Why still no dive acceleration difference?

[BlackBerry]

I drew these pictures. Hope you understand my opinion about excess thrust.

When speed above Vmax, engine thrust is always smaller than air drag force.

w.jpg

q.jpg

[BlackBerry]

Originally Posted by Crumpp

You got it!

One small tweak though....



It is not that you cannot have peak efficiency beyond Vmax. You could design such a propeller but there is little point as all performance the airplane cannot sustain is instantaneous. Of course there would be some serious design trade off's to gain such performance as well. A supersonic propeller design would not work very well on a WWII Piston fighter.

http://www.aerospaceweb.org/question...s/q0031b.shtml

The CSP is designed to maintain peak efficiency through the designs sustainable envelope.

Speed in IAS.

This is my test in Il2 4.11m, Tempest cruising 250km/h @3000 m altitude, and dive to 700km/h, then I tried to maintain 700km/h by lowing my altitude in a shallow dive, after getting on deck at 700km/h, Tempest was slowed down to Vmax-600 km/h.


In this whole process, there is 70 seconds during which tempest speed is between 650km/h and 700km/h(IAS). And it took 40 seconds to slow down Tempest from 700km/h to 600km/h on the deck. Don't forget this is in low altitude where the air is thick--high density, if you dive at high altitude where air density is much less, you could hold a longer time in high speed.


If a La7 follows me from the very beginning with same energy, I can easily drag him down to his low efficciency zone----650-700 km/h IAS for 70 seconds. During this period, I could steal several hundreds of HP from him for more than 1 minutes, it's a HUGE energy loss for La7.

high speed dive.zip

[BlackBerry]

Originally Posted by MadBlaster

come on now. 30 seconds...40 seconds??? in la7 or tempest or whatever, you cruise at 3000 meters then enter dive, it takes just few seconds to hit max level speeds and few seconds more to hit max dive speeds. not that long.

Come on, when my Tempest MKV dives to deck and level fly reaching 780km/h, how long will it take to slow down my Tempest to 600km/h? Only just few seconds?

Are you kidding me?

[MadBlaster]

it depends on what dive angle you took after you turned off your engine at Vmax level and entered your dive. if the dive angle was steep enough, you will continue to accelerate due to gravity. if it is too shallow a dive angle, you will start to slow down. in the latter case, if you are below Vmax in the dive due to too shallow a dive angle and turn on your engine, you have potential excess thrust to tap into and get more acceleration from your prop. However, if you are above Vmax because your dive angle was steep, turning on your engine won't give more acceleration because you can not create excess thrust from your propeller at that point. Your TAS is too high. You are moving down and to the right on your efficiency/advance ratio curve for the given blade angle. Any excess thrust is coming from your dive angle/gravity. When your TAS is high enough that you get no thrust, you should lower your rpms to reduce your drag profile, provided you still have room to your Vne speed.

Come on, when my Tempest MKV dives to deck and level fly reaching 780km/h, how long will it take to slow down my Tempest to 600km/h? Only just few seconds?

Umm, who is chasing who here? Tempest has much more dive room than LA7. Push it to ~ 850 kph in your dive extension and zoom climb out from there. Don't waste the zoom energy to 600 kph unless your exiting the fight. Also, Tempest performs better at lower alt.

Face it BlackBerry, Crumpp won.

[BlackBerry]

Originally Posted by MadBlaster

However, if you are above Vmax because your dive angle was steep, turning on your engine won't give more acceleration because you can not create excess thrust from your propeller at that point. Your TAS is too high. You are moving down and to the right on your efficiency/advance ratio curve for the given blade angle. Any excess thrust is coming from your dive angle/gravity.

You are wrong, LOL. I'll show you with La7's data. When La7 get Vmax=610km/h at low altitude with 1850 HP engine output.

power=thrust*speed

thrust=power/speed=(engine output)*(prop-efficiency)/speed=1850*0.735KW*85%/(610000/3600)=8KN=816kgf

The thrust force is equal to 816kg weight, that is 33% of la7's weight(2.5 tons).

The air drag force is also 816kg force, so la7's speed is steady=610km/h=170m/s.

If La7 dive in a angle of "A" in order to get a portion of gravity for help.

sin(A)=0.33

A=19.3 degree.

So you can turn off your engine/feather your prop and dive in a 20 degree angle, I'll bet that you can sustain 610km/h WITHOUT ENGINE and WITH THE HELP OF GRAVITY.

If you dive in 45 degree, I promise your "dead engine" La7 will be faster and faster untill lost your wing(>730km/h). When you reach 650km/h, you turn on your engine, the thrust is not as big as 816kg, but still around 500kgf, that is, you add "half ton" thrust to your diving La7.

Believe it or not.

In your opinion, la7's propeller will provide zero thrust @650km/h because this is out of "envelope"/Vmax.

Try it in il2 4.11m with your buddy's la7, you shut down engine when speed is above 610km/h and he is still using 110% power, and check if he could pulll away from you or not.

And try to shut down your enigne when you want to escape from the battle field by high speed diving(above Vmax) and when the enemy is chasing you with his 110% WEP. If you dare do that, you'll be caught by him even his aircraft is slower than you@level flight.

Umm, who is chasing who here? Tempest has much more dive room than LA7. Push it to ~ 850 kph in your dive extension and zoom climb out from there. Don't waste the zoom energy to 600 kph unless your exiting the fight. Also, Tempest performs better at lower alt.

Cruising at 3000 meters and dive! In order to prevent the poor la7 from losing his wings, my Tempest MKV will carefully control the speed below 740km/h, and I'll show you how long I can stay in the high speed flying status.

[MadBlaster]

[/QUOTE] In your opinion, la7's propeller will provide zero thrust @650km/h because this is out of "envelope"/Vmax.[QUOTE]


no, that is not what I'm saying. it may provide thrust. but it is not "excess" thrust. that is the key here. excess thrust, excess thrust, excess thrust. it is not excess thrust because Crumpp posted a diagram on csp propellar that shows you can not have peak efficiency beyond Vmax. The only way to get beyond vmax and create excess thrust is to dive at the necessary angle. go back and look how he defined excess thrust. it's the difference between the two force vectors. in level flight, the force vector from gravity has no forward direction. at vmax and level flight, there is no more opportunity to create excess thrust from the prop. you have to dive to create excess thrust and acceleration.