Acceleration and Deceleration G force

[747 forever]

Hello,
We all know takeoff on an airplane is fun, sometimes especially when you get pushed against your seat. And don`t forget deceleration when landing. But I was thinking, how much Gs do we experience during Takeoff and landing? When heavily loaded its less and lightly its like a rocket. Can I compare how much Gs there are on takeoff both fully loaded and lightly loaded. Same with the landing. Im talking about big commercial jets like the 747, a380, 777 etc...
Cessna`s are not much since they are light

[JimmyTAP]

If I recall, a normalish take-off was around 0.2 to 0.4g longitudinally. A landing is normally much gentler unless max braking was used and I think that around 0.5g.....it was a long time ago though.

[Tarnished]

Simple maths I think:

What is the take off speed and how long did it take you to achieve it?

Say it takes a fully loaded 747 about 50 secs to reach 150 kts

"g" = 32.2 ft per sec per sec

150kts = 250 ft per sec (150 x 6000 /3600) 6000 ft in a nm and 3600 secs in an hour

Therefore accel is 5 ft per sec per sec or 0.15 "g"s

Now say a Typhoon hits the same speed in 10 sec (for ease of math, it is quicker than that)

Typhoon accel is 25 ft per sec per sec or 0.77 "g"s

Super Hornet on a Cat shot? About 3"g"s

You do the maths for a Cessna!

A lot of the sensation of high acceleration you "feel" in an airliner is because you are sinking into a comfy seat and you feel it with a lot of you body surface area

[747 forever]

thanks man,
the only thing I find hard in maths like that is what numbers to put in. Ill get the hang of it with some help. Also some airlines don`t have comfy seats so acceleration is not as comfy as some airlines XD

[747 forever]

But can I ask, and I may be stupid. But how can it's be 0.5Gs etc? Cause everyday we experience only 1G on earth when not moving. I thought that it would be higher. Well I might find out later and end up being stupid

[BOAC]

747 - in simple terms it means that you will accelerate along the runway at half the rate at which you would accelerate towards the earth if you stepped off a high structure. (Don't try this at home)

[747 forever]

Ohhhhh so it means it adds 0.5 to the current Gs you experience. So when on earth you only feel 1g but at takeoff you will feel 1.5g

[Tarnished]

We all have 1g acting vertically downwards to keep us stuck to the surface of the earth. The figures we worked out were horizontally acting g forces - along the direction of the runway. The "true" force you feel is actually a vector sum of both (all) the forces acting on you.

[BOAC]

Ohhhhh so it means it adds 0.5 to the current Gs you experience. So when on earth you only feel 1g but at takeoff you will feel 1.5g - I think you are confused! As Tarnished says, we normally accelerate along a runway on take-off and not down towards it. Try drawing on a piece of paper?

[747 forever]

Sorry, now I get it, then how much would you feel on that roller coaster that accelerates then goes up and then straight down?

[Tarnished]

If you get it then you can answer your own question!!!!!

Assuming that you found a roller coaster with a true vertical incline and a true vertical decline (and that after the initial acceleration away from the station) you would experience a deceleration of 1g going up and an acceleration of 1g going down (assumes no friction or wind resistance).

slowing down or speeding up at the rate of 32.2 feet/sec every second

Also ignores the bit going into the climb and out of the dive where we enter the realm of centripetal acceleration which is determined by the speed and turn radius.

[Tarq57]

A rough rule of thumb I've used is to take the static thrust produced (total) and divide it into the loaded weight of the aircraft.

You'll see that most of the big twins have a thrust/weight ratio of about a third, and the big fours about a quarter.

So something like a 74 will accelerate (horizontally) at about a quarter of a g after brake release; a 76 at about a 1/3 of a g. The figures would vary a bit depending on the actual weight of the a/c on the day, and the amount of thrust actually used, which is often less than max available.

The reason a twin has a better ratio than a four is that it still has to be able to climb with a 50% power outage, so it's usually a bit more overpowered so as to be able to achieve that (+ the required margins).

Subjective sensation as a passenger pretty much bears this out. Be aware that the acceleration near the beginning of the takeoff run will be a little higher, as there is little or no wind resistance (drag) to reduce the rate. This is slightly offset by the engines producing a little more thrust as the speed picks up.

The exception appears to be the 777. Thrust/weight ratio is closer to that of a triple, like a DC10. I think it's partly because the autopilot automatically chucks in a bit of rudder to counter yaw if a donk quits, and the wing is a bit more efficient, resulting in an improved climb performance with one out. Perhaps someone who flies one might answer.

The quickest light aircraft I've flown was a PA28-236. It could get to 60kt in about 7-8 seconds, lightly laden. Maybe a quarter of a G.

[Wirbelsturm]

I can only add that the one time I was lucky enough to get a Catapult shot in an F14 it was like being hit by a massive cricket bat!

Never forget it but I'm damn glad I didn't have to do it every day like the nice chap in the back!!!