Author Topic: Some New Data Carts to chew on  (Read 3604 times)

Offline HoHun

  • Gold Member
  • *****
  • Posts: 2182
Some New Data Carts to chew on
« Reply #90 on: January 08, 2002, 01:34:00 PM »
Hi F4UDOA,

>I think the results of these calculations somewhat prove my point.

I'm afraid your message isn't entirely clear on which point exactly they prove.

If I plug in the values for the P-38J, Zigrat's spreadsheet predicts a climb rate of less than 3300 fpm compared to the 3730 fpm listed by the comparison chart you posted, so my conclusion would be that Zigrat's spreadsheet may be inaccurate.

Regards,

Henning (HoHun)

Offline Badboy

  • Silver Member
  • ****
  • Posts: 1230
Some New Data Carts to chew on
« Reply #91 on: January 08, 2002, 02:33:00 PM »
Quote
Originally posted by wells:
Dwarf, the number of blades matters not, only the diameter.

Hi Wells,

I do see your point here, but the equations are misleading. They do make it appear as though what you are saying is true, but it isn't. The number of blades does make a difference. Prop curves are unique to each type of propeller. If you change the number of blades or their design, you need a new propeller polar. I have just checked some real prop curves for three and four bladed props of an otherwise identical type and the difference in efficiency varies between 6 and 10 percent. Change the prop type to that of another manufacturer and the differences might be even more significant.

You can only use the same prop curve, with any hope of reasonable results if you apply it to only those aircraft that have props with the same number of blades of the same type.

The analogy would be if you were trying to use the same lift curve on every aircraft regardless of the type of wing. Different shape wings, or a different number of wings would require that you use the appropriate curves... Similar thing with propellers.

Hope that helps.

Badboy
The Damned (est. 1988)
  • AH Training Corps - Retired
  • Air Warrior Trainer - Retired

Offline Badboy

  • Silver Member
  • ****
  • Posts: 1230
Some New Data Carts to chew on
« Reply #92 on: January 08, 2002, 02:41:00 PM »
Quote
Originally posted by Dwarf:


What I've tried to emphasize is that climb and acceleration differ most markedly at the margins.  That is where you need to be most wary of the Ps value. Ps is a value which is most accurate and believable for both problems precisely when it is needed least and most unreliable when it is needed most - at the margins.


This appears to be complete nonsense.  


 
Quote

At low speed you can safely believe it with respect to acceleration, but not climb.  At high speed you can safely believe it with respect to climb but not accel.  In between, it doesn't matter, because inertia alone will carry you through either transition state.  In fact it's inertia alone which allows you to recover from a terminal velocity dive.  At terminal velocity, Ps is most likely negative.  If that truly meant what it implies recovery would not be possible.

So does this.

 
Quote

Believe me, I wish Ps was more useful.  I'd love for it to be.  It would make performance modeling and prediction so much easier.  

And this.


Badboy
The Damned (est. 1988)
  • AH Training Corps - Retired
  • Air Warrior Trainer - Retired

Offline Badboy

  • Silver Member
  • ****
  • Posts: 1230
Some New Data Carts to chew on
« Reply #93 on: January 08, 2002, 02:51:00 PM »
Quote
Originally posted by Zigrat:
my sheet will not work well for the p38 since it is kind of stacked against it.

I was also going to mention that you can't really use the same prop thrust curve fit for both the P-38 and F4U, but I think you covered it  :)

Badboy
The Damned (est. 1988)
  • AH Training Corps - Retired
  • Air Warrior Trainer - Retired

Offline Badboy

  • Silver Member
  • ****
  • Posts: 1230
Some New Data Carts to chew on
« Reply #94 on: January 08, 2002, 03:02:00 PM »
Quote
Originally posted by Zigrat:
its just that where the hell can you find the activity factor of a bf-109 propeller? damned if i know.

You can work it out yourself if you don't mind spending the time with the propeller (at an aircraft museum) and some measuring equipment.

Badboy
The Damned (est. 1988)
  • AH Training Corps - Retired
  • Air Warrior Trainer - Retired

Offline Badboy

  • Silver Member
  • ****
  • Posts: 1230
Some New Data Carts to chew on
« Reply #95 on: January 08, 2002, 03:09:00 PM »
Quote
Originally posted by Zigrat:
my sheet will not work well for the p38 since it is kind of stacked against it. the fact that the nacelles block a large portion of the spanwise flow will mean that the p38 generates less induced drag but the sheet does not take this into account.

This reminds me, when I came to analyze the P-38 I ended up doing almost a complete rewrite of my modeling tools to allow for the twin engine configuration. It was frustrating, because it was a lot of extra work for only one aircraft.

