Improving an aircraft's descent

[flynryan692]

Hello everybody, I've got a simple question, well I think it is simple.

I have some Airbus aircraft that are fantastic, however they can be problematic when descending. Currently I have to start my descent a little bit early so I do not exceed 1500-1600 fpm, otherwise the aircraft will gain too much speed. The logical thing to do is deploy spoilers and add some drag to the aircraft, however that doesn't slow the aircraft at all, it will keep it from exceeding 260 knots but not any slower. Some airports (I fly on vatsim) tend to have you slow to210 knots or less when you are close to the final approach course, which is something I can't do with the aircraft currently.

So, can you guys tell me how I can fix this or maybe just add some drag to the aircraft/ make the spoilers more effective? I have AirEd but when I open the airfile there is nothing there to edit.

Thank you

 

[Roy Holmes]

Open the air file with AirEd. Look for section 1101.
Under drag per Radian find Cd_ds Drag Coefficient - Spoiler
click on that and increase the value

press enter and save.

Roy

 

[jx_]

It sounds like your drag or idle thrust are the real problem. All current commercial airliners are designed to easily meet a 3 degree descent at normal speed. That's between 3500fpm at highest altitudes and around 1500fpm below 10,000 feet. (dependent on weight of course)

An A320 for example will descend at

2500fpm @ M0.78 / 4000fpm-2500fpm @ 300kias / then 2000fpm @ 250 kias

easily without the need for any airbrakes.

Max speed with the airbrakes extended in the A320 should give roughly a 6000fpm descent.

Make sure your idle thrust is correct, and if so I would recommend adding a little induced drag.

 

[flynryan692]

Thanks for the reply!

Problem is when I open up the air file in AirEd I don't have section 1101, I only have 1, 2, 3 ,4, 100, 101, and 104. Is there something in the aircraft.cfg I could change?

 

[Roy Holmes]

If that is all there is in the air file the sim could be running on default values and my guess is that you are stuck with what you have
Roy

 

[jx_]

Ouch!

Is this a payware? FS9 or FS10?

 

[flynryan692]

It's the Project Airbus actually...FS9/X

Oh well, I'll try different aircraft loads and see what happens. I don't think that changed anything for me a while back, but who knows maybe I'll get it this time.

Thank you!

EDIT: Never mind, I got it! AirEd didn't show anything but this AAM program I got worked and I was able to edit it. Thank you!

http://www.aero.sors.fr/fsairfile.html

 

[hervesors]

It sounds like your drag or idle thrust are the real problem...Be sure your idle thrust is correct, and if so I would recommend adding a little induced drag.

Agree with Jx_..also idle thrust is probably the main culprit as far as induced drag should already be very low during descent (low AoA at normal speeds)

 

[jx_]

(low AoA at normal speeds)

It's actually the lift part that is small and lift is the factor in determining induced drag. More AoA without more lift should only produce more parasite drag.




BTW, could also be ram drag. At idle, net thrust would normally be negative at those speeds.

 

[mgh]

In the normal flight regime, an increase in angle of attack results in a increase in lift coefficient and hence an increase in lift, unless the speed is reduced to cancel it out.

 

[jx_]

Smh.

No sorry, lift, AoA, and speed are all explicitly tied together. You can't change lift produced at a given AoA/speed without changing weight, power, or atmosphere conditions.

At a given weight, the wing will always produce the same lift for a given speed, and therefore will fly a set AoA in those conditions. Changes in pitch require sufficient power or momentum to maintain the current airspeed/lift, and result in climbs or descents at the exact same AoA as level flight during the same conditions and weight. Without changing power, speed will change and THEN AoA and CL change. If speed is maintained, the only way to change AoA is G loading (weight) from maneuvers or vectored thrust lift... If your plane climbs with a higher angle of attack than level flight AoA at the same speed, it is because you have engines giving vectored thrust lift, in which case the wings are still producing the same CL as they would at the regular AoA for the current speed. In modern jets, wings at climbing AoA produce LESS lift than in level flight at the same AoA due to high thrust to weight. If you are transitioning, changes in AoA are due to g loading.

On dead engines, the entire descent would maintain the same CL as level flight CL vs. AoA for your current airspeed. Momentum is power, as long as you have some left you can use it, but you won't be able to exceed the current airspeed CL.


