Lap Times in Wind

[Howard Rush]

I was talking with a name-brand stunt flier yesterday who said his plane maintains the same lap time regardless of wind.  Mine doesn't.  I wondered why.  I plotted lap time change in wind assuming constant airspeed.  Is this a good assumption?  It seems to agree with what my airplane does.  It's kinda interesting, so I've attached the plot.  Now what can I do to have my dog automatically speed up in the wind?

[Mark wood]

I was talking with a name-brand stunt flier yesterday who said his plane maintains the same lap time regardless of wind.  Mine doesn't.  I wondered why.  I plotted lap time change in wind assuming constant airspeed.  Is this a good assumption?  It seems to agree with what my airplane does.  It's kinda interesting, so I've attached the plot.  Now what can I do to have my dog automatically speed up in the wind?

How did you get the lap times? A CL airplane behaves slightly different than an airplane flying a closed course untethered. For an airplane flying a closed course the time it takes to fly the course without wind will always be less than with wind. A tethered airplane must do something very similar so it is likely that in one case the wind component is low enough that the impact is within tolerance of the measurement and difficult to discern. Consider a rectangular course with two legs directly in to the wind and down wind. The time it takes to fly the combined up and down leg is the same as with no wind because the average velocity is the airspeed of the airplane since the wind component cancels.

On the cross wind sides of the course, the airplane must turn in to the wind in order to track the course with an angle of say theta. Then theta is going to be the arctan of Vwind / Vairplane and the on course velocity is going to Vairplane time the cosine of Theta. Since the cosine is always less than one it will always take longer to fly the crosswind portions of the course. Hence an airplane flying a closed course will always take longer when there is a wind present.

For the tethered condition it is likely slightly different depending on how much the airplane yaws with respect to the wind. From my tail cam videos the yaw doesn't appear to be as much as I'd have thought so the time difference is likely less pronounced. To what magnitude, I'm not sure and I would turn to a total energy derivation to do that. It's the old downwind turn argument which I haven't worked out if a few years. Suffice it to say that for two equal airplanes the impact would be same. However for two geometrically equal airplanes of differing mass the impact will be slightly different as the question is one of where is the work coming from and going to. This, in and of itself, can account for the observation you have.

This all is predicated on the airplane flying basically a constant airspeed which is a reasonable assumption for level flight. A total energy discussion will unravel the change in airspeed necessary to maintain constant flight path velocity.


 

[Mike Alimov]

I was talking with a name-brand stunt flier yesterday who said his plane maintains the same lap time regardless of wind. 

If your math model predictions agree with real life measurements, the model must be largely correct.
Were you talking to the same name brand flier who trims CG at 15% chord and uses 2" of leadout sweep? The laws of physics don't apply to his planes, from what I've seen.

[Howard Rush]

Were you talking to the same name brand flier who trims CG at 15% chord and uses 2" of leadout sweep? The laws of physics don't apply to his planes, from what I've seen.

Sounds like the same guy. He's certainly found ways around some of those laws. 

[Serge_Krauss]

Consider a rectangular course with two legs directly in to the wind and down wind. The time it takes to fly the combined up and down leg is the same as with no wind because the average velocity is the airspeed of the airplane since the wind component cancels.

Unfortunately not true, since the plane spends more time flying up wind than it saves going down wind. At the windy 2004 Nats, some laps seemed to take forever. If the wind speed were to equal the plane's airspeed, the lap would never be completed, until the wind lessened.

[Ken Culbertson]

Unfortunately not true, since the plane spends more time flying up wind than it saves going down wind. At the windy 2004 Nats, some laps seemed to take forever. If the wind speed were to equal the plane's airspeed, the lap would never be completed, until the wind lessened.

It is a trick question.  Little Johnnie has to be at his grandmother's house 100 miles away in two hours.  He runs into traffic and only averages 25 mph for the first 50 miles.  How fast does Johnnie need to drive to make it on time?  There must be 2,000 versions of this on applied mathematics tests. (They do still teach it don't they?)

Ken

[Mark wood]

It is a trick question.  Little Johnnie has to be at his grandmother's house 100 miles away in two hours.  He runs into traffic and only averages 25 mph for the first 50 miles.  How fast does Johnnie need to drive to make it on time?  There must be 2,000 versions of this on applied mathematics tests. (They do still teach it don't they?)

