Merged DDWFTTW in a “Real” Wind

[ThinAirDesigns]

Speed known.
Wind speed known
Frontal area known

Mix and match from there to your hearts content. Numerical results may vary.

 

[Llyricist]

Speed known.
Wind speed known
Frontal area known

Mix and match from there to your hearts content. Numerical results may vary.

I take it that it also was not decelerating at that point?

 

[ThinAirDesigns]

I take it that it also was not decelerating at that point?

I guess I don't follow as to which point you're referring to.

JB

 

[ThinAirDesigns]

Our best run (top speed) was 56mph. That however was not an attempt at the record, but rather just an early shakedown run to make sure all the sensors on the vehicle were working. (it also happened to be the run Richard filmed) Since it was not a record run, at that moment there were not ground based anemometers nearby to use as a check reference to the chase vehicle.

I went back and looked at my notes, and the 56mph run was at 18mph, not the 16mph I earlier reported.

JB

 

[Llyricist]

Our best run (top speed) was 56mph. That however was not an attempt at the record, but rather just an early shakedown run to make sure all the sensors on the vehicle were working. (it also happened to be the run Richard filmed) Since it was not a record run, at that moment there were not ground based anemometers nearby to use as a check reference to the chase vehicle.

I went back and looked at my notes, and the 56mph run was at 18mph, not the 16mph I earlier reported.

JB

I do believe that may make the difference.....

 

[Llyricist]

okay 85% drive train, Cd of .3, 88% prop with max at 92%

34 HP, 786 ft lb, 295 lbs of thrust.

 

[Llyricist]

I guess I don't follow as to which point you're referring to.

JB

at the point it was doing 56 mph in 18 mph wind.... but that was the point you told him to slow down, so...

 

[ThinAirDesigns]

okay 85% drive train, Cd of .3, 88% prop with max at 92%

34 HP, 786 ft lb, 295 lbs of thrust.

But of course do remember that just because we used 85% for our drivetrain calcs, that does not make the 85% necessarily correct.

JB

 

[humber]

Well yes, I'm sure we cheated when it comes to weight as well -- even though there are no rules regarding weight.

JB

Meaning that you don't have an official weight, but guessed it. However there is a post where you claimed NALSA weighed the cart.

 

[humber]

You seem to be confusing the limitations of a particular software package with actual physical limitations.

It is not included because there is no theory to support it. It's about time you really accepted that the theory is a collection of unrelated ideas, and complete fabrication.
The numbers don't match the submission calculation, and not the trial either.

 

[69dodge]

(Later, I'll try to work out the torque and thrust at different speeds or RPMS, based on the coefficients given for various advance ratios.)

Ok, I did it for 28 mph in a 10 mph wind. Again, ignoring friction.

If the cart moves at 28 mph, the propeller spins at 133.7 RPM or 2.228 rev/s. The airspeed of the cart is 18 mph, or 26.4 ft/s. And the diameter of the propeller is 16 ft. So, the advance ratio (jj) is 26.4/[(2.228)(16)] = 0.7406.

To get values for the coefficients ct and cq, I interpolated linearly between the two nearest values of jj given (0.7073 and 0.8252), and ended up with ct = 0.0834 and cq = 0.0120.

Then, the forward thrust of the propeller works out to be 61.32 lb, the torque driving the propeller is 141.2 ft-lb, and the backward force of the ground on the wheels is 48.11 lb.

That leaves 13.21 lb to overcome friction and accelerate the cart.

 

[humber]

But of course do remember that just because we used 85% for our drivetrain calcs, that does not make the 85% necessarily correct.

JB

That won't make up for the gaping gaps between the submission calculation, the Javaprop result, and the actual trial data, nor explain the how the cart can operate in the near static conditions either side of WS, when that condition is expressly excluded from the programme.

All you have is a standard prop calculation, when using an engine. Anyone able to operate a keyboard can produce that. It's all guess-work, grab-bag theory, trial and error, and finally, biased trials.

 

[spork]

And the speed of the cart is 25/7 = 3.57 times the wind speed.

We did achieve ratios of just over 3.5X wind speed, but we did nowhere near as much analysis on the data for those runs as we did on the record run. Whether it could sustain 3.5X wind speed in a steady wind on a good surface I can't say for sure. I'm inclined t think it can/did - but I'm perfectly happy with the well instrumented/documented 2.8X.

well taking the drive train down to 85% and increasing the Cd even a little, and I'm right back to running up against max prop efficiency at that speed combination.

I believe the drive train is likely a little over 90% efficient. We initially anticipated making far more extensive measurements of rolling resistance, drive-train efficiency, prop efficiency, etc.; but when it turned out the cart pulled forward on the dyno in all configurations and speed settings, we said screw it and headed for the dry lake.

 

[humber]

Ok, I did it for 28 mph in a 10 mph wind. Again, ignoring friction.

If the cart moves at 28 mph, the propeller spins at 133.7 RPM or 2.228 rev/s. The airspeed of the cart is 18 mph, or 26.4 ft/s. And the diameter of the propeller is 16 ft. So, the advance ratio (jj) is 26.4/[(2.228)(16)] = 0.7406.