The above excludes banking and changing wing, trim, & control configurations, which are changes to 1g weight. Bumpy air doesn't count either because it is a direct change in airspeed relative to the wing and 1g weight.

So the point I was making earlier is that most people believe induced drag is minimal and dependent on AoA when it is dependent on lift and can easily exceed parasite drag. Increasing AoA without increasing lift increases the parasite drag frontal area without adding induced drag. Reducing AoA can do the opposite if the reduction is not due to higher speed. High performance military jets doing full power steep climbs can have zero induced drag at high angles of attack because the wing can be producing zero lift. Flying around with fully extended wing spoilers doesn't produce much induced drag, nor does a stall, but both cases have very high angles of attack. Level flight with spoilers out produces a higher AoA but the same lift coefficient. Level flight with flaps out produces lower AoA, but the same lift coefficient. Induced drag would be unchanged in both cases.


So, in the normal flight regime, an increase in angle of attack rarely increases lift coefficient...because it's not possible to change your lift without first changing speed, power, weight, or atmosphere.

 

[mgh]

In the normal flight regime, an increase in angle of attack results in a increase in lift coefficient and hence an increase in lift, unless the speed is reduced to cancel it out.

is correct

 

[jx_]

lmao... Actually not, but you're entitled to hold on to what you believe even if it's wrong.

This is why you pitch for speed and power for altitude. You don't pitch up to start a climb....

But you already knew that of course......


I really hate all the misinformation that floats around the flightsim community, and you're one of the most frequent contributors I see in the forums.

 

[mgh]

Let's take it step-by-step:

1 Increasing angle of attack increases lift coefficent - Yes or No?

2 Increasing lift coefficient increases lift with the speed unchanged- Yes or No?

3 Therefore, unless the speed is reduced to cancel it out there will be an iincrease in lift - Yes or No?

In the normal flight regime, an increase in angle of attack results in a increase in lift coefficient and hence an increase in lift, unless the speed is reduced to cancel it out.

QED

 

[Roy Holmes]

Any statement must have underlying assumptions and if these are not spelled out in advance there is ample scope for misunderstanding.

If AOA is increased by stick back pressure, lift will be increased, because the increased AOA results in a higher lift coefficient. However this is only true when operating on the straight portion of the CL vs Alpha curve. If at CL max, an increase in AOA will result in less lift.

Increasing AOA by stick back pressure or decreasing it by forward pressure is the fundamental method for controlling an aircraft in the vertical plane.

The increase or decrease in lift will result in a change in pitch angle, which is usually the desired outcome for fore/aft stick movement.

So MGH's statement is true given a fuller definition of normal flight regime.

Roy

 

[jx_]

Sure, let's take it step-by-step:

No No and No!

How in the world would you increase lift without increasing speed???!! POWER! The lift you are thinking of is absolutely impossible in a pure glider.

There is no way to change the angle of attack of a plane in flight without changing speed, weight, power, or environment (or mechanical configuration) FIRST!!!

period.

If you are cruising level at 3 degrees AoA and pull back on the stick to 6 degree pitch, the airplane will maintain 3 degree AoA (and same amount of lift) if speed is maintained. Any climb will come from applying the power that is required to maintain your airspeed... because in order to maintain your airspeed at an incline, you must FIRST have both sufficient level flight airspeed power plus the additional climbing power.

OR... AoA will change with the g load of the transition, then the decaying airspeed FIRST.

You can't climb on lift without power, and you can't change AoA without changing speed. AoA is elastic to airspeed. AoA is a byproduct of airflow and dynamic pressure.


If you were cruising at 3 degrees AoA, then you push the nose down, you will descend at the same AoA if you decrease power to maintain constant speed.

In both cases, lift coefficient, and thus induced drag doesn't change.


If you are in level flight, and you yank back on the stick because you want to prove to me that your theory is right... you would even sound more foolish. The change in angle of attack would be solely a result of the G load increasing the planes weight, which requires more lift... or any change in airspeed.

An unstalled wing on a stable airframe produces the lift that is asked of it as a result of balancing forces.


And if you want to get REALLY technical, the minor oscillations in AoA... where do they come from? Moments about the CoG and moment arms are constantly affecting g loads all over the plane, and the wing responds accordingly.