Ken

Actually Serg is correct. It isn't a trick question but one that has to be regarded within the realm of reason. The bottom line is that the lap time in the wind really can't be equal to a no wind lap time. In my rectangular course example which is a method we use to calibrate the airspeed in an experimental makes an assumption that the wind speed is not a large fraction of the airspeed. The derivation uses a time based on the average speed up and down wind in which the wind velocity cancels. It is not valid when the wind speed is a significant portion of the airspeed. Over a closed course the majority of the time is lost flying across the wind as the airplane must fly in to the wind which reduces the velocity along the flight path in both directions. I didn't bother with too much rigor in the derivation.

[Brett Buck]

It is a trick question.  Little Johnnie has to be at his grandmother's house 100 miles away in two hours.  He runs into traffic and only averages 25 mph for the first 50 miles.  How fast does Johnnie need to drive to make it on time?  There must be 2,000 versions of this on applied mathematics tests. (They do still teach it don't they?)

  205, then wait just off the end of her driveway until it's exactly 2 hours?

    Brett

[Ken Culbertson]

Actually Serg is correct. It isn't a trick question.

Not your question, mine.  Your Avitar changed.  Is that a Pitts?  My little brother had a Pitts a while back.

Ken

[Mark wood]

Not your question, mine.  Your Avitar changed.  Is that a Pitts?  My little brother had a Pitts a while back.

Ken

Yes, it did. That is a Great Lakes 2T-1A which is one of the airplanes I once owned. The GL has a long history beginning in the thirties. The one I had was a 1974 model. It is one of the airplanes that I miss. I' saw Fred Casquin's Smith Mini plane photos and got nostalgic for the GL so I switched out.

[Bob Hunt]

This may or may not be pertinent to this thread, but at the Nats I started using the G force feature in the Fiorotti timer. My initial thought - and it turned out to be a correct one - was that the G force feature would give me better vertical and overhead tension in the high winds we encountered all week. The first practice flights for me - and a large number of fliers - yielded some very, shall we say, interesting moments in those areas of the pattern. And that was true of both glow and electric powered models.

When I activated the G force feature my plane had much more vertical and overhead tension. But, an added byproduct of that was the fact that in level flight I could hear and feel the G force feature adding lots of RPM when the model headed into the wind. It most certainly affected the lap times - in a good way! Did the model slow going into the wind? Yes, but much less than it did without that feature engaged. Those of you who are flying the Fiorotti system might give this a try.

As a brief aside, I've left the G force feature on since the Nats. It just adds a lot to the vertical maneuvers and the overheads even in low to moderate wind.

Later - Bob Hunt     

[Ken Culbertson]

This may or may not be pertinent to this thread, but at the Nats I started using the G force feature in the Fiorotti timer. My initial thought - and it turned out to be a correct one - was that the G force feature would give me better vertical and overhead tension in the high winds we encountered all week. The first practice flights for me - and a large number of fliers - yielded some very, shall we say, interesting moments in those areas of the pattern. And that was true of both glow and electric powered models.

When I activated the G force feature my plane had much more vertical and overhead tension. But, an added byproduct of that was the fact that in level flight I could hear and feel the G force feature adding lots of RPM when the model headed into the wind. It most certainly affected the lap times - in a good way! Did the model slow going into the wind? Yes, but much less than it did without that feature engaged. Those of you who are flying the Fiorotti system might give this a try.

As a brief aside, I've left the G force feature on since the Nats. It just adds a lot to the vertical maneuvers and the overheads even in low to moderate wind.

Later - Bob Hunt   

I am glad to hear that.  I just turned mine on but have yet to be able to fly it.  I have been using a CAM rudder to get that extra overhead tension which works quite well but also affects the rounds unnecessarily.  I am surprised that it also boosts the into the wind speed.  That is a welcome feature.  Not sure how gravity changes with ground speed but I will take all the help I can get!

Ken

[Bob Hunt]

Hi Ken:

One thing I forgot to mention: I'm using a reverse pitch prop, and that gives me increased tension on the outside portions of maneuvers, especially in the vertical maneuvers. I tried the light standard pitch props, and they just did not work for me. I'm using a lightened Cox/Resinger 11 x 5 three blade. Lots of variables...

Bob   

[Serge_Krauss]

"It is a trick question.  Little Johnnie has to be at his grandmother's house 100 miles away in two hours.  He runs into traffic and only averages 25 mph for the first 50 miles.  How fast does Johnnie need to drive to make it on time?  There must be 2,000 versions of this on applied mathematics tests. (They do still teach it don't they?)"

