Climb Vrs Acceleration Part Deux AFDU data

[Badboy]

Originally posted by bozon
F4UDOA,
this acceleration/climbrate problem has enough variables in it to allow just a bout any possible result.

Not true Bozon, the data is simply being misunderstood.

Badboy

[Badboy]

Originally posted by F4UDOA
Wells, like you said the speed has to be the same.

Isn’t that exactly what people have been telling you all along?

And thats not all. How about AoA. That has to be the same as well or you can't compare Cdi either. My point is that there are many variables in to determine best climb because speed and AoA vary when measuring climb but have nothing to do with level acceleration.

Not true, the speed and AoA have just as much to do with acceleration as they with climb, because they both influence the excess power.

So if my last staement is true then you can say that climb and accleration are not directly linked because climb uses more variables

Your last statement was not true, neither is this one.

For instance. How can you say that an A/C has high induced drag if you don't know what the Cl is at the AoA used in it's climb?? And none of that matters in acceleration. Is this not a true staement??

Nope, and I hate to be blunt, because you are obviously struggling to comprehend data you don’t understand, but unfortunately your statements in this post are almost completely incorrect.

Badboy

[Badboy]

Originally posted by F4UDOA
Please do not show me a generic equation. I have Aeronautics books too. But I do not have the backround to put together these equations.So please stop being so condesending and do the math.

Ok, let me try to illustrate what everyone has been trying to explain to you with an example. You, or anyone else, can use the Ps equation to verify the figures. For the sake of the example, let’s compare six different aircraft in two groups. The first three all have different maximum climb rates but their best climb rates all occur at approximately the same speed, those are the first three aircraft in the diagram below. They are ranked in order of their climb rate, and because their climb rates all occur close to the same speed, their level acceleration can be compared also, notice that when you rank them in order of their acceleration, the order is the same.

Now, look at the second group of aircraft. Aircraft 4 has the best climb rate but it appears to have the worst acceleration. The ranking for acceleration now appears to have no correlation with the climb rates at all. This is what you have noticed, and you have incorrectly assumed that something is wrong.

The second group of aircraft don’t have the same ranking because of the variation in airspeeds in the climb. Aircraft 5 appears to have the best acceleration, but if Aircraft 4 had the same climb rate at Aircraft 5’s airspeed it would not have been ranked 3rd it would have had the best acceleration of both groups, as shown at the bottom.

The best climb rates for real aircraft occur at different speeds, acceleration tests for a group of aircraft would normally be conducted at the same speed, that’s why you think there is something wrong. Nothing’s wrong, you are seeing a situation similar to the second group or aircraft below.

Badboy

[F4UDOA]

Standbye.

I'm digesting your information......

Thanks

[niklas]

The fun is that the author wrote down a nice contradiction:


"For instance, the Tempest V which is faster than the Spitfire XIV, takes less time to reach ANY given speed, but the Spit has the best acceleration"

Looks like the author doesn´t have any imagination about physics. When the Spit accelerates faster it will DEFINITLY reach starting speed +10mph earlier.  Things may change at +100mph or so, but at the beginning the Tempest will NOT reach ANY speeds faster.

It´s remarkable that small compact aircraft seem to have better intial acceleration. This can have imo also other reasons:

Large aircraft have large engines with large propeller. Initial acceleration is measured in a reasonable short time period.  That means there are other factors that come into the game:
How fast can the engine run up to full RPM?
How fast can the prop pitch mechanism adjust itself (large props high inertia slow adjustment )
How does the torque change effects movements of the aircraft around the axis(drag) which have to be compensated by large ruder inputs (drag)

And: At what altitude was the acceleration test done btw? At 10000, 15000ft things look different

niklas

[dtango]

Nice post Badboy.

F4UDOA- several of us have been trying to explain it over and over to you and I think we're getting exasperated just as you're getting exasperated with not understanding.  I hope Badboy's chart helps.  He's right on regarding your interpretation of the data which is leading you to believe that there is a lack of correlation between climb and acceleration which is simply not true.

I'm afraid you're next question is going to be "why?" which leads to all the ugly math that seems to have gotten us nowhere in helping with the explanation .  From niklas' responses I think he has gotten it so it wasn't all for not .

To be honest I'm disappointed in my own lack of being able to explain and help you understand the physics behind it all.  As Badboy says there is absolutely nothing amiss.  If needed I can "have another go" at explaining the "why" behind it all again .

Tango, XO
412th FS Braunco Mustangs

[Naudet]

So, let's look if i got everthing right. And i am the prototype of the aerodynamics amateur, no books and no advanced math or physics.

Climb and accel are directly linked to each other cause they both are "results" of available excess power.
The problem now is that this is right only for the same points of both curves.
If i can say Plane A climbs better than all other planes at speed X i can also say that at speed X it will accelerate the best.
But i CANNOT automaticly say that plane A will also accelerate best at any speed different from speed X.
So if i want to proof that Plane A is overall the best climber (and than also the best accelerator) i would have to compare Plane A with all other planes over the whole speedrange (to simplify i asume all planes have the same minimum and maximum flying speeds) at any speed between min and max flying speed.

If i now compare real planes i face the trouble that they all have differnet power curves and propeller designs. Due to this their excess power curves will vary greatly,  but always the plane with more excess power available at one point than the other planes at that point will outclimb and outaccel the others than at this point.

[dtango]

Naudet:

Bingo! You've got it!  

Some points of clarification:

So if i want to proof that Plane A is overall the best climber (and than also the best accelerator) i would have to compare Plane A with all other planes over the whole speedrange...

Key word here being "OVERALL".  I think you've got it down but just want to clarify for others that the plane that's the "overall best climber" does not mean it's also the plane with the "best rate of climb".   "Best Rate of Climb" comparison usually refers to plane to plane comparisons of the best rates of climb of aircraft which occurs at a SINGLE SPECIFIC velocity which is DIFFERENT from aircraft to aircraft.  Another way to look at it is that "best rate of climb" = where the "maximum rate of climb" occurs for an aircraft.

If i now compare real planes i face the trouble that they all have differnet power curves and propeller designs. Due to this their excess power curves will vary greatly...

Again I think you understand the concept but just want to clarify for others that when we speak of power curves it relates to a heck of a lot more than engine brake horsepower, but is a complex relationship of variables including thrust, drag, velocity etc. etc.

Tango, XO
412th FS Braunco Mustangs

[Holden McGroin]

Originally posted by Cogen


There you go again messing everything up by throwing rocks into the fray.

OK, what kind of rock; granite, quartz, or pumie.

I just got interested in the alphabet that was preceeded by the letter "d"  (i.e. dv/dt).  What does "d" mean?

Can it accelerate also?

D stands for Dumbs**t, and the international standard for igneous rock is covalent basalt. for sedimentary; nile sandstone.

[Holden McGroin]

Originally posted by Mino


A rock?  No way, I am sure we are not talking rocks here.  Apples maybe?  Holden are you really I. Newton incognito?

A rock is right between seria and A ran, and who in the hell is this guy incognito?