Air speed v Ground Speed

[R.Perry]

I don’t know if this has been covered, but an acquaintance just crashed his drone because he ran the battery dead trying to get his drone back flying into a head wind.

Air speed and ground speed are two different animals.

Simple example, if you are flying at 30 mph with a 20 mph tail wind your ground speed is 50 mph, adversely if you are flying 30 mph into a 20 mph headwind your ground speed it 10 mph.

The point is if you are flying down wind expect to use more power and less speed returning. Your drone may be able to maintain the ground speed heading into the wind, but it will use more power. Example is if you are flying into a 20 mph a head wind and you want to maintain 30 mph you need to be able to fly 50 mph.

I know this is very basic stuff for most of you, but for those new to the aviation world and drones it may not be.

 

[R Martin]

I don’t know if this has been covered, but an acquaintance just crashed his drone because he ran the battery dead trying to get his drone back flying into a head wind.

Air speed and ground speed are two different animals.

Simple example, if you are flying at 30 mph with a 20 mph tail wind your ground speed is 50 mph, adversely if you are flying 30 mph into a 20 mph headwind your ground speed it 10 mph.

The point is if you are flying down wind expect to use more power and less speed returning. Your drone may be able to maintain the ground speed heading into the wind, but it will use more power. Example is if you are flying into a 20 mph a head wind and you want to maintain 30 mph you need to be able to fly 50 mph.

I know this is very basic stuff for most of you, but for those new to the aviation world and drones it may not be.

Ideally you should plan you missions with the wind on your beam; especially if you are mapping. Headwinds and tailwinds screw with the shot timing. You will wind up with huge gaps in a tailwind situation and compressed overlap in a headwind situation.

 

[clolsonus]

The one that got me was air altitude vs. ground altitude vs. tree altitude. <sigh>

R Martin: I always fly my fixed wing mapping missions as close to 90 degrees to the wind as possible, and plan all my 180 turns starting into the wind to minimize the radius. This seemed to be the most space/time efficient to me, and like you say, gets the most consistent shot spacing in both directions. For quads, drone deploy and pix4d capture seem to fly a constant ground speed and just take the pictures at the right location independent of wind ... but with covid it's been a while since I've been out to map anything so I could be mis-remembering that.

 

[R Martin]

The one that got me was air altitude vs. ground altitude vs. tree altitude. <sigh>

R Martin: I always fly my fixed wing mapping missions as close to 90 degrees to the wind as possible, and plan all my 180 turns starting into the wind to minimize the radius. This seemed to be the most space/time efficient to me, and like you say, gets the most consistent shot spacing in both directions. For quads, drone deploy and pix4d capture seem to fly a constant ground speed and just take the pictures at the right location independent of wind ... but with covid it's been a while since I've been out to map anything so I could be mis-remembering that.

Pix4DCapture allows you to skew the flight plan to put the wind in an advantageous position. I haven't used DD in over four years so don't have a clue how it functions. My current software has a box for wind direction and it will adjust the flight plan accordingly. Works great unless you have a wind shift. I've taken to splitting the difference between the two different directions in that case and that is just the best I can do with a bad situation.

 

[HAWK-i.us]

Adding a piece of trivia -- I had a construction site where the air/ground speed became important to flying the photo and video missions. The geography was such that the site always had a wind tunnel blowing down the the west side of the property. If the wide area wind speed was 10mph, that west side of the site would always be at least 20mph. The wind direction was always the same (S to N) and always stiff, around 20-25 mph. My autonomous flight plan needed a ground speed of about 20mph as constantly as possible around the site perimeter. The first couple of trips to the site, the flight plan was into the wind on that side of the site, and the drone and gimbal just could not cope. It took a while for me to figure out the wind tunnel situation. Reversed the flight direction around the perimter so that side of the site was always flown with the wind, and it worked a treat.

 

[Earthman]

In a uniform air mass that is moving relative to the ground at some speed, if you takeoff and land at the same spot; i.e., make a round trip, you can fly the same distance in any direction and the fuel/energy consumption will be the same as any other direction.

This is clearly true when the speed of the air mass is zero relative to the ground. It is also true when the speed of the air mass relative to the ground is any other number.

However, if the speed of the air mass is not zero, you will consume fuel/energy faster to overcome the effects of the wind, and the additional fuel used to fly the round trip will be higher, but the total will be the same regardless of the direction of your round trip. For rotary aircraft flying in the moving air mass, the amount of additional fuel used should be equal to what it would take to hover over the same spot for the time it takes for the round trip.

Aircraft performance is measured relative to the air mass not the ground. The aircraft doesn’t know the air mass is moving relative to the ground or any other reference frame.

