An aerodynamics question...I think?

RyanB

RyanB

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It’s 11:20pm and I have one of those late night questions.

Driving a car on the road at high speeds requires one to slow down in order to make a turn. In other words, if you’re traveling at 100mph on a straight road and then a 90deg turn to left is ahead, you would have to greatly reduce speed in order to make that sharp of a turn without going off the road and doing extreme damage to the vehicle. So how do airplanes, specifically jets make these sudden heading changes without having to reduce speed? In little airplanes, if we were to do something like this, we may feel some negative g’s, letting us know there’s some stress on the airframe, but sitting in an airliner traveling at speeds that are 10 fold that of a light aircraft and you don’t hardly feel anything at all.

Enlighten me.
 
We experience positive g's in any non-inverted changes in heading which don't involve changes in altitude. Military jets that make sudden changes in heading impose similar g's.

The g's are proportional to V^2/R where R is the radius of the flight path, and V squared is the square of the flight path velocity. The resulting airframe and pilot stresses tend to be very high in high velocity aircraft, and the flight paths use up a lot of space.

Someone have a better explanation?
 
nrpetersen said:
We experience positive g's in any non-inverted changes in heading which don't involve changes in altitude. Military jets that make sudden changes in heading impose similar g's.

The g's are proportional to V^2/R where R is the radius of the flight path, and V squared is the square of the flight path velocity. The resulting airframe and pilot stresses tend to be very high in high velocity aircraft, and the flight paths use up a lot of space.

Someone have a better explanation?
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Thank you. A lot of that is over my head as I didn’t excel in Physics class, but I appreciate you taking a stab at it. It’s mostly one of those things that still amazes me when you think of how fast these jet aircraft are moving through the air and the level of stress that is placed on them.
 
Ryanb said:
It’s 11:20pm and I have one of those late night questions.

Driving a car on the road at high speeds requires one to slow down in order to make a turn. In other words, if you’re traveling at 100mph on a straight road and then a 90deg turn to left is ahead, you would have to greatly reduce speed in order to make that sharp of a turn without going off the road and doing extreme damage to the vehicle. So how do airplanes, specifically jets make these sudden heading changes without having to reduce speed? In little airplanes, if we were to do something like this, we may feel some negative g’s, letting us know there’s some stress on the airframe, but sitting in an airliner traveling at speeds that are 10 fold that of a light aircraft and you don’t hardly feel anything at all.

Enlighten me.
Click to expand...

I got no idea how negative G’s gets into this so maybe I’m not understanding the question. The answer to the question why a fast airplane can make a 90 degree without slowing down and running off the road like a car would is there ain’t no road, the airplane is in the air. How does little airplanes getting negative g’s Get into this? What exactly is your question?
 
To make a turn, something needs to provide centripetal force (that's a force that "pulls" something towards the center of a circular path).
The magnitude of this force is V^2/R.

So "who's doing the pulling"? What force is doing that job?
In a car on a *flat* road, it's friction between the tires and the road, resisting motion sideways, that does the pulling. But friction can only "hold on" with so much force before it'll slip, so it can't hold you in the turn if V is too fast, or if R is too tight. If there's no friction at all (if the road is icy), the turn can't be made at all.

In an airplane, it's the horizontal component of the lift force. And those wings can put out a lot of force! Direct that force horizontally, by banking the plane, and you can exert a lot of force to take a turn that's faster. The centripetal force comes from a completely different animal.

As an added example, consider a car on a *banked* racecourse, rather than on a flat road. In that case, the normal force (perpendicular support force from the road) is at an angle, just like the lift force from an airplane's wings, and the horizontal component is doing the pulling towards the center of the circle. Which means that friction doesn't need to do as much. Which is why a car can take much higher speed or tighter turns if the road is banked. It might even require *no* frictional force at all, like a bobsled on an icy (but steeply banked) track.
 
