Avaition Question

[Lestat]

I'm not sure, but I think when a plane is staring down the runway before takeoff, it fires up it's engines to almost 85% thrust before releasing the brakes. Can anyone confirm this?

My question is, why wouldn't all that thrust cause the plane to skid on its tires? In other words, how does braking the tires stop 50,000lbs. of thrust? I don't believe I've ever even felt a plane lurch forward before takeoff, which I would expect if the plane's engines were almost at full throttle and being held back by the brakes.

Last question: So I don't have to keep bothering you guys, does anyone know of an aviation forum where dumb questions like this can be asked? Thanks.

[madnak]

Thank God. I thought this was going to be another conveyor belt post.

[MidGe]

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I'm not sure, but I think when a plane is staring down the runway before takeoff, it fires up it's engines to almost 85% thrust before releasing the brakes. Can anyone confirm this?

[/ QUOTE ]

When I used to fly, on the planes I was licensed to fly, it was full thrust with the brakes on.

Still not sufficient to overcome the locked wheel friction on the tarmac. OTOH, you could feel the stress with the breaks on, and the lurch on break release.

[Borodog]

The thrust to weight ratios of most planes is somewhat less than 1, some more, but most less. I.e. some planes could fly straight up while most others could not.

Meanwhile, the maximum frictional force that can be delivered by the wheels is the coefficient of static friction (between the rubber of the tires and the runway) times the normal force between the wheels and the ground (i.e. the plane's weight). The coefficient of static friction for rubber to asphalt/concrete is around 0.8.

So as long as the thrust to weight ratio is less than the coefficient of static friction between the wheels and the runway, the plane will not move with the brakes locked. If the engines are a little more powerful, if the thrust to weight ratio exceeds the cofficient of static friction, you will not be able to run the engines up all the way without the plane skidding. You might only be able to run the engines up to say . . . 85%.

[Go Blue]

I don't mean to hijack this post, but I was wondering what the pilot does to make the plane take off. As in, the plane fired its thrusts, it's accelerating, and it takes off at the end of the runway. What actually happens the moment it takes off? It it just because of the higher velocity of does it move some of the flaps or slats outward? I always thought it was the latter but I could never actually observe this from a window that's right over a wing.

[Sharkey]

It’s both. The plane accelerates to a minimum airspeed, then the control surfaces are adjusted to bring the nose up and begin ascent.

[Lestat]

I've sat right over the wings and you absolutely can see the changes of flap (aelerons?) position at different stages of flight, i.e. take off, cruising, and landing. Take a closer look next time.

[Lestat]

Thanks Borodog. I don't understand all the math, but it kinda makes sense now. Actually...

The reason I came up with this question is because I was watching another inside the cockpit video of a Blue Angel flight. Right before takeoff the pilot says," Ok, brakes are on and running 'em up to 85%". I'm not sure what the thrust to weight ratio is here, but obviously this aircraft is capable of an almost verticle takeoff. Yet, the brakes (less tire surface than on a jumbo het) on a F/A-18 Hornet are capable of holding this plane in its tracks at 85% thrust from these powerful engines? I guess. I realize 4 jumbo jets are probably combining to produce more thrust than the twin engines of an F/A-18 Hornet. I love this stuff! I only wish I could understand more.

Btw- I notice during high performance vertical takeoffs, the pilot usually levels out at about 7,000 feet. What would happen if he kept the plane vertical. Obviously, it would eventually stall, correct? But why? If the engines are capable of sustaining sufficient thrust through 7k feet, why not 30k? Why not into orbit for that matter? See how little I understand? -lol

[Lestat]

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I'm not sure, but I think when a plane is staring down the runway before takeoff, it fires up it's engines to almost 85% thrust before releasing the brakes. Can anyone confirm this?

[/ QUOTE ]

When I used to fly, on the planes I was licensed to fly, it was full thrust with the brakes on.

Still not sufficient to overcome the locked wheel friction on the tarmac. OTOH, you could feel the stress with the breaks on, and the lurch on break release.

