Why still no dive acceleration difference?

[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.

[BlackBerry]

Originally Posted by MadBlaster

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.

MadBlaster, When your la7 diving at 20 degree with constant speed of 610km/h, the air drag force is completely counteracted by g*sin(20), and you turn on engine, so that you add extra thrust from engine, although the engine thrust is smaller than airdrag force, this is the excess thrust.

Both gravity---g*sin(20) and engine thrust are allied, the sum of these two, are counteracting against air drag force, since the sum of them is greater than air drag( at 610km/h), your la7 speed increases, when you reach 700km/h, air drag is quite more than g*sin(20), so a portion of engine thrust(eg 40%) will be used in completely 100% counteracting air drag. The excess thrust is from the left portion of engine thrust(60%). The more efficiency, the more excess thrust you get, understand?


If you dive at 60 degree angle reaching 700km/h, of course, your engine thrust could not 100% conteract the air drag by itself, so you need gravity--g*sin(60) to help you, if propeller efficiency is high, you need less gravity to help you, and more gravity will be used as excess thrust. if you turn off engine(efficiency=0), you need the most gravity to help, thus minimum excess thrust.

Therefore, excess thrust comes from the sum of gravity and engine thrust. Gravity and engine thrust help each other, this is teamwork, if one of them performs better, the other will have more ability to counteract air drag, Understand? The more propeller efficiency, the more sum, the faster dive.

[MadBlaster]

Originally Posted by BlackBerry

MadBlaster, When your la7 diving at 20 degree with constant speed of 610km/h, the air drag force is completely counteracted by g*sin(20), and you turn on engine, so that you add extra thrust from engine, although the engine thrust is smaller than airdrag force, this is the excess thrust.

Both gravity---g*sin(20) and engine thrust are allied, the sum of these two, are counteracting against air drag force, since the sum of them is greater than air drag( at 610km/h), your la7 speed increases, when you reach 700km/h, air drag is quite more than g*sin(20), so a portion of engine thrust(eg 40%) will be used in completely 100% counteracting air drag. The excess thrust is from the left portion of engine thrust(60%). The more efficiency, the more excess thrust you get, understand?


If you dive at 60 degree angle reaching 700km/h, of course, your engine thrust could not 100% conteract the air drag by itself, so you need gravity--g*sin(60) to help you, if propeller efficiency is high, you need less gravity to help you, and more gravity will be used as excess thrust. if you turn off engine(efficiency=0), you need the most gravity to help, thus minimum excess thrust.

Therefore, excess thrust comes from the sum of gravity and engine thrust. Gravity and engine thrust help each other, this is teamwork, if one of them performs better, the other will have more ability to counteract air drag, Understand? The more propeller efficiency, the more sum, the faster dive.

yes blackberry. this example is rigged though. if i spawn from 3000 meters and V max with the engine off, I can also create acceleration or de-cceleration simply by changing the angle of the dive and the result is a new terminal velocity/equilibrium. it's the change in the sum of the two vector forces, not simply "the sum of" the two vector forces. the thrust vector is still zero in this case.

But we have been assuming up to now the engine is on, level flight and v max.

[Crumpp]

it's the change in the sum of the two vector forces, not simply "the sum of" the two vector forces.

well said.

You understand Blackberry that is the excess force that moves the aircraft it is new equilibrium point.

The summing of the forces determines the vector of motion but the rate of change in motion along that vector is a function of the excess force.

For example, when the engine is off our thrust = 0 but our drag force remains. The summing of the forces results in a negative vector and our aircraft slows down as it seeks a new equilibrium point.

If the pilot pushes the stick forward to control the angle that he can shift a component of weight in order to counter act that drag force to maintain velocity or even speed up.

[BlackBerry]

To determine aceleration, we need the amount of excess force along our vector of motion. It is that excess force that causes the aceleration.

Right.

However, what is equilibrium point? In my opinion, the equilirium point is the point where all forces are well balanced.

You dive in 45 degree with 110% WEP, your speed is always increasing, BEFORE you reach the so called " equilibrium point", you lost your wings.

Try il2 4.11m, use Tempest, P51D, P47, fw190d,bf109K, La7, to dive in 45 degree with full throttle, you can NEVER find a equilirium point where the speed stops increase.Could you? Could you? Why? The equilirium point is sth 1300km/h! Those planes will explode before reaching 1300km/h, believe it or not. They are not supersonic a/c.

If you dive in a very shallow angle, for example, dive at 10 degree, the equilirium point is sth.700-800km/h which piston planes could withstand. Yes, you can reach this equilirium point.


As I said again and again, dive steeply. If you dive at 45-60% degree, you are always accelarating, you are always increasing your speed, therefore, before you pull to level flight, there is no equilirium point at all in your dive.

Since there is no equilirium point, the Newton 2nd rule tells us:

Force=acceleration*mass

Thus:

dive acceleration=(Excess Thrust in steep dive)/aircraft weight=(engine thrust + gravity along dive- air drag)/(aircraft weight.)

Imagine that there are two la7, "A" is equipped with high efficiency propeller, the other "B" with low efficiency prop, anything else being equal.

At any piont on the 45 degree dive, A always has more engine thrust than B, thus A always has bigger excess thrust than B, finally, A always has better dive acceleration than B.