Learn it inside and out before confusing the poor people who come to these forums for help....... and further spreading misconceptions and misinformation. You make it harder for people who just want to improve their sim experience.

 

[jx_]

If AOA is increased by stick back pressure, lift will be increased, because the increased AOA results in a higher lift coefficient.

Roy, you are making the same mistake he is making.

Moving the stick just changes the gradient. It has no direct effect on AoA. Pitch and AoA would both change by the same amount if everything else stays constant. A 3 degree pitch up in cruise will equate to a 3 degree AoA climb or descent if speed is equal. (excluding the exceptions I pointed out before.)


Applying back pressure does what?

Elevator up puts added g load on the wing through a tail down moment while adding elevator drag. If you were to trim this to net out the pitch moment and add power to net out the drag, AoA would increase at the same airspeed and so would CL, but not because AoA increased.... but actually because weight demand on the wing has changed.

There is no real way to change angle of attack directly. If you pull on the stick and don't add power --- AoA increases (and CL) BECAUSE speed is dropping (or looked at in other terms, energy of momentum is being drained to make up for the lack of power) and once again, the wing is accommodating a higher lift demand.

But if you add power, then pull back to maintain speed AoA WILL NOT CHANGE WHILE YOU CLIMB, nor will CL. Climb comes from power (or momentum) only. AoA comes from lift demand. CL is not produced, but derived.


If it worked how MGH says so, you could pull back on the stick and the airplane would rotate about it's CG to a new AoA and climb without changing power, since it is producing a higher CL that is solely a product of higher AoA according to him. If that's the case why don't we just fly at higher AoA with no power and climb away like hot air balloons.....?

Because it's not possible. A normal plane doesn't climb or descend that way. The climb power is what gets you up. Not lift. MGH is wrong.

 

[WarpD]

I'm not going to wade very deep into this discussion... except to state that your statements and conclusions are quite flawed 'jx'.

If it worked how MGH says so, you could pull back on the stick and the airplane would rotate about it's CG to a new AoA and climb without changing power, since it is producing a higher CL that is solely a product of higher AoA according to him. If that's the case why don't we just fly at higher AoA with no power and climb away like hot air balloons.....?

MGH is correct in his statement. Increased elevator results in a pitch up, which results in increased AoA, which results in an increase in altitude. Because of the increase in lift from the increased AoA, drag is also increased. This is why more power is required. The only reason more power is required.

Power does not, ever... ever generate lift.

 

[mgh]

Why not simply explain which of my 3 statements is wrong.

 

[jx_]

WarpD,

That's humor. Elevator never changes AoA directly, it only aims it. Speed and weight change AoA.

Increased elevator results in a pitch up, that changes the vector path of the AoA.... AIMS IT! Doesn't change it. Speed changes change AoA.


If you were cruising along with 5 degrees nose up, and I said reduce your speed by three knots but maintain 5 degree pitch... what would happen? You would descend at a gradient equal to the increase in AoA due to lower speed. No elevator, more CL, same pitch... but descending.

Conversely, if you were cruising along at 5 degree nose up and I said, pull back on the stick and maintain speed, AoA would not change, but you would climb... and you would climb at a gradient equal to the increase in pitch from 5 degrees... but the AoA would NOT change. If you believe otherwise I suggest you do some research.


This simple basic principle is what all pilots rely on when the lose all elevator and stab control. Remember the DC10 in Sioux City in 1989??? If you can't change pitch, you can't aim AoA, but you can't definitely change it by changing speed.

Power does not, ever... ever generate lift.

Ignorant statement... Rockets, fighter jets, helicopters, harriers, and all jets that can climb at 5000 per minute at 150 kias are climbing on power. What do you think an engine is??? A forward lift machine. A wing is a stationary upward lift machine. Point an engine upward and you get an upward lift component. The difference between engines and wings is wings cant be throttled, their lift is tied to airspeed.

Also, changing from level flight to climb at the same airspeed is still an acceleration. Acceleration ALWAYS requires power. Lift can not accelerate more than it is already without increasing power or stealing from momentum. Power or momentum can without help. Gravity, thrust, thermal heat, and such are forms of power. Speed is momentum.