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Guru

Join Date: Sep 2006
Location: El Lago, Texas, USA
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Lateral Force Due To Wind

05/02/2016 11:08 AM

How would you go about determining the lateral force on an aircraft (or boat, or automobile) due to a sideways wind? It seems like you could do it by measuring the differential in air pressure on either side of the body?

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Guru

Join Date: Dec 2008
Location: Long Island NY
Posts: 15283
#1

Re: lateral force due to wind

05/02/2016 11:43 AM

A change in trajectory seems to me to be the simplest method to me. A differential air pressure measurement will only indicate the pressure differences at the points one is measuring. This wont tell you the area that this pressure is being applied to.

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Guru

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#2

Re: Lateral Force Due To Wind

05/02/2016 11:57 AM
Guru

Join Date: May 2009
Location: Richland, WA, USA
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#3

Re: Lateral Force Due To Wind

05/02/2016 12:05 PM

Here are some sources:

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Guru

Join Date: May 2009
Location: Saint Helens, Oregon
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#4

Re: Lateral Force Due To Wind

05/02/2016 2:29 PM

Here's a link regarding the effects of wind while flying, "The Plane and The Wind" it also includes Pythagorean Theorem and SOH CAH TOA

an airplane traveling at 100 miles per hour and 20 mile per hour wind, (1) shows tail wind (2) shows a head wind (3) shows a cross wind and it's vector

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Guru

Join Date: Oct 2008
Posts: 42308
#6

Re: Lateral Force Due To Wind

05/02/2016 6:18 PM

Remind me to never fly with pilot #3.

Guru

Join Date: Dec 2008
Location: Long Island NY
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#7

Re: Lateral Force Due To Wind

05/02/2016 10:07 PM

The way all three fly straight down tells me that none of them will be flying for very long. Splat, Splat, Splat.

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Guru

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#5

Re: Lateral Force Due To Wind

05/02/2016 2:34 PM

An airplane is a bad example. An airplane flying through the air has a crosswind only if it is flown with the controls (aileron and rudder) crossed. Airplanes are not normally flown in this matter.

For a boat or automobile with a cross wind, the resultant wind direction would be a vector sum of the crosswind and headwind due to the motion of the vehicle. It would be equivalent to the body of the vehicle acting as an airfoil flying at an angle of attack determined by this wind direction.

The flow of air past an airfoil, especially a badly shaped one such as a car or boat body, is complicated. The pressure varies from point to point and you would have to make pressure measurements in many places to come up with an answer. For example, the pressure pattern around a simple airplane wing is shown below:

Probably the best tried and true method is to use a wind tunnel and a model car or boat angled at the correct angle to simulate the crosswind. The results can then be scaled up.

Guru

Join Date: May 2009
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#10

Re: Lateral Force Due To Wind

05/03/2016 1:54 AM

"An airplane is a bad example"? The OP asked, "How would you go about determining the llateral force on an aircraft (or boat, or automobile). I gave him/ her an example of an airplane flying in a 3 dimensional space. It comes down to how much surface area is exposed to the wind, rather it be a (1) tail wind, (2) head wind, or (3) a cross wind. Example (1) works great, saves a lot of fuel, but could be a bitch if your stuck waiting an hour for your ride! Example (2) could also be a bitch, if your an hour late getting to the airport and your ride gets tired waiting and leaves. And (3) could be disastrous for your flight, if you didn't compensate for the cross wind component. You could end up being over 100 miles off course, out of fuel, and no place to land.

Cars, trucks and boats suffers the same effects as examples 1-3, as an airplane, when it comes to navigation, ETA's or how much "crabbing" in-put is applied to the steering wheel/ joy-stick.

Airplanes are a good example of the effects of wind components, tail, head or cross winds. The biggest factor is, 'how much surface area is exposed the the wind and the corresponding vector resulting because of it. And why I include the links Pythagorean Theorem and SOH CAH TOA.

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Power-User

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#12

Re: Lateral Force Due To Wind

05/03/2016 6:55 AM

Rixter is correct you won't find a differential unless you do something very stupid like fly with crossed controls. And without crossed controls there will be no pressure differential on the sides.

Guru

Join Date: Apr 2010
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#13

Re: Lateral Force Due To Wind

05/03/2016 9:24 AM

With a crosswind, an airplane "crabs" with respect to the ground path but flies straight ahead with respect to the air. Once the wheels have left the ground, there is no force on the airplane from the crosswind. The airplane is just being carried by the air mass the same as a boat is carried by the flow of a river.

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#8

Re: Lateral Force Due To Wind

05/02/2016 11:40 PM

Tandem rotor helicopters like the CH-47 Chinook or the CH-46 Sea Knight are notoriously difficult to fly straight (parallel to their center line) through even calm air. Thus, they are outfitted with SAS (stability augmentation system). This system, among other things, measures differential air pressure along the side of the helicopters through an expanded pitot system and provides subtle control inputs to help the pilot without impairing his ability to cross control the aircraft when he wants to. Without this system optimum performance would (and does) suffer significantly.

The SAS does not preclude the pilot from "crabbing" into a cross wind to maintain a heading.

Hooker

Guru

Join Date: Sep 2006
Location: El Lago, Texas, USA
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#14

Re: Lateral Force Due To Wind

05/03/2016 10:07 AM

Thanks, I'll check that out.

Power-User

Join Date: Sep 2007
Posts: 152
#9

Re: Lateral Force Due To Wind

05/02/2016 11:59 PM

Build an accurate scale model, find a wind tunnel you can test it in, determine the drag coefficient for all orientations/wind vectors you are interested in, then compute the drag force that the full scale item will experience.Or get a ton of money, a super computer (or many super computers), a good computational fluid dynamics math model of the item you are interested in and then turn the crank and make iterative changes for an indeterminate amount of time until you get an acceptable answer or lose interest in the problem.

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#11

Re: Lateral Force Due To Wind

05/03/2016 6:13 AM

Using a scale model a and wind tunnel.

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