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Headwind and gliding

Asked by: 2265 views Aerodynamics

Hi All

I have a hypothetical which I'm struggling to understand... hopeing one of the Masters out there can help me out :-)

 

Let's say we are in a gliding descent at the recommended glide speed of 70Kts on a still day. All of a sudden, we encounter a headwind of 20Kts. Are the following statements correct (?):

 

1. When the headwind is encountered, assuming there is no change in attitude, the IAS would increase to 90Kts

2. The maximum glide range will still be achieved at 70Kts, because this represents the best lift to drag ratio

3. The attitude of the aircraft should be increased to to get the IAS back to 70Kts to maintain the best glide speed (because the best lift to drag ratio hasn't changed)

4. Even though our ground speed is now 50Kts (20Kts slower than the start) the glide range will remain the same as the start, it will just take longer to get there

 

Any help would be much appreciated.

6 Answers



  1. Nathan Parker on Mar 16, 2013

    “1. When the headwind is encountered, assuming there is no change in attitude, the IAS would increase to 90Kts”

    If the headwind is sudden, you’d see an increase in IAS, but with no AOA change of the aircraft, it would seek out its trimmed airspeed, which we’ll assume is 70 knots. If the wind increase is sudden, the aircraft will undergo an oscillatory motion called a “phugoid”, but that will eventually settle down. The pilot might help, but it isn’t absolutely necessary.

    “2. The maximum glide range will still be achieved at 70Kts, because this represents the best lift to drag ratio”

    No, if you measure best glide with respect to the ground, a headwind requires an increase in your glide speed. Imagine, for instance, a headwind of 70 knots. At the best glide speed, you won’t get anywhere.

    “3. The attitude of the aircraft should be increased to to get the IAS back to 70Kts to maintain the best glide speed (because the best lift to drag ratio hasn’t changed)”

    The aircraft will resume its trimmed airspeed without the pilot’s aid and the attitude of the aircraft will be no different from before; however, the glide angle with respect to the ground will be steeper. (Don’t think of attitude controlling airspeed, but rather AoA.)

    “4. Even though our ground speed is now 50Kts (20Kts slower than the start) the glide range will remain the same as the start, it will just take longer to get there”

    No, the glide range with respect to the ground is reduced. Again, with a 70 knot headwind and a 70 knot glide velocity, the range into the wind is zero.

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  2. Christopher Ian on Mar 22, 2013

    Interesting. As glider pilot & student SEL these ponderances come up readily.
    With gliders they talk about best speed to fly for purposes of getting somewhere- such as out of sink to the next thermal or in competition to the next turnaround point.
    - That speed is not necessaily best L/D.

    Question for Nathan Parker.
    Point # 2. You are correct, best Lift over Drag (L/D)- that is best (farthest distance) in a glide is ‘written’ into the design/ shape of the aircraft/ airfoil.
    Best L/D is a specific AoA achieved at a specific speed at a given weight.
    Most manufacturers tell you best glide ratio is Ex. 9/1 at 56 knts at gross weight.

    -True, a headwind alters that best (farthest) glide speed necessitating an increase in speed to achieve max glide distance. We know this in gliders but i do mot understand the physics or more importantly, how that NEW best glide speed is determined.

    POI: Best glide ratio (angle) is NOT effected by aircraft weight.
    The heavier aircraft achieves max glide distance at a higher speed than the lighter aircraft.
    POI: Of dozens of books on flying, i have but one that gives this rule of thumb;
    ~ To achieve best glide speed, start at best glide speed at max gross weight, stated by the manufacturer. Reduce glide speed 5% for each 10% below max gross weight.
    ~ – Not sure if this rule of thumb can be used to determine best glide speed into a headwind.
    BTW, i do not believe a 20knt tailwind simply pushes you farther in a glide. I believe you must reduce your glide speed w a tailwind to achieve max glide distance.
    - – There are times in gliders or engine out scenario, we do not want best/ longest distance glide, which might shorten our flight; – rather we want longest time aloft and not greatest distance to allow more time to prepare for landing or allow glider ground traffic to clear the runway which is right below us.
    That greater time aloft speed would be slower – Stick and Rudder book discusses this in detail.
    Thanks.

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  3. Nathan Parker on Mar 22, 2013

    “True, a headwind alters that best (farthest) glide speed necessitating an increase in speed to achieve max glide distance. We know this in gliders but i do mot understand the physics or more importantly, how that NEW best glide speed is determined.”