An aircraft (fixed-wing, multi-rotor, ballon, or spec-of-dust) is carried with/within the air mass. This is similar to a boat sitting dead in the water while being carried down stream by a current - the boat is not moving relative to the water but it is moving relative to the bottom (ground). The boat and the people in it don’t know that they are moving unless they look at the shore (or the bottom).

So, it doesn’t seem to matter how you orient the ground track of your survey if it’s a round trip circuit in a uniform air mass.

Flying a survey in a moving air mass will cost more fuel/energy, but it’s going to be the same amount regardless of the ground track.

Flying a survey in a moving air mass will mess up the spacing and overlap of your photos if your camera triggering depends on simple timing, but not if your triggering is based on position.

Choosing a ground track so your turns are “into the wind,” i.e., counter the direction of the moving air mass, may be advantageous if the sensors and PIDs in the autopilot of your UAS are not well tuned or otherwise able to handle “precise turns” in other directions relative to the ground. Level turns at a given bank angle are the same relative to the air mass, but will take a different track relative to the ground when performed in a moving air mass.

It’s all relative to your reference frame.

 

[HAWK-i.us]

Agree with all above. In my case, the air mass was moving at distinctly different speeds on different sides of the site . The constant mission speed is based on GPS, which is ground speed, so the speed of the bird through the air in my example was not the symmetrical opposite calculation on the two opposing sides of the site. Against a 10mph wind on one side and with a 20 mph wind on the other. Total energy for the circuit won't be the same either for CW versus CCW directions. Photo intervals were OK because they were triggered by waypoint locations.

 

[Meta4]

Simple example, if you are flying at 30 mph with a 20 mph tail wind your ground speed is 50 mph

Does the Inspire work this way?
The Phantom doesn't unless you fly in atti mode.
In P-GPS with a tailwind, you can only pick up a small bonus in a strong tailwind.
DJI have the Phantom programmed such that it puts the brakes on for tailwind flying.

 

[HAWK-i.us]

DJI have the Phantom programmed such that it puts the brakes on for tailwind flying.

In my case it was a Litchi mission with programmed ground speed, so it was spending a little energy to slow down on the downwind side of the site. That particular example had a mission speed of 20 mph, so a 25 mph tail wind caused an equivalent energy burn from braking of around a 5mph flight speed, which was fine compared to trying to fly 20mph ground speed into a 25 mph headwind when in the opposite circuit direction.

Somewhat off topic (apologies), but it's an appropriate place -- Before I figured out what the problem was and reversed the circuit direction, I discovered that while trying to buck that heavy headwind, the P4P gimbal could control when facing the incoming wind but could not control the camera at all while trying to fly sideways. Never did explore that to see if the gimbal was getting weak because changing the circuit eliminated that problem as well. I guess the gimbal isn't designed to resist a heavy side-wind. Dang thing was bouncing all around. Thought I'd damaged it, but it was fine after I stopped trying to fly 20mph sideways (to look into the center of the site) into a 25 mph on-coming wind.

 

[Earthman]

In my case it was a Litchi mission with programmed ground speed, so it was spending a little energy to slow down on the downwind side of the site. That particular example had a mission speed of 20 mph, so a 25 mph tail wind caused an equivalent energy burn from braking of around a 5mph flight speed, which was fine compared to trying to fly 20mph ground speed into a 25 mph headwind when in the opposite circuit direction.

Somewhat off topic (apologies), but it's an appropriate place -- Before I figured out what the problem was and reversed the circuit direction, I discovered that while trying to buck that heavy headwind, the P4P gimbal could control when facing the incoming wind but could not control the camera at all while trying to fly sideways. Never did explore that to see if the gimbal was getting weak because changing the circuit eliminated that problem as well. I guess the gimbal isn't designed to resist a heavy side-wind. Dang thing was bouncing all around. Thought I'd damaged it, but it was fine after I stopped trying to fly 20mph sideways (to look into the center of the site) into a 25 mph on-coming wind.

I avoid flying in high winds so the extra energy spent “breaking” to counter act a strong tail wind didn’t occur to me. But I can see how this is a factor for multi-rotors.

Nor had I realized that I might have experienced the problem that you describe with a gimbal while “flying sideways” relative to a strong crosswind with a multi-rotor. I have had a few blurry photos in surveys and this seems like a likely explanation.

More reasons to avoid flying in strong winds.

Thanks!

