Making posts in the General Discussion area does not increase your post count. (Mine doesn't change here either).

- Jack

 

There is a big difference between these two crashes. In the first (latest) one the aircraft was in a dive and had downward momentum, the pilot had no option to lower the nose to gain airspeed, that would have just sped up the crash.

In the second (earlier) clip, the plane was neither gaining nor losing altitude and the pilot could have lowered the nose without losing altitude, it was a recoverable condition similar to the Air Florida crash in DC.

Lift is linearly proportional to the angle of attack and exponentially proportional to speed. Double the angle of attack and you double the lift(constant speed). Double the speed and you quadruple the lift (constant angle of attack). In the second clip, if the pilot had cut the angle of attack by 25% by lowering the nose, and lost 25% of the lift, the resulting increase in speed would immediately recover the lost lift and more, it was a recoverable situation, the first was not, the plane was going to strike the ground no matter what the pilot did.

 

I'm sorry, we seem to be looking at different videos. The Canadian crash that prompted this thread happened when a CF-18 pilot was practicing a slow speed, high angle of attack, level pass, maybe 50 to 100 feet above the ground. The "dive" occurred after he punched out.

The Sabre Dance was also low, level and slow. But, he was VERY close to the runway (20-30 ft?) and might have survived the impact if he had lowered the nose before the aircraft started skidding sideways.

Neither of these guys really had the option of lowering the nose without hitting the ground - to do so would have reduced the angle of attack, reducing lift and causing a descent before the airspeed built up enough to get them out of trouble. Both would have been on their bellies on the ground, due to that loss of lift almost immediately.

And Tyson, my Bachelor's degree is Aerospace Engineering from Ga Tech. I'm VERY familiar with lift and drag equations. I also have about 4500 hours as a Pilot/IP/Flight Examiner in Air Force planes, most of it in jets. Additionally, I have a sailplane license.

The error you are making in your discussion of the lift equation is that you are assuming there will be an almost instant increase in speed if you reduce the angle of attack. An old gent named Newton said that just ain't gonna happen. But, there will be an instantaneous decrease in lift due to the reduction in alpha, so you will drop, like a stone. And, even if you DO have enough altitude to build up the airspeed sufficiently, you still have to pull out of the dive you're now in, requiring even more lift (and maneuvering room)!

In an Air Force fighter, if you lose control below 10,000 feet, you are supposed to eject. I think the Navy has a similar rule. Notice this does not say, "Try to recover and if that's unsuccessful, eject." When the aircraft is no longer responding to your input, you get out. Above 10,000 feet you get to try to regain control, but if you don't see a response by 10,000, you've just bought an airplane. The Canadian pilot did just that when he no longer had control.

You'll notice too I did not say your discussion of density altitude was incorrect. The term "rho" in the lift and drag equations incorporates this factor. Of course your takeoff roll is longer in the summer or at someplace like Colorado Springs, and, your true airspeed is higher for landing under these conditions, because to get the same lift, you need a higher V to counter the reduced rho. Your thrust is also reduced under the same conditions, so it's a bit of lose-lose, isn't it?

Anyway, peace! I was not trying to start any arguments. The crash looked like more than simple "stall" to me and the IP in me came out.

- Jack

 

the sign in front of the fireball....entering vehicle control area.... NOT.

 

Dang that was a bad one. I agree with Jack, basically as soon as the pilot lost control he was screwed. He was very technical with it, bit it all boils down to he just didn't have the room to regain control.

On another not, the power to weight ratio of these planes are amazing. The "High Alpha" maneuvers pilots can perform and pull out of them are amazing to watch.

 

I wonder if he lost his career for that

 

I guess I didn't see the full clip on the Canadian, I just assumed that he was doing an aerobatic maneuver since a 'slow' pass in an F18 ain't too slow.

And I'm not trying to argue, at least needlessly, I learn from these discussions.

My point is, that with power on, at a high angle of attack, the thrust from the motor is balanced by aerodynamic drag. Lower the nose and the drag is dramatically lowered resulting in an almost instantaneous increase in speed. The increase in speed immediately replaces the lift lost by the lower angle of attack without a loss in altitude. At least that's how it works in my little plane that weighs 1600 pounds. It is conceivable to me that this may be slightly different in a plane that weighs over 20,000 pounds.

 

Absolutely correct! (And this was my point). However, if you allow the aircraft to get slower, the drag increases sharply requiring more thrust. And, at a higher angle of attack, the thrust vector is rotated more towards the vertical so that we're actually trading forward thrust (to overcome the drag) for lifting thrust. The aircraft is suddenly kind of "balanced on its tail" and needing more power to fly slower! You don't have enough power in a general aviation airplane to do this (to this extreme).

It took me a while to find this and I was almost ready to eat my words about dropping into a region of insufficient thrust, but I finally found what the Canadians are flying. It's the F-18A and here's a link to that information: http://en.wikipedia.org/wiki/CF-18_Hornet

Basically, it retains all the weight of the original Navy carrier version and the early engines. The max static thrust is 32,000 lbs and the empty weight of the machine (no fuel, pilot or stores) is 23,049. The "loaded weight" is 37,150, so I think we can safely assume this plane was probably at or near a 1:1 thrust:weight ratio. If the pilot was not quick enough with the power, he would get behind the drag rise, and this was my whole point. Things happen real fast in that region.

If he had rotated the nose down, he loses not only the lift provided by angle of attack, but also the lift vector provided by the engine thrust. I didn't see the burners light, so either they were already on, or things happened too fast for him to do it. Regardless, a "recovery" at this point was going to cost him altitude and he didn't have that available.

As I said before, this is a standard maneuver in airshows and I wish they wouldn't do it. It has no tactical value whatsoever. If you're at low altitude, you want to be at "best corner speed", which gives you the ability to maneuver in a confined area. Pilots learn to control their aircraft at low speed (high alpha) of course, since it teaches "finesse", and an appreciation for how quickly things can deteriorate, but that training is done at altitude.

In a fighter, "Speed is life!"

- Jack

 

Agreed, the thrust to weight ratio is way different than anything that I've flown.

I think that it's (low speed flight) exceedingly valuable at low altitude too, because that's how you land the plane, although this guy was probably going much slower that an average approach speed and relying on vector thrust for lift. I feel bad for him, I would imagine that it's a career ending mistake.