The myth busters deal was a plane on a conveyor belt running in the opposite direction. the plane took off because its air speed was take off speed, regardess of the tires spinning 2x the normal "relative" "conveyor" ground speed because of a opposite moving conveyor belt. Big difference. the conveyor could be running 1000mph in the wrong direction, and as long as you have enough thrust to over come the rolling and bearing friction, the plane would take off at the normal, 50-70mph indicated air speed.

Cessnas cant take off at 0 indicated air speed but might be able to fly at 0 indicated hanging on the prop. , but it can take off at 0 true air speed. Big difference


Originally Posted by S4ordie
All of this makes me think about the "Myth Busters" episode where the put a plane on a conveyor belt to see if it would take off without forward progress. Amazingly, to me, it did. Does this somehow correlate?

A Cessna 150 will fly at zero indicated air speed with full power and full flaps, a very entertaining moment in my flight training.

All this dyno racing seems like harmless fun, but I don't see the point of using estimated HP as some figure of merit. Run the car down a 1/4 mile track and you will KNOW how much power it makes.

 

Did you read the article? Did you see the graph?? Am i the only one that sees the misleading representation here? the graph was hp vs mph. notice the shift inthe curve looks like you have gains everywhere, but if you shift the curves back to the diameter difference ratio, you see there is only a max hp gain of about 3hp as mentioned (could be due to anything, usually tempurature or a host of other factors) The tell tale reason that this test shows the wheels DONT do much of anything, is that if you do shift the curves to represent the same rpm of the engine, you would see the gains are ONLY at the top, which contradicts basic physics here. The gains are going to be based on rate of change of speed. SO, any inertial gains would be found as greater hp loss or gains in the lower rpm ranges, not the top, where the rate of accelertion would be lower.

EDIT: look at the curve again. the article mentinos a great HP loss at 70mph, but fails to show that the HP curves are based on MPH, not rpm, so they need to be shifted together, showing this 70mph HP difference is really nothing, (1-2HP)and there is only a peak gain up top. again, contraditing the fact that if there was a difference, it would be shown at the lower rpms and speeds as well. In the end, the gains are just as they would be expected they would be. the 3hp peak gains are believeable for redline gear of 110mph, and near a 10lb tire change with near 1.5" diameter change. in a taller gear, as we use for dyno runs, the gains would be much less.

its really simple, and not disputable. you give me a rate of acceleration, a comparable wheel and tire set up, and you can accurately determine the hp gain or loss, JUST as accurately as a dyno measures HP for your car.

Larry , what was the wheel weight difference between the stock wheels and larger wheels tested? repeatable, meaning you did the test and then put the stock wheels back on, while on the dyno? what gear were you in? 3rd 4th? you will get very different results based on the rate of dyno drum acceleration over any speed range.
In the magazine article, they did a 3rd gear run to near 110mph it looked like. HP was 110hp range, so the acceleration rate over the dyno test was probably over a 7 second period. 20lbs, 24" diameter (for 2 tires) rpm from 500 to 1000rpm, (actual wheel and tire rpm after gearing), might be 1 to 1.5hp. Im sure Glen could go through the rough calculation.

 

why do you say this? it absolutely measures absolute hp. you are measuring the cars ability to accelerate the drums rotation, giving the defintion of HP. (rate of change of kinetic energy). now, does it factor out inertia in the lower gears? yes. Is the friction of the road the same as the drum to tire contact? no.

yes, i agree, you do need to know the weight distribution. however, having most of the mass in the outer 3rd of the diameter is what Ived used for my calculations. generally, you can equate wheel and tire weight as 2x as if it was sitting in the car. For a rim change only , that factor would be more like 1.5 :1. (for a 18" rim).

very simple. take that weight, play with your hp /weight ratio and see the hp effects of 40lbs sitting in the car. could be like 4 hp as would be measured by the dyno. 10lbs per wheel and tire, x 2 tires. of course, since there is 4 of them, it could be an effect of 8hp, on the drag strip but not measured on the dyno, unless you were dynoing a 4x4.

It really depends on the rate of acceleration and the range of speeds. 1st gear effects will be a lot more than in 5th gear for example.

 

Calculations are never wong in this area, unless you are inputing the incorrect weights, diameter and distribution of the weight.

My tires and wheels weigh 48lbs for 305s on 18" rims. 20lbs rim, 28lb tire, unless its a hoosier, then its 25lbs diameter is 26". I forget what stock rims weigh, but my old C2 rims were 27 for the rim and then 26lbs for the 275 tire. going to the larger rims and tires saved 5lbs but were 1.5" taller.

go through the calculation and its near a wash. say you compare the stock 928 rims and tire combos which are near 40lbs , for a savings of 8 lbs or so per wheel.

again, the dyno uses the exact same math with the drums as we are talking about with the wheels and tires. It all based on rate of acceleration over any speed range.

HERE IS THE FORMULA for comparing weight on the wheel with weight in the car. Its very basic!

KE (rotation) = 1/2Iw^2
KE (linear) = 1/2M(wxR)^2
If you solve for M, you get a rule of thumb formula:

M=I/R^2

so, if you have a lbs of weight added on a tire, its like about 2lbs if it was sitting in the car.