 

[HAWK-i.us]

One final point from me on the OP's post- Fixed wing RC pilots become keenly aware of this ground speed/air speed deal early on. You'll normally have a throttle set for a nice cruising speed and don't have to mess with it much unless your doing 3D or pattern work. But when a wind comes along, if you're not paying attention to it, you'll be flying your race track on a downwind leg and think "Man, I"m haulin butt" and cut back on the throttle. Then turn into the wind on the return leg and get surprised with a sudden stall. As Earthman so well described, the fixed-wing bird doesn't care what it's ground speed is, just that it's got enough air speed to keep flying. But with only the eyeball for telemetry, it's easy to forget that what what you see from the ground isn't what the bird feels in the air.

R.Perry - great discussion. Thanks for starting it. Hope All Ya'll have a great July 4th weekend. Stay safe.................. R

 

[Earthman]

One final point from me on the OP's post- Fixed wing RC pilots become keenly aware of this ground speed/air speed deal early on. You'll normally have a throttle set for a nice cruising speed and don't have to mess with it much unless your doing 3D or pattern work. But when a wind comes along, if you're not paying attention to it, you'll be flying your race track on a downwind leg and think "Man, I"m haulin butt" and cut back on the throttle. Then turn into the wind on the return leg and get surprised with a sudden stall. As Earthman so well described, the fixed-wing bird doesn't care what it's ground speed is, just that it's got enough air speed to keep flying. But with only the eyeball for telemetry, it's easy to forget that what what you see from the ground isn't what the bird feels in the air.

R.Perry - great discussion. Thanks for starting it. Hope All Ya'll have a great July 4th weekend. Stay safe.................. R

Just a point of clarification. If the RC fixed-wing pilot reduced throttle on the downwind leg and trimmed for level flight, the aircraft may be close to aerodynamic stall speed in that configuration. But if the pilot manages to turn 180-deg without stalling, the aircraft would continue to maintain level flight, but at lower ground speed. The air speed would be the same on both legs.

In this scenario, if the aircraft stalls, it’s most likely to happen in the turn. Stall speed increases with bank angle (an accelerated stall), and if the turn isn’t performed with coordinated aileron and rudder, the aircraft can enter a stall-spin where one wing or the other stalls causing the aircraft to roll over as it enters a spin with the wings stalled. The stall-spin often happens when the pilot adds rudder to tighten the turn, which can be disastrous when done close to the ground - like when turning from base on to a short final while landing.

I almost entered a stall-spin once while turning a Cessna 152 onto a short final, but recognize it as the controls got sloppy and the inside wing began to dip. I instinctively did the right things to stop the stall, recovered, and managed to land safely, but with an ego that still hurts today. I’ve also done it twice while turning an RC Pitts S-1S onto short final, and crashed nose first into the weeds both times. The Pitts is designed to be relatively unstable and stalls really well. Fortunately the Pitts is also built like a tank and there was no damage.

Yesterday, I was landing a Timber (an RC STOL fixed-wing aircraft) into a headwind that was equal to the aircraft’s airspeed, so with full flaps, the aircraft had a ground speed of zero. I was able to hover it over one spot and my glide slope was vertical. That was fun.

 

[Mavic Mac]

When I was a LEO I investigated (state level) a fatal aircraft accident and the cause was wing stall. Not being an aviation guy I was clueless to what a wing stall was.The NTSB investigator was gracious enough not to laugh at me and explained it to me. The pilot was flying below a peak of a mountain and a young couple were looking down onto his plane. The guy told his girlfriend that if the plane continued to do what it was doing it would crash. Unfortunately his words came true quickly when the plane tried to climb over the peak and turn at the same time. Nose first into the ground, the cone in front of the prop made a perfect indentation into the ground and it was visible where the prop spun for a split second before the plane tip onto it's back and incinerated. Both souls on board never had a chance

 

[HAWK-i.us]

Agree with all above. Counterpoint: Under reduced throttle on the downwind leg with apparent-to-the-observer good speed, the plane is actually descending due to the reduced power, but with good speed that descent angle is deceptively shallow, especially with the plane moving away. The stall in the turn will happen pretty easily if the pilot is controlling for constant altitude, but with a descending flight path in the approach to the turn, he may continue the descent in the turn and not experience the stall until he turns upwind. Then it gets ugly in a hurry.

Ain't this fun?

 

[R.Perry]

Does the Inspire work this way?
The Phantom doesn't unless you fly in atti mode.
In P-GPS with a tailwind, you can only pick up a small bonus in a strong tailwind.
DJI have the Phantom programmed such that it puts the brakes on for tailwind flying.

No, the Inspire uses GPS to control it's ground speed otherwise mapping would be a real problem. You are correct in atti mode wind does effect ground speed. Either way, if you are flying into the wind, you are using more power.