[/ QUOTE ]

Interesting, because I never felt this lurch on a commercial flight. It almost seems as if the pilot releases the brakes before the engines whine to full power. I'll definitely have to pay more attention on my next flight.

[MidGe]

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Interesting, because I never felt this lurch on a commercial flight. It almost seems as if the pilot releases the brakes before the engines whine to full power. I'll definitely have to pay more attention on my next flight.

[/ QUOTE ]

In some instances the tower gives authorisation to line-up and go without wait. This is done to increase the number of take-offs and landings (within safety limits). You will note however that there is another point, a stop of a plane prior to lining up on the runway (ie off the runway) where there is a moment of full thrust (maybe less then full for military and heavy jets) applied. That is also part of normal pre take-offs checks.

[Borodog]

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Thanks Borodog. I don't understand all the math, but it kinda makes sense now. Actually...

The reason I came up with this question is because I was watching another inside the cockpit video of a Blue Angel flight. Right before takeoff the pilot says," Ok, brakes are on and running 'em up to 85%". I'm not sure what the thrust to weight ratio is here, but obviously this aircraft is capable of an almost verticle takeoff. Yet, the brakes (less tire surface than on a jumbo het) on a F/A-18 Hornet are capable of holding this plane in its tracks at 85% thrust from these powerful engines? I guess. I realize 4 jumbo jets are probably combining to produce more thrust than the twin engines of an F/A-18 Hornet. I love this stuff! I only wish I could understand more.

Btw- I notice during high performance vertical takeoffs, the pilot usually levels out at about 7,000 feet. What would happen if he kept the plane vertical. Obviously, it would eventually stall, correct? But why? If the engines are capable of sustaining sufficient thrust through 7k feet, why not 30k? Why not into orbit for that matter? See how little I understand? -lol

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As the air gets thinner the maximum possible thrust drops. I'm not sure at what height it occurs (it must be different for different engines), but eventually the thrust to weight ratio will drop belowe 1, and the plane will no longer be able to fly straight up.

[Lestat]

I wish there were somewhere I could learn more about this. I know thrust doesn't "push" off air, so why does thinner air effect thrust? Thrust can work in the vacuum of space, which has no air, right?

I did find a couple of sites that explain thrust. I'll read and see what I can understand. Thanks again Borodog!

[Borodog]

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I wish there were somewhere I could learn more about this. I know thrust doesn't "push" off air, so why does thinner air effect thrust?

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Well, thrust doesn't push off air, but the plane does. A propeller or a jet engine thrusts air back, which thrusts the plane forward. If the air is thinner, there is less air to thrust back thus there is less thrust exerted on the plane. This is offset somewhat, I believe, by the lower drag, which also drops as the air gets thinner.

I think what hat really lowers the maximum thrust as the air gets thinner is the fact that engines run off of combustion, which requires air, and as the air gets thinner the engine's maximum power decreases.

[purnell]

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I wish there were somewhere I could learn more about this. I know thrust doesn't "push" off air, so why does thinner air effect thrust?

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Well, thrust doesn't push off air, but the plane does. A propeller or a jet engine thrusts air back, which thrusts the plane forward. If the air is thinner, there is less air to thrust back thus there is less thrust exerted on the plane. This is offset somewhat, I believe, by the lower drag, which also drops as the air gets thinner.

I think what hat really lowers the maximum thrust as the air gets thinner is the fact that engines run off of combustion, which requires air, and as the air gets thinner the engine's maximum power decreases.

[/ QUOTE ]

It's easier if you think of the plane being pulled forward by the difference in air pressure between the area ahead of the prop/intake and the area behind it. The pressure of the "thinner" air can't be reduced as much as can the more dense air at lower altitude, because it starts out at a lower pressure.

[MrMon]

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Thanks Borodog. I don't understand all the math, but it kinda makes sense now. Actually...