While Petersen nailed the physics of it, maybe a less technical explanation is the sharper the turn, the higher the g load for a given speed. Think of the car on a long sweeping high-speed turn rather than your 90deg turn. That’s more like what the fighter jets are doing. An airliner can’t do that many g’s so it makes a wider turn for the same speed. So think of the fighter with the stronger airframe as the Porsche 911 with wide sticky race tires versus the SUV on skinny all-seasons as the airliner (to stay with jets). That help?
 
luvflyin said:
I got no idea how negative G’s gets into this so maybe I’m not understanding the question. The answer to the question why a fast airplane can make a 90 degree without slowing down and running off the road like a car would is there ain’t no road, the airplane is in the air. How does little airplanes getting negative g’s Get into this? What exactly is your question?
Click to expand...
That’s part of the problem, I’m not sure how to precisely ask it.

Think of it this way. You’re in a dune buggy in the middle of the desert..so no road. You’re driving it in a straight line at 100mph but you want to make a left or right turn, so you’ll have to slow down to make the turn without the buggy flipping over and rolling. How do these jets traveling at 500+ MPH make sudden heading changes without having to slow down? I’m just imagining the stress that must be placed on the airframe to do so.
 
Ryanb said:
That’s part of the problem, I’m not sure how to precisely ask it.

Think of it this way. You’re in a dune buggy in the middle of the desert..so no road. You’re driving it in a straight line at 100mph but you want to make a left or right turn, so you’ll have to slow down to make the turn without the buggy flipping over and rolling. How do these jets traveling at 500+ MPH make sudden heading changes without having to slow down? I’m just imagining the stress that must be placed on the airframe to do so.
Click to expand...

Precisely like you do in your airplane. Radius of turn is bigger. Road’s wider. It ain’t sudden. It can only be so sudden or you and/or the airplane breaks. Is that what you meant? Doing it in the same radius of turn as a slower airplane?
 
Kath has it nailed down.

You could design a road that would allow that car to make a 90 degree turn at 100 mph. The limiting factor is the tire/road contact. The Hot Wheels track sets demonstrate that quite well.

For a plane the sky IS the road, and the flight surfaces are the tires.
 
This may help blast the whole “turning on a dime” thing out of your head...

Remember turn radius increases with speed in the aircraft.

A 9G turn in a fighter airplane at low speed is a smaller radius turn than a 9G turn at high speed. Faster you are going the wider the turn for the same forces imparted on the airframe and the meatbags inside.

In the car, you’re trying to force a specific ANGLE of turn via the tires and friction contact with the road surface.

In the airplane you can bank and pull but you’ll limit out on the force your body can take or the airplane can take before you can “force the angle”.

Or... you’ll trigger an accelerated stall. The wing will exceed its critical angle of attack and not “lift in the direction of the bank” anymore.

The accelerated stall could be said to be somewhat analogous to the tires losing grip/friction with the road. You exceeded their design limits.

Kinda. The analogies fall apart but sometimes they help you visualize what’s happening to both.
 
Ryanb said:
It’s 11:20pm and I have one of those late night questions.

Driving a car on the road at high speeds requires one to slow down in order to make a turn. In other words, if you’re traveling at 100mph on a straight road and then a 90deg turn to left is ahead, you would have to greatly reduce speed in order to make that sharp of a turn without going off the road and doing extreme damage to the vehicle. So how do airplanes, specifically jets make these sudden heading changes without having to reduce speed? In little airplanes, if we were to do something like this, we may feel some negative g’s, letting us know there’s some stress on the airframe, but sitting in an airliner traveling at speeds that are 10 fold that of a light aircraft and you don’t hardly feel anything at all.

Enlighten me.
Click to expand...


If the road were adequately banked inwards, you will not have to slow down. In fact, if you look closely, many highway ramps are banked, but not enough to support driving at full speed. If you go too fast, you will skid outwards. If you go too slow, in principle, you will slip inwards, but in real life the tire friction prevents you from slipping. In the case of an airplane, you can bank at the appropriate amount to prevent skidding or slipping. That's the idea of a co-ordinated turn.
 
Ryanb said:
That’s part of the problem, I’m not sure how to precisely ask it.

