Well Deino, you are a chemistry teacher, so elementary math should be well within your ability. Let me lay it down to you again as simply as I can, humanly.
To suspend a
20 tons plane (included fuel) in mid air, requires 20 tons of force to push it up. To make it climb vertically, it requires 20+ tons of forces. Faster it climbs or accelerate, greater the force is required.
Force = Mass x Acceleration
Now, if a plane has two engines, that means each engine must supplies
10 tons of force each.
Still following me, so far? 20 divided by 2 = 10. I hope everybody got it. If not use a calculator.
Now, a turbofan jet generates thrust by burning fuel and compressed air in the combustion chamber, this thrust is called
Dry Thrust or military power. The pilot could also activate the afterburner and greatly increase the engine's thrust or power.
View attachment 365263
"Afterburning is achieved by injecting additional fuel into the jet pipe downstream of (i.e.
after) the turbine. The advantage of afterburning is
significantly increased thrust; the disadvantage is its
very high fuel consumption and inefficiency, though this is often regarded as acceptable for the short periods during which it is usually used.
Pilots can activate and deactivate afterburners in-flight, and jet engines are referred to as operating
WET when afterburning is being used, and
DRY when not.
An engine producing maximum thrust
WET is at
Maximum Power, while an engine producing maximum thrust
DRY is at
Military Power". --Wikipedia
So DRY or Military Power is
less than WET or
Maximum Power. Dry thrust or military power is usually 50-60% of Wet or Maximum Power.
Now, if an airplane is able to climb vertically, without the use of the Afterburner, or seeing a long long blue flame from the exhaust, then it is using Dry or Military Power only.
That means 10 tons of thrust generated by each engine is solely Dry thrust. Now, we have said Dry thrust is usually 50-60% of Maximum Thrust. Supposed, its
50%, instead of 60%, I usually used. The F135's dry thrust is 67% of maximum Thrust. Very high.
That means Maximum Thrust is
double of Dry Thrust,
Max. Power = Dry Thrust / 0.5,
20 = 10 / 0.5 , so the Maximum Thrust of
each engine is
20 tons.
1 ton is 9.8 kilonewtons. So 20 tons is 20 x 9.8 =
196kN.
So this engine Maximum Power is at least
+196kN, if Dry is 50% of Max. Power.
Now, we are assuming a plane to be weighting 20 tons including fuel.
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The F-22 is weighting 19.6 tons
empty. And J-20 is much bigger than F-22, in fact, it's body is at least 3m longer. So we need to add 2 tons of weight to make it more realistic and add 3 tons of fuels for the demo or testing.
So, now the weight of J-20 is 20 + 2 + 3 =
25 tons. It's two engines need to generate a total of 25 tons of Dry Thrust to lift it vertically, without afterburner.
Using the above calculation, I have arrived the figure of 25/2 =
12.5 tons of Dry Thrust for each engine. If the engine efficiency is 50%, we can find the Maximum Thrust by double the Dry Thrust, 12.5 x 2 =
25 tons. or
245kN.
Now, if the engine efficiency is higher than
50%, say,
60%. Then we can use the formula,
Max Thrust = Dry Thrust/0.60.
20.8 = 12.5/0.6
So, the Maximum Thrust of the engine required is 20.8 ton or
203.8kN, for a plane like J-20, weighting 22 tons
plus 3 tons of fuel.
It all depends on how much J-20 actually weights, how much fuel it was carrying when it was doing the vertical climb, and its engine's efficiency.
I am guessing those numbers, by using F-22's weight (20 tons + 2 tons) as reference, and using a low fuel weight of 3 tons, and an engine efficiency of 50-60%.
Sounds clear?
Do you got it this time?
Show the math to someone, if you still have doubts.
This is the third or fourth time that I done the calculation. I don't think I can do it better.