guys got any suggestions on how i might handle this let me know, maybe you've all flown with someone that fits this mold...

Simple solution...

Get a new roommate

Lead Sled

 

[enigma]

Jmmccutc,

But what if you have a plane that's in mushing flight with a nose level attitude or higher? Does it not descend without having a nose down attitude?

Dave

Now you've done it. You seem to have forgotten that were looking at the trees in this discussion.

OK, I've got a question for the CFI. Will an airplane climb from the runway without the engine running? What about if you pull back on the stick? What about if you put a couple of cases of oil on the horizontal stab to weigh it down and make the nose pitch up?

Those ought to make him pull his hair out!


enigma

 

[Ailerongirl]

oh about the dog and the pail of water, it's about 22hours of constant humping to make the water even slightly warmer...

*LMAO*

You kill me!

 

[mattpilot]

Jmmccutc,

But what if you have a plane that's in mushing flight with a nose level attitude or higher? Does it not descend without having a nose down attitude?

Dave

I assume your talking about something like a plane on an ILS with a constant rate of descent, but yet a positive angle as compared to the horizon?


Well - lets analyze that. Assuming the ILS has a standard 3 degree glideslope. Thus, the relative wind comes from 3 degrees below the horizon. Now it will be a constant airspeed descent - right? Say 80 knots in a 172? Lets 'assume' this takes a 8 degree AoA to give us sufficient lift to overcome our weight (there are actual formulas to calculate that, but i'm to lazy to look it up ). That would put the longitudinal axis 5 degrees above the horizon, giving us a nose up attitude - right?

So what makes the plane go down with a nose up appearence? Downward acceleration - or gravity. Because the throttle is pulled back, we can't maintain altitude with the given airspeed. Thus, the airplane will begin a descent - relative wind starts to come from below the horizon - this would increase the AoA so that LIFT = WEIGHT. Remember, gravity provides the additional acceleration required to maintain speed.

As you slow down and approach the AIrcrafts stall speed, you will need to increase your AoA to create more lift to support the weight. The slower you get, the higher your nose will be raised. So if your on a 3 degree ILS flying at Vso, then your AoA will be roughly 18 degrees. Comparing that to the horizon, you would appear to be 15 degrees nose up.

But didn't we say that if the nose points up, the airplane would climb, and if the nose points down, the aircraft would descend? Well yes, but you must also have sufficient power/thrust available. Its not really where the nose points, more the end resultant vector of your accelaration. If you reduce power/thrust lower than the L/D max power required point, the aircraft can no longer sustain level flight, and will thus use gravity to get the acceleration needed so that LIFT = WEIGHT.

I'm sorry if i'm not good at explaining things, but thats because english is my second language.


(edit... hmm i seem to have qouted the wrong guy .. was that rhetorical question dave? i'd figure you know the answer. ah hell its late - good night )

 

[Bryan D]

I suppose it would be a deficit of thrust then?

You must be a f'ing CFI dude! It's true, you can climb and descend with a positive angle of attack by simply adding or removing thrust. CFI's rock!

 

[FN FAL]

You must be a f'ing CFI dude! It's true, you can climb and descend with a positive angle of attack by simply adding or removing thrust. CFI's rock!

Pitch plus power equals performance.

 

[Singlecoil]

Quite a popular thread! I'm going with a deficit of thrust horsepower, not just a deficit of thrust. Sorry to be technical... Rate of climb is a function of excess thrust horsepower, angle of climb is a function of excess thrust. Ergo, rate of descent would be a function of a deficit of thrust horsepower.

Gravity is wrong, especially for those who were discussing acceleration. If you you are being accelerated/deccelerated, you are feeling a g-force, either positive or negative. In this problem, we are dealing with a constant airspeed, constant rate descent. Ergo, no acceleration involved.

 

[mattpilot]

Gravity is wrong, especially for those who were discussing acceleration. If you you are being accelerated/deccelerated, you are feeling a g-force, either positive or negative. In this problem, we are dealing with a constant airspeed, constant rate descent. Ergo, no acceleration involved.

perhaps the word acceleration is wrong, but it is still gravity that is causing the downward FORCE. Potential energy (Mgh) is being converted into kinetic energy (1/2mv^2) which the airplane uses to maintain airspeed.

 

[Singlecoil]

perhaps the word acceleration is wrong, but it is still gravity that is causing the downward FORCE. Potential energy (Mgh) is being converted into kinetic energy (1/2mv^2) which the airplane uses to maintain airspeed.

Yes, but as we have all learned, the four forces are balanced in straight and level flight, a constant speed climb, and a constant speed descent. Therefore, if Gravity and lift are balanced, how can gravity cause the descent?

 

[100LL... Again!]

Could you invite your cocky little pal to come join us here?

We'll wipe the smirk off his face right quick.

Make sure he has his momma's phone number ready.

If I had a buck for every incompetent, ignorant CFI I met, I'd have at least ten dollars.

 

[100LL... Again!]

