Greater speed influence, head or tail wind

[Truckedup]

  Will a tail wind add more speed than an equal head wind will slow you down?

[Malcolm UK]

My amateur view is by looking at other sports such as ski jumping, downhill skiing, running, and gravity events such as luge.  The human form is more efficient in a head forward in the direction of travel position (neglecting that running backwards is so difficult). Therefore, on a naked machine I conclude that the effect of wind direction will be slightly less with head receiving it first. The push on the rear will be better to gain speed.

Of course with any streamlining or fairing the results could be changed. But with bikes where the fairing design is often to improve just going forward, through the wind, again a tail wind will give a pick-up in speed.


 

[tortoise]

 Will a tail wind add more speed than an equal head wind will slow you down?

Usually not. A machine could be built to prove me wrong, but it'd have to be geared stupid.

[jacksoni]

Plugging numbers into Bonneville Pro, it consistently says you lose slightly more with a headwind than you gain with a tailwind. (trial of 2 with a 154mph car running at Wilmington) FWIW

[RansomT]

Plugging numbers into Bonneville Pro, it consistently says you lose slightly more with a headwind than you gain with a tailwind. (trial of 2 with a 154mph car running at Wilmington) FWIW

and to add ... doing this in windy conditions for awhile now on a bike:  [on a stock or near stock fairing bike] Headwind loss is about 1 mph per 2.5 mph of wind.  Tailwind gain is about 1 mph per 3 mph of wind.  As you increase aero, the headwind drop starts reducing to 1 mph per 3.0 mph of headwind; however, tailwinds make the bike more and more unstable.  To mix it up even more, if you have to lean (cross headwind or cross tailwind) to keep the bike straight it scrubs off a lot of mph even with a cross tail.

[bbarn]

Depends totally on the shape of the object that is placed in the wind stream. Aerodynamics do not necessarily behave the same on forward flow as it does in reverse flow. Separation drag and stagnation drag in reverse flow will likely be different unless the object being tested is symmetrically shaped.
 

 

[manta22]

Depends totally on the shape of the object that is placed in the wind stream. Aerodynamics do not necessarily behave the same on forward flow as it does in reverse flow. Separation drag and stagnation drag in reverse flow will likely be different unless the object being tested is symmetrically shaped.
 

The headwind is just slower due to the tailwind, the airflow never goes backwards over the vehicle unless the tailwind speed is faster than the vehicle... unlikely. 

Regards, Neil  Tucson, AZ

[bbarn]

Depends totally on the shape of the object that is placed in the wind stream. Aerodynamics do not necessarily behave the same on forward flow as it does in reverse flow. Separation drag and stagnation drag in reverse flow will likely be different unless the object being tested is symmetrically shaped.
 

The headwind is just slower due to the tailwind, the airflow never goes backwards over the vehicle unless the tailwind speed is faster than the vehicle... unlikely. 

Regards, Neil  Tucson, AZ

That difference depends more on the influence of the air on the shape than anything else. If you have an efficient shape pointing into the wind, the negative effect of a headwind can be less than the positive effect of a tailwind that reduces separation drag.

Think of an equilateral triangle presented in two ways to the airflow. In the first we point one of the convergence points of the triangle directly into the the air stream. This would give you a low stagnation drag and a high separation drag. Now reverse the flow 180 degrees and we would have a high stagnation drag and dare I say a high separation drag as well.

If that triangle were travelling across the ground with a constant HP and rolling resistance, a headwind (Low stagnation/high separation drag) would reduce the speed. However, a tail wind of the same speed under the same condition would lower the stagnation drag which in turn would reduce the separation drag resulting in a larger differential of speed.

Look at some of the cars out there and the tail fins and spoilers designed to generate down force. The work by creating a vacuum under and behind the car. If you take one of these cars and reverse them at high speed, those devices work like sails (or kites) why? What happens there? (rhetorical, they catch air)

When a tail wind blows up behind those same cars they in fact do catch air or, more precisely stated, the tailwind reduces the separation drag that is generated allowing the shape to move more freely through the air. (<-Specifically ignoring the loss of traction on the effect of speed)

My thoughts without doing a study - A tailwind in most designs will yield a higher gain in ground speed than a headwind will reduce speed in a shape that is designed to go fast straight ahead.

[Rex Schimmer]

Isn't it all really about relative wind velocity? If it is a direct head wind then the air going over the car is going faster and there is more drag if it is a direct tail wind then the air velocity over the car is slower and therefore less drag. The shape of the car is probably inconsequential unless the speed is so low that the head or tail wind constitutes a large percentage of the total air velocity.

Rex

[bbarn]

Isn't it all really about relative wind velocity? If it is a direct head wind then the air going over the car is going faster and there is more drag if it is a direct tail wind then the air velocity over the car is slower and therefore less drag. The shape of the car is probably inconsequential unless the speed is so low that the head or tail wind constitutes a large percentage of the total air velocity.

Rex

The only thing I would add is that if the car is symmetrically shaped I believe your statement is completely accurate. However, I would add that if it weren't symmetrical, the level of influence the relative wind has may be greater as a tail wind than as a headwind. (<-- The assumption here is that the shape is better optimized for forward travel aero than it is in reverse flow.) 

[jdincau]

The point is there is no reverse flow only slower or faster flow

[Truckedup]

 I'm thinking of the tail or head wind on a naked frame motorcycle in the 130 mph range...A 10 mph tail wind might add 2-3 mph ?

[jacksoni]

As it happens Bonneville Pro has a sample bike data that just about match your question. No wind, 130.7. 10mph head wind, 125.5. 10 mph tailwind, 136. FWIW

[Stan Back]

Here's the way I figured it out (just a reword of Jim's).

Say you're going 100 miles per hour with no wind.  You have a 100 mph headwind.

Say you're going 100 miles per hour with a 20 mph head wind.  It's like a 120 mph head wind.

Say you're going 100 miles per hour with a 20 mph tail wind.  It's like a 80 mph head wind.

Your results may vary.  Results dependent on keeping the front in front.

[distributorguy]

Correct but the variation in effect has more to do with individual vehicle shapes and the speed they are capable of.  At 350 mph, a headwind isn't going to matter as much as a vintage car running a top speed of 100 mph.  In other words, the loss isn't a percentage, so to quantify in simple terms is all but impossible other than to say a 10 mph head wind is going to have the same effect as the wind resistance of going 10 mph faster and vice versa - whatever effect that is in your situation.  In some cases it may be overcoming 20 hp, in other cases 100 hp or more based on vehicle speed and shape - not so much about a flat 1.5% variation of loss of by a head wind versus tail wind.   
Please correct me if I have this wrong.