Badboy
The Damned (est. 1988)
  • AH Training Corps - Retired
  • Air Warrior Trainer - Retired

Offline Badboy

  • Silver Member
  • ****
  • Posts: 1230
Some New Data Carts to chew on
« Reply #96 on: January 08, 2002, 03:22:00 PM »
Quote
Originally posted by Dwarf:
BTW, my source does include generic graphs for a "typical 3 blade propellor".  I'm unwilling to try to extrapolate much from those for any specific 3 blade propellor or any 2 or 4 blade prop.

I think that is the first thing I have seen you say, that actually makes good sense!

Badboy
The Damned (est. 1988)
  • AH Training Corps - Retired
  • Air Warrior Trainer - Retired

Offline Dwarf

  • Zinc Member
  • *
  • Posts: 67
Some New Data Carts to chew on
« Reply #97 on: January 08, 2002, 04:21:00 PM »
Quote
Originally posted by F4UDOA:
[QB]Dwarf,

Two quick things.

1. How does the P-38 compare well with F7F? The F7F is nearly 60MPH faster at sea level where there is no danger of compressabilty. I'm not sure if I understand what you are comapring?

QB]

I was comparing range.  If I made out the numbers correctly, both carry 360 gallons of fuel and the P-38 actually has a slightly better radius of action.

Dwarf

Offline Dwarf

  • Zinc Member
  • *
  • Posts: 67
Some New Data Carts to chew on
« Reply #98 on: January 08, 2002, 04:29:00 PM »
Quote
Originally posted by HoHun:
Hi Dwarf,

>V Max for the 38 was due to compressibility.

Though the P-38 had a lower critical Mach number than most contemporary fighters, it still was unable to reach it in level flight.

Regards,

Henning (HoHun)

Agreed.  I didn't mean to be cryptic, but what I meant to convey was that the documented speed was, at least in the case of some 38's, lower than what it could actually achieve.  Intentionally so.  The AAF didn't want its pilots trying to go any faster than about 415 in the 38.  

From my reading of the history of the plane, the AAF imposed a lot of restrictions on how it could be flown.  Some justifiable and some not.  My bet is that the documented numbers for the airplane are based at least as much on those restrictions as they are on the planes actual capabilities.  Just a hunch.

Dwarf

Offline Dwarf

  • Zinc Member
  • *
  • Posts: 67
Some New Data Carts to chew on
« Reply #99 on: January 08, 2002, 04:39:00 PM »
Awwww, stop it Badz, yer makin me blush.

That's probably the nicest thing you've ever said about me.

Care to offer some proofs?  Or do ya just want to snipe?

Dwarf

Offline Guppy

  • Zinc Member
  • *
  • Posts: 89
Some New Data Carts to chew on
« Reply #100 on: January 08, 2002, 05:09:00 PM »
Quote
Originally posted by F4UDOA:
Also in regard to the Yamamoto mission. I wouldn't put to much stock in "It was the only A/C for the Job". More decisions where based on interservice rivalies and politics than on what was the best for the Job.

True enough in general, but I doubt this in the case of the Yamamoto mission.

As far as interservice rivalry goes, the Marines were the dominant force on Guadalcanal, not the Army. The P-38 flight leader afterwards said that there was no way his squadron would have been picked if F4Fs or F4Us could have made the trip.

Moreover, the Marine fighter ops exec who helped plan the mission specifically stated that the P-38s were the longest-range fighters  available.

Offline Dwarf

  • Zinc Member
  • *
  • Posts: 67
Some New Data Carts to chew on
« Reply #101 on: January 08, 2002, 05:09:00 PM »
Quote
Originally posted by HoHun:
Hi Dwarf,

Your low speed example is misleading because you implicitely assume that initially, the plane is in level flight and has to pull up to enter a climb. You're right it can't pull up while flying at 1 G stall speed without stalling. However, if the initial situation is a climb, this is not necessary, and aircraft can climb quite well at 1 G stall speed.

I agree entirely.  Once a climb has been established it can be maintained, even at speeds down to Stall.  

And once either climb or accel has been established, Ps will do a useful job of providing rate data.

All I've been trying to do is get across the idea that Ps will NOT tell you whether you can get from your current flight condition to that established state in all cases.

     
Quote

Your high speed example is slightly flawed, I'm afraid. At top speed, an aircraft uses all of its power to overcome drag, and there's no power left to climb. Only by going slower, the aircraft can begin to climb - just like it could accelerate by descending.

The math isn't that complicated:

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

=> acceleration: a=(Ps*g)/v, climb rate: Vv=Pe/W=Pe/(m*g)=Ps

The conclusion:

a=Vv*g/v

Acceleration and climb are directly and linearly interdependend.

The derivation of the above formula requires no knowledge of aerodynamics at all, it's a fairly simple application of Newton's axioms.

Regards,

Henning (HoHun)[/qb]

Please reread my posts.  In the first instance, I specified that some part(s) of the structure had encountered critical mach and its attendant drag rise.  Whether the equation(s) we have for deducing Ps accurately would take that drag into account isn't apparent.  At that point, the aircraft could no longer accelerate, but it could still climb.

