Some New Data Carts to chew on

[F4UDOA]

Wells,

Would an A/C with a measurably lower aspect ratio climb as well as one with a high aspect ratio with wing area, power and everything else being equal?

I thought that a high aspect ratio assist in glide, lift and long range IE. Voyager, U-2, B-52 and P-38.

[Dwarf]

Originally posted by wells:
Dwarf, for density, you should be using 0.00237 slug/ft^3

That's sea level density.  Not sure how that applies if I'm flying at 8000 ft.

In any case it's just a constant, and though it might change the magnitude of the two halves of e, it won't change their difference.  The answer at the end would still be 1.

Dwarf

[ 01-07-2002: Message edited by: Dwarf ]

[Dwarf]

Originally posted by F4UDOA:
Wells,

Would an A/C with a measurably lower aspect ratio climb as well as one with a high aspect ratio with wing area, power and everything else being equal?

I thought that a high aspect ratio assist in glide, lift and long range IE. Voyager, U-2, B-52 and P-38.

A. Maybe.
B. Yes.

The reason A is maybe is that, a high aspect wing will produce more lift per unit of drag and therefore requires less power to achieve flight or any given rate of climb.  BUT, for any given AoA the faster you go the more total lift a wing produces.  The more lift you produce the more induced drag you produce.  So, the high aspect wing would perform best at low speeds.  A lower aspect wing, while relatively less efficient, would be inherently faster.  So while the high aspect wing might permit a 3k ROC at 120, the low aspect wing might allow a 3k ROC at 180.  As long as the low aspect plane has enough power to sustain 180, they would climb at the same rate.  

The U-2 is a high altitude wonder, but you wouldn't want to dogfight with it.  The wings wouldn't take the stress.

As long as you are willing or able to fly slowly, the high aspect wing will permit long duration missions because they require less power for level flight.

Dwarf

[HoHun]

Hi F4UDOA,

>It is just an extreme example to prove the point I was trying to make. That you can't directly link climb and acceleration.

Here's the explanation for your example:

Ps=Pe/(m*g)=(Pt-Pd)/(m*g)

If you increase the necessary power to overcome drag (Pd) while leaving the power available as thrust (Pt) constant, you have an aircraft with different Ps.

Two aircraft with different wing shapes, but equal thrust and drag in a specific flight state are equal in climb and acceleration.

Climb and acceleration are directly and linearly linked.

Regards,

Henning (HoHun)

[F4UDOA]

Two aircraft with different wing shapes, but equal thrust and drag in a specific flight state are equal in climb and acceleration.

Climb and acceleration are directly and linearly linked.  

HoHun,

I am not an engineer but I do understand some of the more basic equations and principles such as lift, drag, thrust etc. but I rely on engineer types like yourself and Wells to clue me in on the rest. But Somehow I think something is getting lost in the translation.

For instance. I have speadsheets, one done by Wells and another done by Zigrat (whom has not been on the boards in a while) calculating Climb, turn, thrust and drag through out the speed range as long as you know the wing area, Aspect ratio, HP, wingspan and 1G stall speed (clean, power on). It gives a very realistic out come based on input. If I put in the F4U-1 data from the spreadsheet I have posted above I will get a climb rate of approx. 3150FPM at Sea level. However if I change one characteristic of the F4U-1, Wingspan the Aspect ratio increases and the climbrate increases several hundred FPM.

Why??

I will post this Spreadsheet on my Webpage and then supply a link.

Standbye.

BTW Zigrat if your out there I will credit you on my Webpage.

[ 01-07-2002: Message edited by: F4UDOA ]

[HoHun]

Hi Dwarf,

>Now, we have a situation where the aircraft can climb, but can no longer accelerate despite what the Ps number says.

You're trying to compare different flight conditions.

An aircraft very close to its top level speed can either climb or accelerate. If it starts to climb, it still can climb "one second later". The flight condition has not changed*.

(* Neglecting altitude since a one-second climb will make little difference.)

If it starts to accelerate, drag will increase, specific excess power will decrease, and it will hit top speed "one second later". The flight condition has changed.

I think what's confusing you is are the different results of applying specific excess power for climbing or for acceleration for a finite time.

Regards,

Henning (HoHun)

[HoHun]

Hi F4UDOA,

>However if I change one characteristic of the F4U-1, Wingspan the Aspect ratio increases and the climbrate increases several hundred FPM.

Does the spreadsheet contain a column for acceleration? That one should increases proportionally.

Regards,

Henning (HoHun)

[F4UDOA]

HoHun,

Your saying an A/C will accelerate and climb faster with all other factors being equal if the wingspan is increased?

Why?

Here is the link to the spreadsheet. It is in Excel.

I hope Zig didn't copyright it or anything    


Performance calculator

[ 01-07-2002: Message edited by: F4UDOA ]

[Dwarf]

Originally posted by HoHun:
Hi Dwarf,

>Now, we have a situation where the aircraft can climb, but can no longer accelerate despite what the Ps number says.

You're trying to compare different flight conditions.

Regards,

Henning (HoHun)

Not really.  All I've specified is that the aircraft has encountered a drag rise due to mach.  At that point the pilot has only one option - reduce speed.  He can do that by either reducing power, or climbing.

Maintaining his current power and attitude just beats his head against a wall.  Further accel just went out the window.  

At the point 0.001 fps before he encountered mach, your Ps equation might lead him to believe he had another 20 mph in the bank.