Think of it this way. You’re in a dune buggy in the middle of the desert..so no road. You’re driving it in a straight line at 100mph but you want to make a left or right turn, so you’ll have to slow down to make the turn without the buggy flipping over and rolling. How do these jets traveling at 500+ MPH make sudden heading changes without having to slow down? I’m just imagining the stress that must be placed on the airframe to do so.
Click to expand...
I think the problem is your conception of 'sudden' heading changes as applied to an airliner. The 'sudden' turn could span miles. I'm sure someone will be along at some point to do the math for us. A standard rate turn takes 2 min. no matter how far you fly. (30 seconds for a 90 degree standard rate turn) And your passengers may not even notice it.
 
sarangan said:
If the road were adequately banked inwards, you will not have to slow down. In fact, if you look closely, many highway ramps are banked, but not enough to support driving at full speed. If you go too fast, you will skid outwards. If you go too slow, in principle, you will slip inwards, but in real life the tire friction prevents you from slipping. In the case of an airplane, you can bank at the appropriate amount to prevent skidding or slipping. That's the idea of a co-ordinated turn.
Click to expand...

Speaking of banked roads. I was looking up Daytona which is known for its bank when I found this.

https://abcnews.go.com/US/story?id=93412&page=1
 
"So how do airplanes, specifically jets make these sudden heading changes?"

They don't make sudden heading changes. Probably on most jets at altitude it's more like 17 degrees of bank, which is much less than standard rate.
Not much stress in a 17 degree banked turn.
 
Ryanb said:
That’s part of the problem, I’m not sure how to precisely ask it.

Think of it this way. You’re in a dune buggy in the middle of the desert..so no road. You’re driving it in a straight line at 100mph but you want to make a left or right turn, so you’ll have to slow down to make the turn without the buggy flipping over and rolling. How do these jets traveling at 500+ MPH make sudden heading changes without having to slow down? I’m just imagining the stress that must be placed on the airframe to do so.
Click to expand...
Bad premise. You don’t have to slow down to make the turn. You just make the turn at a very slow rate. Maybe it takes you miles to make the turn..... like it does in a 500+ mph jet. Add the bank angle as mention by fearless and you can turn faster without losing traction.

Also mentioned above is a 2 minute turn. A plane going 100 mph is going to do a 2 minute turn in far less distance than one doing 500 mph. Stretch your turn in your dune buggy to have a 10 mile radius and you can go pretty darn quick during the turn.

Ps> this isn’t aerodynamics.
 
I've never seen an airliner make the taxiway turnoff at 100mph. Are you honestly asking about the difference between banked turns in the sky and lateral tire grip?
 
Pfftt! NASCAR doesn't slow down for turns. They do aerodynamics too.

image.jpeg
 
exncsurfer said:
I think the problem is your conception of 'sudden' heading changes as applied to an airliner. The 'sudden' turn could span miles. I'm sure someone will be along at some point to do the math for us. A standard rate turn takes 2 min. no matter how far you fly. (30 seconds for a 90 degree standard rate turn) And your passengers may not even notice it.
Click to expand...

Exactly what I was thinking of typing.
 
RoscoeT said:
I've never seen an airliner make the taxiway turnoff at 100mph. Are you honestly asking about the difference between banked turns in the sky and lateral tire grip?
Click to expand...

I think he is. Ryan, the best of street cars pull about 1.0g in the corners, due to friction limits between the tires and the road. F1 race cars, at high speeds where aerodynamics provide very high downforce, can approach 8g in a turn, which is approaching the 9g a fighter jet can pull.
 
I got it. When making the comparison between car and plane, put the car on a lake, a big one. Its frozen. It was calm and flatter than pizz on a plate when it froze. Then ya bring in a fleet of Zambonis and polish it for miles. Here's the car, you put Teflon tires on it.

For your listening pleasure,
 
I'd venture a guess that, an automobile (or even a boat) would have a tighter turn radius at 100mph than an aircraft could at the same velocity.
 
Ryanb said:
n little airplanes, if we were to do something like this, we may feel some negative g’s, letting us know there’s some stress on the airframe
Click to expand...

Why are you feeling negative G's when you turn?
 