 

An airplane on a REAL conveyor belt would NEVER take off. A plane standing still is functionless without lift under it's wings generated by the force of air.

Dan, just put a floor jack under your car while it's on the dyno, instant weight reduction...

 

[QUOTE=BB79;6796474]An airplane on a REAL conveyor belt would NEVER take off. A plane standing still is functionless without lift under it's wings generated by the force of air. =QUOTE]

Blythe - Should read - "lift 'over' it's wings"

Mark - IIRC the Myth Busters experiment only put the plane on a converyor belt that was indeed running backwards (to me this means the belt moved from the prop towards the tail) and the damn thing lifted off. Of course engine was at top rpm but nonetheless it surprised the hell out of me as there should have been no/little airflow over the wings in my mind.

BTW you are a hero to me. I just love the discussion and passion you bring to these debates. Even if I don't agree/understand what you are saying. Keep it up

 

actaully, the mythbusters test was running a conveyor belt pulled by a truck. the plane was easily able to overcome the rolling and bearing friction of the wheels spinning 2 x the take off speed, but forward movement was the same as it would be with it on the pavement directly. in otherwords, the airplane could careless what the wheels are doing, all the prop needs to do is pull the airplane forward, and overcome the slight braking force that the wheels would have on the reverse moving conveyor. YOu have to watch it again, but you might have missed the part that the airplane took off at normal take off speed through the air.

BTW, Thanks for that last comment. Its all good stuff!


[QUOTE=S4ordie;6796562]

 

are you sure about that?? think about it, as I just said to S4ortie, the ONLY thing that a conveyor would do to a boing 747 taking off , would be the wheels to roll faster. the air frame would not care, nor would the jet engines. even if the converyor was 4x the speed of take off in the reverse direction, it could never generate enough force to the wheels even spinning hundreds of mph, to keep the plane from taking off. (as long as they didnt blow up )

 

Exactly, the plane was indeed moving forward.

 

Um....no. You get an F

There is no way for a conveyor belt to prevent the plane from moving forward. Unless the belt is going so fast it burns out the bearings in the wheels causing the plane to crash.

 

Mark, so what your saying is the main reason the experiment worked was due to engine speed and not lift?
So this theory could have been proven by suspending a jet plane motionless while letting it come to full power and letting go?
This could also be called a missile.
I understand you if you if this is correct

 

Hacker, what is a tread mill? the plane from the show was rolling faster than the cloth it was on top of. If it was a "real" conveyor belt it would have stood still, like a jogger on a tread mill.
We are talking in theory...

 

Mark, all I can say is that I was in the plane with an excellent view of the ground not moving out the side window, and airspeed needle resting on zero, yet here I am typing this message so it did fly. It did not fly WELL, but airspeed slowly built up and I made it around the pattern and landed.

I suppose a Dynojet could measure real HP, but none of I know of are calibrated to any kind of standard. Most telling thing to me are the lists of timeslips found on many sites. Sort by 1/4 mile trap speed and then look at the range of claimed RWHP for cars around 125 mph. Mustang site I like the range is about 400 rwhp to 800 rwhp, and the weight differences are nothing close to explaining it.

We could argue all week about the technical reasons dynojet HP is a fact or fantasy, but how do you argue with so much empirical evidence of inaccuracy?

BTW do you think trap speed is a better indicator of torque or HP?

 

EXACTLY! think of the conveyor belt as a slight drag of the emergency brake for take off, thats it. whatever the rolling friction and internal friction of the wheels and bearings provide, the thrust has to over come this. no problem up to some pretty serious conveyor speeds in the opposite direction.

 

Tell me you cant be serious!

number 1 rule of aviation. airplanes need lift and forward momentum to fly if they dont have a 1:1 thrust ratio. Here are the facts:

1. You were in the airplane. Ill trust that
2. You did look at the airspeed indicator and saw 0 Ill trust that too.
3. The airspeed indicator was not showing the true airflow over the wing. REMEMBER, the pitot tube faces in the same direction as the fuselage. as the angle of attack rises, its ability to measure forward airspeed is severly compromised. in otherwords, if I clog your pitot tube, you will be going 45mph in a cessna ,just above stall, and maybe 30mph with full power and full flaps, but your "indicated airspeed" will be "0". will you be going 0 knots? NO!! you will be flying at the stall rating of the air frame, unless you have enough power to fly like a helecopter. Im sure other pilots here can concur.
4. your true air speed might have been 0, but that means you were flying in a pretty stiff headwind. This could have been why you looked out the window and didnt see the ground moving. But, for a plane to fly, you need air moving over the wings to the requirements of just above full power, full flaps stall speed.

DynoJets. There is no "calibration" nesessary. If we take the actual output, there is some smoothing, but no calibration. either you accelerate the drums or you dont. its that simple. the only calibration goes into the fine tuning of the output. (i.e. SAE output). So, if someone accelerates the drums from one speed to another, thats "X" HP, because of the rate of change of kinetic energy determined by only the size and mass of the drums, nothing else but some minimal bearing friction matters. (for actual output) . As you know there are too many factors to list for drag racing results. If there wasnt, you would just turn in your dyno sheets for a trophey.

mk