It's a poor predictor.  I don't know that any better predictor currently exists, but that doesn't change the fact that (especially) at the margins the tools we currently have are not adequate to the task.

Ps is just a number and there are situations at both ends of the flight speed spectrum where it may lead you to try things that just aren't possible.  Especially if you believe that climb and accel are the same problem and are linearly linked.

Their differences at the margins differentiate them.  If they didn't (to use a gross and unfair example) the aircraft would launch verically into the air as soon as you start the engine.  Instead you must accelerate to a speed where flight is possible.  At the low end, you can accel when you cannot climb and at the high end you *may* encounter a situation where you can climb but can no longer accelerate.
(Which is exactly what you see in a terminal speed dive, to use another example.)

Climb and acceleration have very different entry requirements.  When you include those entry requirements and not just limit yourself to conditions that obtain after those requirements have been met, an aircraft can see a Ps with a zero or negative value with respect to climb while the same power setting, airspeed, altitude etal will yield a positive Ps number with respect to acceleration.  If you ignore the entry requirements, the Ps number will be the same for both climb and accel, but, depending on where you are in the flight envelope, trying the wrong maneuver will fail.

The idea that if it can accelerate it can climb and if it can climb it can accelerate is not borne out by 100 years of flying history.

Dwarf

[ 01-07-2002: Message edited by: Dwarf ]

[gripen]

Well, at 1g stall speed the plane flys at Clmax  of that speed so certainly it can't climb but it can accelerate ie aerodynamic restriction for climb exists. So basicly this point is different than the max climb rate and max acceleration speed when there is no aerodynamic restriction but available energy can be used for what ever wanted. At max speed all thrust is used to counter drag so if it starts to climb it will slow down until balance is reached again.

Generally HoHuns system is pretty neat and actually it is related to old thrust/drag system but calculation is done backwards. And it also is accurate if there is no huge difference between climbing and accelerating flying condition (and there is not) and used data is accurate.

gripen

[ 01-07-2002: Message edited by: gripen ]

[Badboy]

Originally posted by Dwarf:
Anybody else got a set of formulae that isn't circular (unlike the set I already had)?

The equations you had were perfect, they are not circular, they are intended to be used with a set of prop curves for the specific prop in question. So for example, you could calculate the trust from the equation you posted for the thrust coefficient, you would just need the thrust coefficient curve to do it.

Badboy

[Dwarf]

Originally posted by Badboy:


The equations you had were perfect, they are not circular, they are intended to be used with a set of prop curves for the specific prop in question. So for example, you could calculate the trust from the equation you posted for the thrust coefficient, you would just need the thrust coefficient curve to do it.

Badboy

Thanks, Badz.

Now... anybody got prop curves for the props on the AH planes?  

So, basically, I'm just stuck with my generic 80% without those curves.

Dwarf

[Badboy]

Originally posted by F4UDOA:
For instance. I have speadsheets, one done by Wells and another done by Zigrat (whom has not been on the boards in a while) calculating Climb, turn, thrust and drag through out the speed range as long as you know the wing area, Aspect ratio, HP, wingspan and 1G stall speed (clean, power on). It gives a very realistic out come based on input. If I put in the F4U-1 data from the spreadsheet I have posted above I will get a climb rate of approx. 3150FPM at Sea level. However if I change one characteristic of the F4U-1, Wingspan the Aspect ratio increases and the climbrate increases several hundred FPM.

Why??

Because they are using the wing aspect in their calculations to estimate the lift related drag coefficient, and as you increase the wing aspect, the lift related drag coefficient gets smaller and the acceleration and climb rate increase accordingly.

Badboy

[Badboy]

Originally posted by F4UDOA:
HoHun,

Your saying an A/C will accelerate and climb faster with all other factors being equal if the wingspan is increased?

Why?

Yes. Because the other factors don't stay equal, if you increase the wingspan, the induced drag coefficient is reduced.

 

Here is the link to the spreadsheet. It is in Excel.

I hope Zig didn't copyright it or anything      

Firstly, bear in mind, that spread sheet is only doing a generic thrust calculation and also an approximate estimate (using the same Oswald factor for every aircraft) of induced drag, so although the results are good for getting a feel for the behavior and trends involved, it won't actually be correct for any particular aircraft.

Having said that, let me tell you how to edit that spreadsheet to provide acceleration data.

Open the spread sheet and go to the cell R12 and type in the following:

=O12*32.2/(E12*129)

That will give you the acceleration of the aircraft in mph every second.

Basically that calculation takes the climb rate, or Ps displayed in cell O12 and multiplies it by g/v to get acceleration and the 129 is a conversion factor so that the units come out in miles per hour, every second.

Once you edit that cell, copy it, then paste it down the entire column.

Badboy

[Badboy]

Originally posted by Dwarf:
Ps is just a number and there are situations at both ends of the flight speed spectrum where it may lead you to try things that just aren't possible.  Especially if you believe that climb and accel are the same problem and are linearly linked.

Ps is a very important number, and it is related to climb rate and acceleration. HoHun is absolutely correct.  

 

The idea that if it can accelerate it can climb and if it can climb it can accelerate is not borne out by 100 years of flying history.

Dwarf, most of your statements here (including those I've snipped) are incorrect. Acceleration and Climb rate are directly related, they only differ by a factor that includes (v/g) and so it is possible to make direct comparisons. Trust me on this, HoHun is correct.

Badboy