Merged DDWFTTW in a “Real” Wind

[CORed]

Does Javaprop accept negative values for the airspeed? That must be the case when above WS.

Humber's confused again. What a shock.

 

[ThinAirDesigns]

And they are the numbers known and checked by NALSA?

NALSA has no weight rules for the class.

JB

 

[Subduction Zone]

Does Javaprop accept negative values for the airspeed? That must be the case when above WS.

Wrong again humber. When an airplane is flying it is always flying at a speed "above WS". So one more time, humber wrong, try again.

 

[Llyricist]

What do you get for 56mph in a 16mph wind?

JB

I get it bumping right up against the max theoretical prop efficiency. Using the same 89 percent the max theoretical comes up as 89.287%. Any higher and the max theoretical is lower, and too much lower and the max theoretical is lower.

but the power is 53 hp at the shaft, 1131 ft lbs of torque, thrust of 442 lbs.

That all seems too high to me, I'm thinking the drive train might be better than 92%.

I had done it before at 54 MPH in a 20 MPH wind and the numbers came out much friendlier.

 

[Llyricist]

Does Javaprop accept negative values for the airspeed? That must be the case when above WS.

1. I am not using JavaProp, I am using the generic formula that you are familiar with.

2. Airspeed is negative below wind speed, not above.

 

[humber]

NALSA has no weight rules for the class.

JB

Do you have a check weigh result?

 

[humber]

1. I am not using JavaProp, I am using the generic formula that you are familiar with.

2. Airspeed is negative below wind speed, not above.

Sure it is, but the answer is the same, Cored, No.

If you are using the generic formula then you agree that power must be added to the prop shaft, Because the is Dr Hepperle's assumption, yet you have no power to do that, and whatever the case, the results must agree with Javaprop, and the trial.

 

[ThinAirDesigns]

I get it bumping right up against the max theoretical prop efficiency. Using the same 89 percent the max theoretical comes up as 89.287%. Any higher and the max theoretical is lower, and too much lower and the max theoretical is lower.

but the power is 53 hp at the shaft, 1131 ft lbs of torque, thrust of 442 lbs.

That all seems too high to me, I'm thinking the drive train might be better than 92%.

I had done it before at 54 MPH in a 20 MPH wind and the numbers came out much friendlier.

Try using 2 sq meters frontal. We're a bit less than that even.

 

[ThinAirDesigns]

Do you have a check weigh result?

Do you have a question that makes sense?

JB

 

[humber]

Do you have a question that makes sense?

JB

Some recognised evidence of the cart's weight. Weight can make a lot of difference.
And the prop data for 132rpm?

Javaprop won't work for negative airspeeds, and you have not posted the data for start-up at all. If the theory fails to show that prop thrust is responsible for motion from a standstill, then it must be something else.

 

[CORed]

Do you have a question that makes sense?

JB

There's always a first time, but probably not.

 

[69dodge]

No problem. I'll be happy to answer questions.

I did some simple calculations, based only on the airspeed and RPM near the top of your post. (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.)

120 propeller RPM corresponds to 25 mph. So, to get an airspeed of 18 mph requires a tailwind of 7 mph.

The torque at the propeller is given as 124.1 ft-lb. So the torque at the wheel is (124.1 ft-lb)(23/60) = 47.57 ft-lb. As the wheel has a radius of 13.5" = 1.125', the force at the wheel's interface with the ground is (47.57 ft-lb) / (1.125 ft) = 42.28 lb.

Which is less than the propeller thrust of 49.92 lb.

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

Of course, I haven't accounted for any frictional losses. But, clearly, it's wrong to say that, in principle, the propeller can't produce more thrust than the force which turns the wheels.

 

[Llyricist]

Try using 2 sq meters frontal. We're a bit less than that even.

Well that takes it too far down I think, maybe the Cd is a bit higher?

Now I'm getting 17 HP, 366 ft lb, 150 lbs of thrust

using 93% prop efficiency with max theoretical of 95.8%

 

[CORed]

Some recognised evidence of the cart's weight. Weight can make a lot of difference.
And the prop data for 132rpm?

Javaprop won't work for negative airspeeds, and you have not posted the data for start-up at all. If the theory fails to show that prop thrust is responsible for motion from a standstill, then it must be something else.

Ya think?

 

[humber]

There's always a first time, but probably not.

Java prop can't be used for negative values, and that is a problem for explaining how the cart moves at all. But even at 18mph above WS, the prop will be turning at 132rpm, and that takes a lot more power.

 

[ThinAirDesigns]

Weight can make a lot of difference.

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

JB

 

[Llyricist]

I did some simple calculations, based only on the airspeed and RPM near the top of your post. (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.)

120 propeller RPM corresponds to 25 mph. So, to get an airspeed of 18 mph requires a tailwind of 7 mph.

The torque at the propeller is given as 124.1 ft-lb. So the torque at the wheel is (124.1 ft-lb)(23/60) = 47.57 ft-lb. As the wheel has a radius of 13.5" = 1.125', the force at the wheel's interface with the ground is (47.57 ft-lb) / (1.125 ft) = 42.28 lb.

Which is less than the propeller thrust of 49.92 lb.

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

Of course, I haven't accounted for any frictional losses. But, clearly, it's wrong to say that, in principle, the propeller can't produce more thrust than the force which turns the wheels.

And the energy to feed that feedback loop comes from slowing down the wind.

 

[CORed]

Java prop can't be used for negative values, and that is a problem for explaining how the cart moves at all. But even at 18mph above WS, the prop will be turning at 132rpm, and that takes a lot more power.

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

 

[ThinAirDesigns]

Well that takes it too far down I think, maybe the Cd is a bit higher?

Now I'm getting 17 HP, 366 ft lb, 150 lbs of thrust

using 93% prop efficiency with max theoretical of 95.8%

I'm confident of frontal area. All others will be educated plug-ins.

While we certain did quite a bit of fairing, there is still a lot of dirty all over that chassis. Rolling resistance on a broken dirt surface = ?? Drivetrain efficiency -- (that includes all bearings (9) in a dirty environment plus twisted chain) I believe we used 85%. Prop efficiency - it was our first prop so I wouldn't expect the high end of possible first round.

JB

 

[Llyricist]

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.