Ryanb said:
Think of it this way. You’re in a dune buggy in the middle of the desert..so no road. You’re driving it in a straight line at 100mph but you want to make a left or right turn, so you’ll have to slow down to make the turn without the buggy flipping over and rolling. How do these jets traveling at 500+ MPH make sudden heading changes without having to slow down?
Click to expand...
you could still make that 90 degree turn in your dune buggy, it might just take several miles to do so if you don't want to slow down, heck, you can even do a 180 turn or a full 360. Take a look at FlightAware and look at aircraft tracks. They don't make those turns on a dime.. they can take several miles. you can make the turn a lot sharper, and then you'll start to feel the Gs more
 
Keep in mind, the g's whether plus or minus come from the elevator and not the ailerons.
 
mscard88 said:
You're doing it wrong if you're feeling negatives Gs in a steep turn.
Click to expand...
I must not be up to speed with my various g forces. What’s the term for the g felt during a steep turn?
 
mscard88 said:
Nonexistent? :D
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Nah, I’m thinking of the times that the nose may fall through the horizon so you add back pressure while you’re in the turn. Isn’t that negative?
 
SoonerAviator said:
I'd venture a guess that, an automobile (or even a boat) would have a tighter turn radius at 100mph than an aircraft could at the same velocity.
Click to expand...

Not if the road is flat. Friction between tires and a road will max out at some fraction of the vehicle's weight, because the force = (mu)*N, where "mu" is a number generally between zero and one (like 0.2 or something), and "N" is the upward force necessary to hold the car up**.

Airplane wings, on the other hand, will max out at something like 3.8 times the vehicle's weight (if flying within normal category!). More, if utility/acrobatic. Even at a modest 45-degree bank, where the horizontal component is only 1/sqrt(2) of this, this is better than the car.

The normal force from a banked piece of pavement, on the other other hand, will max out at some astoundingly huge multiplier of the vehicle's weight.

So in the hierarchy of turnability (assuming the speed is fixed at 100 mph or whatever): wheels on the flat road is the worst performer, airplane does better, and banked road can do the tightest turns assuming it's banked steep enough (like a luge track!)


Now, boats, on the other other other hand, are going to be complicated... ;)

<Physics, yay!>

** Edit: ..."N" is equal to the car's weight, for most everyday cars.
 
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Ryanb said:
Nah, I’m thinking of the times that the nose may fall through the horizon so you add back pressure while you’re in the turn. Isn’t that negative?
Click to expand...

Nope, that's just garden-variety "positive" g's. The kind that make you "feel heavier".

Negative g's would make you "feel lighter". Like if you do a "zoom over the top maneuver" and everything in the plane starts floating. :)
 
The normal force doesn't have to be a fraction of the car's weight, though for you and me it always is. Interesting bit of trivia... your Honda Civic (or whatever) is aerodynamically much more efficient than an Indy/Formula 1 race car. The reason is that the Formula One cars are designed so that the aerodynamics at speed forces the car down onto the pavement, both using the top surfaces of the car and by creating a partial vacuum underneath. The entire reason is to increase the normal force and allow faster cornering without having to lose speed.

At race speeds, the track could go inverted and the cars would still stick to it. This is also why when they do a road race through a course on city streets, they have to go over the course and weld down any manhole covers along the route; if they didn't, when a car went over one that 70-lb cover would go flying in the air.
 
Ryanb said:
It’s 11:20pm and I have one of those late night questions.

Driving a car on the road at high speeds requires one to slow down in order to make a turn. In other words, if you’re traveling at 100mph on a straight road and then a 90deg turn to left is ahead, you would have to greatly reduce speed in order to make that sharp of a turn without going off the road and doing extreme damage to the vehicle. So how do airplanes, specifically jets make these sudden heading changes without having to reduce speed? In little airplanes, if we were to do something like this, we may feel some negative g’s, letting us know there’s some stress on the airframe, but sitting in an airliner traveling at speeds that are 10 fold that of a light aircraft and you don’t hardly feel anything at all.

Enlighten me.
Click to expand...

If you are asking why cars need to slow down and planes don't, the answer is the radius of the turn. If your 90 turn on the road was over 5 miles, you would not need to slow down in the car either.

Airliners take many miles to make a turn for that exact reason. Not to stress the plane and occupants
 
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