Why still no dive acceleration difference?

[BlackBerry]

The efficiency is nearly constant in any portion of the envelope that design can sustain flight....

That is the beauty of a CSP.

Crumpp, this thread is about DIVE, in a high speed dive, you can easily reach very high TAS which, of course, you can not sustain. Finally, as you pull out of dive and begin to level, the aircraft will slow down to its max. level speed, isn't it? The late dive peroid is a temporary flight process in which you may have 25% efficiency advantage over your opponent bacause both propellers could not maintain constant efficiency(85%), namely your 4-blade prop get 80% and your enemy just have 55%, this is a huge tactic factor. Forthermore, from dive terminal speed to max. level speed, that is the slowdown peroid, you still have efficiency advantage. So how long is your advantage peroid? 40seconds? 1 minute? It depends on your altitude to dive from. You are bleeding his energy in this peroid even if your enemy is the best pilot in the world! You are stealing his energy for 40 seconds by the "hand of God". Now you can run away with ease, out of his shooting range. This is called "OUTDIVE".

The very nature of power producers is such that the faster they go, the less thrust they produce. The reverse is also a characteristics of power producers. The lower the velocity, the more thrust they produce. That efficiency drop occurs because the propeller blades are stalled just like in very high speed flight. The reason is different but believe me, both realms, high and low speed, produce stalled blade portions. In the low speed realm, we are looking at speeds at taxi and the first part of take off but our thrust force is extremely high at low velocity. Therefore, in the scheme of things, it is a useless detail to include the reduction in efficiency in a dive. The performance is not sustainable in the first place and our reduction in thrust with velocity is already well approximated by:

As I said above, if you are stealing 500HP from your enemy for 30 seconds, he loses "hundreds meters altitude" energy! Imagine that in a il2 combat you decrease throttle to 65% for 30 seconds, that's suicide.

In a high speed flight, a little thrust will give you a lot power, don't forget:

Power=thrust*speed

It is output power not thrust determines your energy state.

[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.

[Crumpp]

As I said above, if you are stealing 500HP from your enemy for 30 seconds

Who is stealing 500 hp???

How much excess excess thrust does a P47 have at 20,000 feet in level flight at Vmax?

NONE

How much excess thrust does any propeller airplane have at Vmax?

NONE

If our airplanes Vmax is 420mph EAS and we dive to Vne at what speed does our Excess Thrust produced by the Propeller = 0?

Vmax

How much Excess power does a CSP propeller aircraft have from cruise flight to Vmax to devote to a dive excess thrust?

All of the excess thrust produced to Vmax...at Vmax our excess propeller thrust = ZERO

What speed does a CSP equipped aircraft traveling at cruise flight reach zero excess propeller thrust at in a dive?

At Vmax....the same speed as in level flight

What is the design propeller efficiency for a CSP in an aircraft envelope from Vs to Vmax?

n = .85 from Stall to Vmax

[MadBlaster]

Crumpp, that doesn't seem quite right based on the diagram you posted a few pages back. Peak efficiency 2.2 at Vmax. but, you can still move down and to the right of the curve towards less efficient (advance ratio increasing for a given blade pitch) as the tip speeds increase and approach mach??? As long as the efficiciency value is > zero, don't you have some thrust being produced by the propellar?

edit:
referring to post #143.

[Crumpp]

As long as the efficiciency value is > zero, don't you have some thrust being produced by the propellar?

Sure you have some thrust being produced but it is not EXCESS thrust. It is excess thrust that accelerates the airplane.

In a dive from Vmax, the only excess thrust is the component of weight.


Propeller thrust only accelerates the aircraft within its envelope.

[MadBlaster]

okay, I think I get it. you said it three posts back, I just didn't sink when I read that the first time.

[MadBlaster]

btw, this has been a very interesting thread. thnks for your input guys. i learned some things.

[BlackBerry]

Originally Posted by Crumpp

Sure you have some thrust being produced but it is not EXCESS thrust. It is excess thrust that accelerates the airplane.

In a dive from Vmax, the only excess thrust is the component of weight.


Propeller thrust only accelerates the aircraft within its envelope.

Crumpp, I totally disagree with you. Imagine that you are now flying a la7 with 110% throttle and you get Vmax speed. The efficiency of propeller is 80%, yes, you propeller are producing thrust(not big) which is equal to La7's drag force. Namely, La7 is in equilibrium. You have no excess propeller thrust, right? Till now, we should have no bifurcation.

And then you suddenly turn off your engine and feather your propeller, It's obvoius that you are about to lose speed because your engine is dead, but you push the stick forwards and begin to dive. A portion of gravity will help you counteract the dargforce and make you maintain Vmax during your "dead enigine" dive. If you dive in a more steep angle, you could even faster than Vmax. in another word, you are spending altitude to maintain your Vmax or get even more speed.

During your "dead enigine" dive, if you suddenly turn on your 1850HP engine, will you dive faster? Will engine give you excess thrust?

In my opinion, definitely, you will get greater dive acceleration because your propeller efficiency is 80-60%.

In your opinion, prropeller efficiency suddenly become zero from 80% when aircraft speed is somehow higher than Vmax.

Propeller thrust only accelerates the aircraft within its envelope.

Propeller thrust could accelerate the aircraft beyond its envelope, as long as gravity force comes to help aircraft-----DIVE.

[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.