  205, then wait just off the end of her driveway until it's exactly 2 hours?    Brett

Well, it's sort of a trick question, because it encourages solvers to take an incorrect short cut. The answer is that there is no way then to get there in two hours. The two hours have been used to go 50 miles. There is no time left.

There actually isn't much difference in lap times for "normal" winds, but in higher winds where people still fly, like, say, 15 mph it's noticeable. If the plane flies 55 mi/hr, and the distance is 140ft + 140 ft = 280 ft,

V_w = 15 mph = 15 x 88/60 = 22.00 ft/s,
V_p = 55 mph = 55 x 88/60 = 80.66... ft/s
Ground Speeds: Upwind V_u = 80.67 - 22.00 = 58.67 ft/s; Downwind V_d = 80.67 + 22.00 = 102.67 ft/s

V = d/t => t = d/v: t_u = 140 ft/ 58.67 ft/s = 2.39 s; t_d = 140/102.67 = 1.36 s

t_tot = 2.39 + 1.36 = 3.75 s in the wind.
t without wind = 280/80.67 ft/s = 3.47 s

'gotta run. 'hope I didn't multiply wrong. Edit: I may multiply right, but as I ran out of the house, it appears that I did not subtract right. So I have corrected the upwind and downwind speeds and the rest accordingly. 'sorry!

[Ken Culbertson]

"It is a trick question.  Little Johnnie has to be at his grandmother's house 100 miles away in two hours.  He runs into traffic and only averages 25 mph for the first 50 miles.  How fast does Johnnie need to drive to make it on time?  There must be 2,000 versions of this on applied mathematics tests. (They do still teach it don't they?)"

Well, it's sort of a trick question, because it encourages solvers to take an incorrect short cut. The answer is that there is no way then to get there in two hours. The two hours have been used to go 50 miles. There is no time left.

We have a WINNER!

Ken

[Mark wood]

"It is a trick question.  Little Johnnie has to be at his grandmother's house 100 miles away in two hours.  He runs into traffic and only averages 25 mph for the first 50 miles.  How fast does Johnnie need to drive to make it on time?  There must be 2,000 versions of this on applied mathematics tests. (They do still teach it don't they?)"

Well, it's sort of a trick question, because it encourages solvers to take an incorrect short cut.

Guilty

The time it takes to travel a distance X up wind T = X/(V-v)  As v approaches V the the denominator approaches 0 which means the time it takes approaches infinity.

[Howard Rush]

This may or may not be pertinent to this thread, but at the Nats I started using the G force feature in the Fiorotti timer. My initial thought - and it turned out to be a correct one - was that the G force feature would give me better vertical and overhead tension in the high winds we encountered all week. The first practice flights for me - and a large number of fliers - yielded some very, shall we say, interesting moments in those areas of the pattern. And that was true of both glow and electric powered models.

When I activated the G force feature my plane had much more vertical and overhead tension. But, an added byproduct of that was the fact that in level flight I could hear and feel the G force feature adding lots of RPM when the model headed into the wind. It most certainly affected the lap times - in a good way! Did the model slow going into the wind? Yes, but much less than it did without that feature engaged. Those of you who are flying the Fiorotti system might give this a try.

As a brief aside, I've left the G force feature on since the Nats. It just adds a lot to the vertical maneuvers and the overheads even in low to moderate wind.

Later - Bob Hunt     

This really isn’t related, but the red LEDs I put in BICAS units are no longer made. They were pretty bright, but the replacement parts are half again as bright. I figure the extra side thrust in the nose will help overhead line tension.

[Mark wood]

This may or may not be pertinent to this thread, but at the Nats I started using the G force feature in the Fiorotti timer. My initial thought - and it turned out to be a correct one - was that the G force feature would give me better vertical and overhead tension in the high winds we encountered all week. The first practice flights for me - and a large number of fliers - yielded some very, shall we say, interesting moments in those areas of the pattern. And that was true of both glow and electric powered models.

When I activated the G force feature my plane had much more vertical and overhead tension. But, an added byproduct of that was the fact that in level flight I could hear and feel the G force feature adding lots of RPM when the model headed into the wind. It most certainly affected the lap times - in a good way! Did the model slow going into the wind? Yes, but much less than it did without that feature engaged. Those of you who are flying the Fiorotti system might give this a try.

As a brief aside, I've left the G force feature on since the Nats. It just adds a lot to the vertical maneuvers and the overheads even in low to moderate wind.