The reason I came up with this question is because I was watching another inside the cockpit video of a Blue Angel flight. Right before takeoff the pilot says," Ok, brakes are on and running 'em up to 85%". I'm not sure what the thrust to weight ratio is here, but obviously this aircraft is capable of an almost verticle takeoff. Yet, the brakes (less tire surface than on a jumbo het) on a F/A-18 Hornet are capable of holding this plane in its tracks at 85% thrust from these powerful engines? I guess. I realize 4 jumbo jets are probably combining to produce more thrust than the twin engines of an F/A-18 Hornet. I love this stuff! I only wish I could understand more.

Btw- I notice during high performance vertical takeoffs, the pilot usually levels out at about 7,000 feet. What would happen if he kept the plane vertical. Obviously, it would eventually stall, correct? But why? If the engines are capable of sustaining sufficient thrust through 7k feet, why not 30k? Why not into orbit for that matter? See how little I understand? -lol

[/ QUOTE ]

The F/A-18 C/D Models have 36,000 pounds of thrust. Takeoff weight is between 16,651 kg and 25,401 kg or 36,632 lbs and 55,882 lbs. For the Blue Angels, assume the lower weight. With a nearly 1:1 thrust to weight ratio, the 85% is probably just short of the static coefficient of friction. If they went higher, it would skid down the runway.

F/A-18 E/F models have 44,000 lbs of thrust.

The Boeing 747-400 has total thrust of 63,300 lbs using the most powerful engines, but has a max takeoff weight of 875,000 pounds!

As far as going into orbit, you'd run out of air eventually. The engines are air breathers, not rockets that carry their own oxidizer. Why not climb to 30,000 ft? Highly inefficient use of fuel. You could do it, but you'd have to come back down quickly as you'd be out of fuel. A quick takeoff to 7,000 ft. is a combat maneuver, I believe, after that you use the wings. Much better use of fuel.

[Sharkey]

Both jet and rocket engines work by accelerating a reaction mass backward, which, due to the conservation of momentum, creates a recoil on the vehicle forward.

[Lestat]

Thanks Mrmon! Just curious how you know so much about this? Is this your field? Do you know of anywhere I learn more? Thanks again.

[Lestat]

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Both jet and rocket engines work by accelerating a reaction mass backward, which, due to the conservation of momentum, creates a recoil on the vehicle forward.

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Right, but air wouldn't/shouldn't affect thrust. I think Mrmon's explaination makes sense. The engines "use" and breath air. So thrust wouldn't fail in thinner air, but the engines eventually would.

[Sharkey]

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Both jet and rocket engines work by accelerating a reaction mass backward, which, due to the conservation of momentum, creates a recoil on the vehicle forward.

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Right, but air wouldn't/shouldn't affect thrust. I think Mrmon's explaination makes sense. The engines "use" and breath air. So thrust wouldn't fail in thinner air, but the engines eventually would.

[/ QUOTE ]

This is probably an easy one to answer for someone who knows the technical details. I can say this much, that the acceleration the engines provide comes from the combustion of fuel using atmospheric air, and the reaction mass itself is available air forced out the engines. Seems like a thinning of the atmosphere could only reduce engine power.

[MrMon]

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Thanks Mrmon! Just curious how you know so much about this? Is this your field? Do you know of anywhere I learn more? Thanks again.

[/ QUOTE ]

Google. It took 30 seconds.

[Lestat]

I gotta learn how to google.

[HLMencken]

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The reason I came up with this question is because I was watching another inside the cockpit video of a Blue Angel flight. Right before takeoff the pilot says," Ok, brakes are on and running 'em up to 85%". I'm not sure what the thrust to weight ratio is here, but obviously this aircraft is capable of an almost verticle takeoff. Yet, the brakes (less tire surface than on a jumbo het) on a F/A-18 Hornet are capable of holding this plane in its tracks at 85% thrust from these powerful engines?

[/ QUOTE ]

I believe that 85% thrust in this case is non-afterburner. If afterburner is used for a full military power takeoff, then afterburner is not lit until after brake release. I think. Google may help but I didn't see anything right off the bat.