La7 speed at 13k is 10mph too fast

[Hamish]

 

"Well, Whoop dee doo, What does it all mean Basil?"

[MANDOBLE]

Originally posted by Zigrat
henning i believe you are on crack. your statements make no sense. fly at a higher angle of attack for the same lift? huh?

Not for the same lift,  but to keep aircraft flying level at its maximum level speed, where lift will be greater than weight u will need different "nose" angles depending on your weight to keep the aircraft flying at level. And most probably than not, the lighter aircraft will need to overcompensate with a slight more nose down attitude than the heavier one.

[Naudet]

I am just curious if there are not a few things you oversee.

Location of the fuel tanks will have influence to, cause if the tanks are far way from the CG, the plane needs more trimming to compensate for that away from CG weight.

Also i think reduced weight will generally increase speed, cause you get a better power to weight ration, also accel, climb and turning will be better.

I have an FW document that includes a comparrison calculation for the D9 at 3500kg (normal take-off weight 4270kg), the plane achieved a max speed of 698 km/h at its climb and combat power setting.

A normally loaded D9 managed speeds equal to that only with Special Emergency Power.

The difference in PS output looks like this:

SWEP: 2100 PS
Climb&Combat: 1680 PS


I think considering weight differences the power-weight-ratio effect is much larger than the drag effect through a little change in the angle of attack.

[MANDOBLE]

Naudet, all this is considering no variations in the CG due fuel load. It is true that some planes have the tanks "far" from the CG, but, for example, this is not the case with 190A5 and D9. No idea about La7 fuel tank placement.

The extra weight will affect climb and acceleration negatively, but not top speed at level.

When u are flying level at your max speed u need to:

1 - Fight the extra lift with a nose down attitude. At these speeds your aircraft will tend to climb and you want to keep at level. More weight implies less overcompensation to keep flying at level.

2 - Fight the drag with your engine power.

Think that at these speeds u are fighting your lift to fly at level, not your weight, your weight will help u to keep at level with less compensation (nose down trim), just the opposite than at slow speeds.

[HoHun]

Hi Zigrat,

>fly at a higher angle of attack for the same lift?

I meant to say "the same equilibrium".

Regards,

Henning (HoHun)

[hitech]

Zigrat, Might be a different use of terms, i would call trim drag the addition drag created by the trim tab to move the control surface where other wise you would generate this force from the stick, This we do not model, it realy would be a very small effect.


Now if you using Trim drag to meen with a more forward CG the horizontal stab must produce more down force, and the wing must produce more lift to compensate for this extra down force. and hence more drag, yes we do.

HoHun

As to the faster with while being heaver, would have to do some big analisis to be sure, but would find it hard to belive that the reduction in profile drag would be greater than the increase in induced drag. The only way I could see this is with a very thick/high lift wing.

HiTech

[Hooligan]

A few comments:

1) Rather than all this discussion about whether or not an La-7 with full fuel should be faster, just measure it ferchrissakes.

2)  As stated before, measure it with the autotrim method and realize that this is only accurate to within a one or two mph.

3)  Take a close look at the chart.  Eyeballing the 10k speed as being 390 is not that accurate.  I enlarged the chart and printed it out.  The first thing to notice is that the speed lines are about 2-3mph thick so there will necessarily be some uncertainty in the figure you get off the chart.  When I used a ruler to measure the speed off the enlarged chart I got a speed range of 394-397 for the La-7 at 10k.

Hooligan

[FLS]

There's no noticable difference in speed or AoA between 25% fuel and 100% fuel in the AH La-7.  I'm guessing 50% fuel would be the same too.

If you want to debate the affect of weight changes on maximun level speed in RL you have to keep in mind the affect of speed on lift and drag. Changing the CG is a seperate issue.

--)-FLS----
Musketeers

[thrila]

Why did you guys have to hijack my whine thread into a science lesson?

[Naudet]

Hitech

can u give me a short defeniton what is induced and what is profile drag?

[funkedup]

RAM - Lame of you.

Naudet - http://www.grc.nasa.gov/WWW/K-12/airplane/drag1.html

And http://wright.grc.nasa.gov/WWW/Wright/airplane/drageq.html

What HT calls Profile Drag is called Form Drag in those explanations.

[thrila]

Drag?......did someone say drag, you mean this sort of drag?


Oh wait that's not drag:eek:  it's a real ugly chickl:eek:

Also, can someone help me out with this, i'm having a hard time calculating it's aerodynamic properties-

[Tilt]

Originally posted by MANDOBLE
No idea about La7 fuel tank placement.

 

La 7 had a three tank set ......2 in the inner wing areas and one on the same axis in the fuselage. Fuel was therfore held on a nuetral axis re nose / tail heavy.

Nose / tail heavy trim adjustment apart........... weight has to be over come by lift and so induces drag.

I suppose it is plausable that an AC could be so badly designed as to require such trim correction at lesser weights for level flight as to induce such drag.

Obviously such a plane would be a useless fighter aircraft for one it would never be able to successfully "unload"

Tilt

[HoHun]

Hi Hitech,

>As to the faster with while being heaver, would have to do some big analisis to be sure, but would find it hard to belive that the reduction in profile drag would be greater than the increase in induced drag. The only way I could see this is with a very thick/high lift wing.

I'm afraid my explanation wasn't as accurate as I'd hoped :-)

It's not the wing by itself that causes the effect, it's the fuselage. Angle of incidence is (for example) selected for best level speed at full pressure altitude, so below that altitude, the fuselage has a nose-down attitude at top speed in level flight. Adding weight will require more lift and a greater angle of attack, decreasing the nose-down attitude, aligning the fuselage with the airstream and decreasing parasitic drag.

I don't think the effect has a big impact, but it's interesting for the impact being opposite of what one would normally expect.  

Regards,

Henning (HoHun)

[WhiteHawk]

The heavier a plane is, the more power required by the
engine to maintain x mph.  Since the max power of the
engine is finite,  any weight added at max power would either
slow the plane or cause it to lose alt.  Or we can say lose
E where E is energy which is some function of altitude
airspeed and weight of the plane.  Those of u who have
mastered vector calculus should easily see this.  
  At any fixed thrust, any weight added to any part of the
plane bleeds e.  If the alt is fixed AND the thrust is fixed
(for ex.  full throttle and 1 inch above the ground) any additional
weight, even a bird poop on the window, will slow the plane
down a bit.  So, to the guy who cant understand why a
couple hundred pounds of fuel has no effect on top speed of a  
plane, rest assured, it does.

Next,  I am going to learn to use those faces to the left.