Later - Bob Hunt   

Interesting. I had the same thing happening but in revers. flying in to the wind my power decreased and with the wind increased. Turned out my sensor was not perfectly aligned and when I went back and aligned it the surging went away.

[Tim Wescott]

The time it takes to fly the combined up and down leg is the same as with no wind because the average velocity is the airspeed of the airplane since the wind component cancels.

Erm -- absolutely true, except for the bit about it being wrong.

Consider the extreme example, of a plane flying 25 m/s on a 25 meter square course, aligned with a 25m/s wind.  For the downwind leg, the plane has a ground speed of 50m/s, and that leg takes half as long to fly.  For the upwind leg, the plane has a ground speed of 0m/s, and that leg takes -- oh, damn, there's a divide by zero error here while computing the time it takes to fly the leg...

If you take the average velocity as being the time average of velocity, then the upwind and downwind ground velocities don't average out to the airspeed, because the plane takes longer flying the upwind leg (even if it's not taking an infinite amount of time).

This would actually be a good question for a highschool AP algebra class, with extra credit for doing the calculation for a plane going in a circle.

• Downwind: ground speed = airspeed + wind speed; time = distance / (airspeed + wind speed)
• Upwind: ground speed = airspeed - wind speed; time = distance / (airspeed - wind speed)
• Crosswind: ground speed = sqrt(airspeed² - (wind speed)²); time = distance / (all that stuff)

[Tim Wescott]

When I activated the G force feature my plane had much more vertical and overhead tension. But, an added byproduct of that was the fact that in level flight I could hear and feel the G force feature adding lots of RPM when the model headed into the wind. It most certainly affected the lap times - in a good way! Did the model slow going into the wind? Yes, but much less than it did without that feature engaged. Those of you who are flying the Fiorotti system might give this a try.

That's interesting -- because I would expect that a Fioretti system working off of just the gyro would have the same effect, at least if the control were aggressive enough.  Reason being that it's servoing the average yaw angle to the target value; if it did that "perfectly"* then your lap times would always be dead on.

* "perfect" is always a dangerous word in engineering -- you often end up being perfectly wrong everywhere other than the one thing you just made "perfect".

[Mark wood]

Erm -- absolutely true, except for the bit about it being wrong.

Consider the extreme example, of a plane flying 25 m/s on a 25 meter square course, aligned with a 25m/s wind.  For the downwind leg, the plane has a ground speed of 50m/s, and that leg takes half as long to fly.  For the upwind leg, the plane has a ground speed of 0m/s, and that leg takes -- oh, damn, there's a divide by zero error here while computing the time it takes to fly the leg...

If you take the average velocity as being the

time average of velocity, then the upwind and downwind ground velocities don't average out to the airspeed, because the plane takes longer flying the upwind leg (even if it's not taking an infinite amount of time).

This would actually be a good question for a highschool AP algebra class, with extra credit for doing the calculation for a plane going in a circle.

• Downwind: ground speed = airspeed + wind speed; time = distance / (airspeed + wind speed)
• Upwind: ground speed = airspeed - wind speed; time = distance / (airspeed - wind speed)
• Crosswind: ground speed = sqrt(airspeed² - (wind speed)²); time = distance / (all that stuff)

Yeah Serge already busted me on that one. You're late to the party Tim. [/list]

[Howard Rush]

That's interesting -- because I would expect that a Fioretti system working off of just the gyro would have the same effect, at least if the control were aggressive enough.  Reason being that it's servoing the average yaw angle to the target value; if it did that "perfectly"* then your lap times would always be dead on.

I don't know what Fiorotti's control law is, nor what he means by G force, but I considered (and we probably talked about it) using yaw rate, which the TUT measures, as a surrogate for ground speed.  Controlling to yaw rate would boost the throttle going uphill and into the wind.  Alas, the airplane with lines has a yaw oscillatory mode at a frequency you'd like to use for the ground speed control.  I think Igor knew this long ago. 

[Mark wood]

I don't know what Fiorotti's control law is, nor what he means by G force, but I considered (and we probably talked about it) using yaw rate, which the TUT measures, as a surrogate for ground speed.  Controlling to yaw rate would boost the throttle going uphill and into the wind.  Alas, the airplane with lines has a yaw oscillatory mode at a frequency you'd like to use for the ground speed control.  I think Igor knew this long ago.

I don't either. The only thing I know is when I zeroed all the "axis'", and they weren't, the power surging mostly went away. I put all of my learning efforts on that down a while back when I decided there were existing solutions that work reasonably well enough. The Fiorotti certainly fits the well enough zone.