To get values for the coefficients ct and cq, I interpolated linearly between the two nearest values of jj given (0.7073 and 0.8252), and ended up with ct = 0.0834 and cq = 0.0120.

Then, the forward thrust of the propeller works out to be 61.32 lb, the torque driving the propeller is 141.2 ft-lb, and the backward force of the ground on the wheels is 48.11 lb.

That leaves 13.21 lb to overcome friction and accelerate the cart.

The NALSA submission has the drag and rolling resistance as 13.4lbs at 20mph, so will be more at 28mph. Then there are the 3 efficiencies of that calculation to be included within yours, Dodge69.

 

[humber]

I believe the drive train is likely a little over 90% efficient.

But not the overall efficiency. You allowed a generous 85% for the prop in the submission report.

We initially anticipated making far more extensive measurements of rolling resistance, drive-train efficiency, prop efficiency, etc.; but when it turned out the cart pulled forward on the dyno in all configurations and speed settings, we said screw it and headed for the dry lake.

Which you did not actually do. The dyno proves nothing, and you never got the instruments to work. In the end, the entire dyno moved as a result of the 6hp engine, moving it. So, the "screw it" attitude, also applied to the trial.

 

[69dodge]

Ok, I did it for 28 mph in a 10 mph wind. Again, ignoring friction.

And now for 56 mph in an 18 mph wind.

If the cart moves at 56 mph, the propeller spins at 267.3 RPM or 4.455 rev/s. The airspeed of the cart is 38 mph, or 55.73 ft/s. And the diameter of the propeller is 16 ft. So, the advance ratio (jj) is 55.73/[(4.455)(16)] = 0.7818.

To get values for the coefficients ct and cq, I interpolated linearly between the two nearest values of jj given (0.7073 and 0.8252), and ended up with ct = 0.0784 and cq = 0.0117.

Then, the forward thrust of the propeller works out to be 230.5 lb, the torque driving the propeller is 550.3 ft-lb, and the backward force of the ground on the wheels is 187.5 lb.

That leaves 43 lb to overcome friction and accelerate the cart.

 

[profpangloss]

Thanks.

I thought so too.



I haven't seen I Ratant's video.

But, why can't a propeller produce thrust with a tailwind blowing through it?

Suppose the wind is blowing at 15 mph, but because the propeller is trying to blow air backwards, air moves through it at only 10 mph. I figure that the propeller will produce just as much thrust as if it were in stationary air, blowing air backwards through itself at 5 mph. No?

No. Thrust force is measured relative to the cart and has nothing to do with relative velocity of air blowing through the prop to the surrounding air. There must be an air mass passing through the propeller from front to back in order for it to be producing thrust force on the cart. If the air is passing through from back to front (tailwind blowing right through the spinning propeller) there is no thrust.

 

[profpangloss]

And now for 56 mph in an 18 mph wind.

If the cart moves at 56 mph, the propeller spins at 267.3 RPM or 4.455 rev/s. The airspeed of the cart is 38 mph, or 55.73 ft/s. And the diameter of the propeller is 16 ft. So, the advance ratio (jj) is 55.73/[(4.455)(16)] = 0.7818.

To get values for the coefficients ct and cq, I interpolated linearly between the two nearest values of jj given (0.7073 and 0.8252), and ended up with ct = 0.0784 and cq = 0.0117.

Then, the forward thrust of the propeller works out to be 230.5 lb, the torque driving the propeller is 550.3 ft-lb, and the backward force of the ground on the wheels is 187.5 lb.

That leaves 43 lb to overcome friction and accelerate the cart.

43 lbs isn't much! Have you calculated the rolling friction of the cart (650 lbs with rider) moving at 56 mph? Have you considered the force of the resultant headwind blowing at 38 mph, it will be considerable? (Sorry if you have already done all that, I just now tuned in and saw your post and have not caught up on the rest of the thread.)

 

[profpangloss]

I believe the drive train is likely a little over 90% efficient.

I seriously doubt that! A very short chain that is perfectly aligned can achieve theoretical efficiencies of over 90%. The chain on your cart is very long ( I'm guess over 5 meters total length?) and it is twisted. I'l go out on a limb and say it is less than 80% efficient, probably less.

 

[69dodge]

No. Thrust force is measured relative to the cart and has nothing to do with relative velocity of air blowing through the prop to the surrounding air. There must be an air mass passing through the propeller from front to back in order for it to be producing thrust force on the cart. If the air is passing through from back to front (tailwind blowing right through the spinning propeller) there is no thrust.

I disagree that "there must be an air mass passing through the propeller from front to back in order for it to be producing thrust." As long as the propeller accelerates air backward, it will produce a forward thrust. If the air is moving forward through the propeller, but the propeller slows that forward motion, it does accelerate the air backward.

The tailwind is not blowing "right through" the propeller; it's being slowed as it passes through. This is all that's required for the propeller to produce thrust.