In the second instance, I specified terminal velocity, not merely top speed.  The aircraft is in a dive and can go no faster no matter what you do, but it still has enough altitude to effect recovery.

For the sake of discussion, let's take a hypothetical aircraft.  It has a top level speed of 400 mph.  Due to it's configuration, not even WEP and gravity can get it to exceed 600 mph.

It enters a dive at near top speed.  Once it passes above that 400 mph top speed, Ps becomes negative.  Soon it reaches its 600 mph terminal velocity.  Now nothing can make it go faster, but it can still recover (climb).  Not because there is some Ps hiding somewhere, but because of the inertia it carries.  So long as the stresses don't cause disintegration, inertia will carry it through the transition into climbing flight, after which it will slow until Ps again becomes positive and it can sustain some rate of climb.  

At the point at which recovery is begun, it can climb but it can no longer accel.  Ps would say it can do neither.

At all points between (slightly above) Stall Speed and Terminal Velocity, the value of Ps doesn't really matter so far as entering a maneuver goes.  Both climb and accel can be initiated at will.  Both can be sustained at some rate and for some period of time.  

Even at those speeds between top level speed and terminal velocity, the pilot can still accel or climb at will.  At such speeds Ps can no longer provide any accel rate information and can only tell you that climbing will reduce your speed.  It may assist somewhat as to how much accel is being retarded and decel would occur in the climb, but probably not all that much.
 
Dwarf

[ 01-08-2002: Message edited by: Dwarf ]

Offline HoHun

  • Gold Member
  • *****
  • Posts: 2182
Some New Data Carts to chew on
« Reply #102 on: January 08, 2002, 05:54:00 PM »
Hi Dwarf,

>All I've been trying to do is get across the idea that Ps will NOT tell you whether you can get from your current flight condition to that established state in all cases.

Yes, you have to think of a specific Ps value as a snapshot of the situation. Transitions between specific situations involve a range of Ps values - if it were a movie, you'd have to examine Ps for every frame :-)

>In the first instance, I specified that some part(s) of the structure had encountered critical mach and its attendant drag rise.  

I didn't miss that, but for the Ps concept, the source of the drag is not of interest. If level top speed is reached, and we look at the movie frame of that instant, we'll see that both acceleration and climb are impossible at that speed. The frame before that, he could have done both - and though the acceleration value might have looked good enough from that speed, the sudden drag rise at compressiblity ate it up in the next frame. Ps didn't tell us in advance, the aircraft had to go there to find out :-)

>In the second instance, I specified terminal velocity, not merely top speed.  [...] Now nothing can make it go faster, but it can still recover (climb).  Not because there is some Ps hiding somewhere, but because of the inertia it carries.

Now I begin to see your point :-) Of course, this is a transition and accordingly more complex, but if we only look at the zoom climb following the climb-out, we certainly see an aircraft climbing at negative Ps for a moment.

Ps indeed describes sustained climbs and instantaneous accelerations, which looks a bit strange to the untrained eye ;-) But a more complete description is that Ps is the rate at which the sum of kinetic and potential energy changes in a specific flight condition. We've seperated this into pure climb and pure acceleration, but in real life, it's usually a combination of both.

Regards,

Henning (Hohun)

Offline Dwarf

  • Zinc Member
  • *
  • Posts: 67
Some New Data Carts to chew on
« Reply #103 on: January 08, 2002, 06:00:00 PM »
Although this wan't where I was headed when I started, I seem to have arrived at the idea that a new way of looking at climb and accel needs to be devised.

HoHun is absolutely correct when he says that current practice considers climb and accel to be the same problem.

Since the equations to prove it don't currently exist, it's rather hard to conclusively demonstrate that they are not.

When considering the problem of climb, I believe we need something that accounts for the entry cost as well as the cost of maintenance.

Anybody got any ideas how we could do that?

Dwarf

Offline Dwarf

  • Zinc Member
  • *
  • Posts: 67
Some New Data Carts to chew on
« Reply #104 on: January 08, 2002, 06:05:00 PM »
Quote
Originally posted by HoHun:
Hi Dwarf,


Yes, you have to think of a specific Ps value as a snapshot of the situation. Transitions between specific situations involve a range of Ps values - if it were a movie, you'd have to examine Ps for every frame :-)

...

Ps indeed describes sustained climbs and instantaneous accelerations, which looks a bit strange to the untrained eye ;-) But a more complete description is that Ps is the rate at which the sum of kinetic and potential energy changes in a specific flight condition. We've seperated this into pure climb and pure acceleration, but in real life, it's usually a combination of both.

Regards,

Henning (Hohun)

And, I'm probably expecting too much from Ps  ;)

What I want is something that can predict as well as confirm.  Of course I'd like to be rich and handsome, too  :D

Dwarf