    The rule of thumb in the gliding community is 1/2 the wind velocity; in the example of the OP, you’d add 10 knots to the best glide speed of 70 knots. I’ve seen a mathematical analysis of the actual optimum speed and I recall that this rule of thumb isn’t very accurate, but it’s better than doing nothing. If you had a tailwind, you’d subtract the 10 knots from the best glide speed.

    Conceptually, this issue arises because the airplane isn’t aware of steady wind; the only wind the aircraft experiences is produced by its forward velocity through a particular body of air. If that body of air is itself moving, it has no aerodynamic effect on the airplane. Unless you were trying to land on SHIELD’s hover carrier, you don’t really care about maximizing the distance flown through the body of air, you want to maximize the distance flown with respect to the ground. It’s a different problem, unless the body of air isn’t moving.

    In theory, a speed polar diagram (AKA hodograph) can show you the proper speeds to fly under any wind conditions, but these don’t exist for powered aircraft. I would think you’d have them for gliders.

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  4. Christopher Ian on Mar 25, 2013

    Yes, polars do exist and one pilot colleague recently opined that glider Best L/D ratio that i compare; 21/1, 28/1, 42/1 on our gliders is only part of the performance. The polars describe – minimum sink rate. I gather the relationship between best L/D and min. sink rate is not a direct correlation from glider to glider. That is 2 gliders w the same L/D may have different values for min sink.
    A low speed at Min sink rate allows a glider to circle in a thermal and gain best advantage climbing while a faster glider at min sink rate allows covering distance which is needed for competition.
    Any competition that involves a course seeks speed as the advantage whether it is best time to complete the course or how many ‘laps’ in a given time slot, both seel speed.
    A competition that measures most distance climbing- that is climb as high as possible, return to 2,000 ft and do it again, or longest time aloft would seel best climbing performance.
    -
    Yes i do recall the 1/2 wind speed number as an adjustment + or – for head and tailwind.
    There is nothing more than guess work as to wind speed aloft. Especially w gusts.
    A pilot could glide in one direction at a steady speed for 1 or 2 minutes, note the altitude loss. Measure distance covered on a map, DME or GPS. Then glide again at a different speed for identical time and take measurements.
    - I still would like to see a computer graphic app that shows a plane w every conceivable parameter that effects lift & speed & performance & glide angle. Wind, air density, aircraft weight, temperature, humidity, pitch, flaps, even slip- for a specific plane (Cessna 172) or a generic plane. Let the user adjust each parameter and show the difference in speed, glude, distance, etc.
    I did find one online app that allowed 3 or 4 parameters but i want them ALL. And show the aircraft attitude and glide angle and path/ distance to the ground.
    - – would be an eye opener to most pilots.

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  5. Nathan Parker on Mar 25, 2013

    “The polars describe – minimum sink rate. I gather the relationship between best L/D and min. sink rate is not a direct correlation from glider to glider. That is 2 gliders w the same L/D may have different values for min sink.”

    Just to be clear, best glide is NOT minimum sink rate. Minimum sink rate will always be slower than best glide; in theory, it’s .76 * the speed of best glide. Best glide occurs at minimum drag, minimum sink occurs at minimum POWER required.

    “I still would like to see a computer graphic app that shows a plane w every conceivable parameter that effects lift & speed & performance & glide angle. Wind, air density, aircraft weight, temperature, humidity, pitch, flaps, even slip- for a specific plane (Cessna 172) or a generic plane. ”

    In theory, it’s easy enough to do if you had a drag polar diagram for the airplane, which we never do. Probably could generate one with enough flight tests, measuring descent angles at various airspeeds. I’d remove lift from one of the parameters, because lift = weight in unaccelerated flight, and I’d remove air density (including temp and humidity) since using IAS removes the influence of these values. I’d remove pitch, too, because it’s a result of the calculations, not an input. I’d probably throw in load factor or bank angle.

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  6. Nicolas on Jun 19, 2013

    Nathan,

    “Best glide occurs at minimum drag, minimum sink occurs at minimum POWER required.”

    In fact, best glide occurs at best L/D (VY), and minimum sink, or best endurance (VX) occurs at minimum drag = minimum power required= minimum fuel consumed= max time in the air.

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