Flight Mechanics Lectures - Snippet #2

Added: 2026-05-19

[00:00:03]Let us actually dwell into that before we go any further. I think it'll be mathematically simpler.

[00:00:09]So, >> [clears throat] >> the diffuser, right? I'm going to draw it here.

[00:00:17]Well, I have the I have the picture.

[00:00:20]I don't need to draw anything.

[00:00:30]The diffuser here, this is the diffuser.

[00:00:38]It's going to increase our static pressure.

[00:00:44]It's going to increase our static pressure as a function of, of course, the dynamic pressure.

[00:00:51]Not all of the dynamic pressure is going to be converted to static pressure. That would be impossible. That would require the air to become entirely stationary, which is not what you want. But, you know, you're going to get a certain increase of pressure, delta pressure, that is going to be more or less equal to the dynamic pressure.

[00:01:16]Dynamic pressure multiplied by um an efficiency um factor.

[00:01:22]Uh let's call it delta D uh eta D for diffuser.

[00:01:28]So, it means Hmm.

[00:01:39]That is equal to if we substitute this.

[00:01:47]This multiplied by 1/2 V squared inlet multiplied by the density.

[00:01:57]Now then, I am going to go right ahead and state that for World War II kind of manufacturing plant efficiencies, they they knew about diffusers quite well, but you know, in serial production things cannot be ideal and they didn't have computers to make it as accurate as possible. I'm going to assign it a value of 0.80 80% efficiency for this um for this diffuser.

[00:02:29]We'll just roll with that and see if it gives us a reasonable answer or not.

[00:02:34]Um so we have here an increase of pressure derived from the shape of the diffuser.

[00:02:45]But of course, this air is going to impact the radiator.

[00:02:49]And the radiator is essentially a collection of tubes and small passages for fluid to be in contact with the air.

[00:02:57]But whether you like it or not, even if you make the most brilliantly manufactured streamlined beautiful radiator, it is something you're putting in front of the air. The air literally has to go through it. So it's going to incur some losses.

[00:03:11]The beauty of the Meredith effect is that you have a positive pressure differential.

[00:03:16]This contribution here which just in case we're clear, we're talking about this side of the equation first. We'll deal with the first one later. But on this one, to have a positive differential of pressures where the exit pressure is better or more beneficial than the atmospheric pressure, you need to make sure that the extra pressure gain that you get from the diffuser is higher than the losses that you are going to be suffering from making the air go through the radiator. That obstacle you're putting in front of the air.

[00:03:49]So can you make the arithmetic so that your gain of pressure is superior to your losses of pressure? Well, let's let's mathematically find that out. So, I'm going to call the delta of losses delta L.

[00:04:05]And that's going to be equal, well, more or less approximately equal to K, which is a factor of losses, multiplied by 1/2 of the density times the velocity at