Ask him if an airplane experiences p-factor to the right when in a descent (with some power on).

 

[VNugget]

In any situation where the airspeed is constant, you must be in equilibrium (ie. NO unbalanced forces). This is clearly true because if there was an unbalanced force there would be an acceleration (i.e. a change in velocity).

Really? What about in a constant-airspeed turn?


Yes, but as we have all learned, the four forces are balanced in straight and level flight, a constant speed climb, and a constant speed descent. Therefore, if Gravity and lift are balanced, how can gravity cause the descent?

When the path of the plane is inclined downward, a component of the weight vecttor is aligned with the thrust vector.

 

[Cat Driver]

Geeses, I know I shouldnt do this, but here goes.

Judging from the quality of aircraft handling skills that I find among todays pilots I think the time has come to find instructors who can put all this gobbldy gook formula stuff to use by going and getting further training so they can actually fly the thing themselves.

Then maybe the students would also know how to fly.

 

[uwochris]

Vnnuget,

You are correct. I was, however, speaking of non-turning flight, in response to the original question. I acknowledge that you are not in equilibrium while turning, even if the IAS is constant. This is because your flight path is constantly changing, which requires an acceleration (change in direction or speed).

In response to the slow flight question... you are descending, despite having a high nose attitude, because of the deficit of power. In this regime, the power required is quite high because of the high induced drag. In this case, the power required exceeds the power available, which implies a negative "excess" of power.

An a/c can have a positive AOA while climbing, descending, or turning. Just because the AOA is positive, this does not imply a climb is occuring. Think about the space shuttle when it is landing. The power (or thrust setting for jets) is not sufficient to maintain S&L flight at that particular IAS. The a/c has no choice but to descend, in order to gain an equilibrium between the forces.

In a climb and a descent, there is an upward component of thrust. In a climb, there is a rearward component of weight, which acts along with drag to retard the a/c. In a descent, there is a component of weight acting along the flight path, which acts along with the forward component of thrust.

 

[enigma]

Gravity is wrong, especially for those who were discussing acceleration. If you you are being accelerated/deccelerated, you are feeling a g-force, either positive or negative. In this problem, we are dealing with a constant airspeed, constant rate descent. Ergo, no acceleration involved.

Singlecoil, after the chalk clears, you might be technically correct. But gravity is not the wrong answer. You're arguing what technically overcomes gravity, while I'm trying to apply the KISS principle. The original question asked what makes the aircraft descend. That's a pretty simple question to answer.

Fear not, Singlecoil. I agree that your answer best describes the science behind the descent, we're just on slightly different planes of reason.

regards,
enigma

 

[Mr. Cole]

Matt,

My question to Jmmccutc was rhetorical in that we know that a plane can be in mushing flight with a nose up attitude and descend with never having its nose pointed down. I just wanted to give an example of a time when you can have a descent with downward pitch. I think much of what you've written in your follow-up post agrees with my original post.

But I wanted to add a couple of points to your post. Most of the discussion has centered around what happens when you pull the yoke back or push it forward. But you mentioned the case of pulling back the power first which is also illustrative. We know that if the power required equals power available at a particular airspeed the plane will fly S&L at the particular airspeed. We know that there are two outcomes of shifting the power available curve downward: S&L at a lower airspeed or a descent at the same airspped. If you were to suddenly pull the power, drag would immediately start slowing the plane down. The CL would remain the same momentarily and therefore a descent would start at the lower airspeed. If the yoke is released then it would appear that the plane would pitch down to achieve the trimmed CL, again due to long stability. If it does achieve the trimmed condition eventually it must return to the original airspeed, but in a descent. This is the case of a deficit of power for the airspeed.

The other case is when you hold the pitch attitude where it is and don't allow the plane to drop its nose. It will slow down and since the attitude doesn't change, the descent will cause an AOA increase, which is what you mentioned. You may also pull back more to keep the plane from descending, further adding to the AOA increase that the initial drop gave you. If you keep the plane from re-establishing the trim condition at the old airspeed but with a descent rate, it will find a new airspeed such that power required at that speed is equal to the power available, which is a function of where you pulled the throttles. But in all cases that initial drop is due to W > L, even if just for a moment.


Dave

 

[USMCAirWinger]

Doens't this entire thread belong in the Dorky Pilot Series?

 

[Mr. Cole]

USMCAirWinger,

You can be so cruel when you're sober.

Dave

 

[VNugget]

Vnnuget,
You are correct. I was, however, speaking of non-turning flight, in response to the original question. I acknowledge that you are not in equilibrium while turning, even if the IAS is constant. This is because your flight path is constantly changing, which requires an acceleration (change in direction or speed).

Fair enough. It's just that I always get giddy when people use words like "any" and don't provide the caveats of their arguments, even when they cross into the "obvious" zone. A bit of a character flaw, I guess.

Right now, for example, I'm having a hell of a time fighting off the urge to give another counterexample, which does not involve turning flight.