

Jun 28, 2002, 04:18 PM  

Not the limit
Been waiting Paul for something more than “Facinating!”. More akin to your other aero postings.
In the meantime here are some of my thoughts: Have written before that I don’t think pitch speed limits the maximum airspeed, and that a prop adds velocity to the incoming flow. Found the following reference that indicates the same: http://beadec1.ea.bs.dlr.de/Airfoils/propuls4.htm . Also Ecalc indicates the same thing, in that flight speed is where thrust equals drag and that can be greater than pitch speed. 
Jun 28, 2002, 06:48 PM  

An illustration from my aero text.. "Technical Aerodynamics", K.D. Wood, 1955..
What Mark said... And I doubt than anyone other than a propellor manufacturer has real information on model airplane propellors and thrust. Inflight airspeed and rpm are required at the very least.. The forces involved are so small that a precision device would be needed to determine the numbers.. 
Jun 28, 2002, 10:44 PM  
Walled Lake, MI, USA
Joined Feb 2000
11,182 Posts

Tim, now see what you've started  all of the math and science nerds from high school are all grown up and they still remember all of the equations that I never learned.
I can't argue with any of the technical stuff. But I can tell Dick Huang that I have run billions and billions of computations in PCalc, and I have found many combinations that have virtually identical thrust figures for both static and inflight. I accept no blame for any incorrect data that may come out of any *Calc program. 
Jun 28, 2002, 10:56 PM  
California
Joined Jan 2002
859 Posts

Hi Dave, :) I just love Ezone and Ezoners for whatever it is... It is very educational and highly interesting. I love to learn what I don't know and review what I forgot... So the fun is there and many nice people in the zone... What more do I want? Oh yeah... more time for my planes... is that sensible? :)

Jun 29, 2002, 12:40 AM  

If you really wanna get into prop design, you need information like this..
Frankly, it's way too much effort for me, on a toy airplane, to do this much work. Let the geniuses at Landing Products, or Graupner scratch their heads bald trying to get this information at model scales and build the props. Which they do quite nicely however they develop them. As models at the sport level of flying are far from precision instruments, it's easiest to try it and fly it.. take a bucket full of props to the field, and fly them. Your plane will necessarily be different from all the other planes, including those from the same kit. What works on your plane may not work as well on next guy's "identical" plane. 
Jun 29, 2002, 10:40 AM  

Dave Hederich
"I can't argue with any of the technical stuff. But I can tell Dick Huang that I have run billions and billions of computations in PCalc, and I have found many combinations that have virtually identical thrust figures for both static and inflight. I accept no blame for any incorrect data that may come out of any *Calc program." Really,show me on sample and the velocity for the in flight thrust. Yours, Dick Huang 
Jun 29, 2002, 10:06 PM  

The inflight thrust must exactly equal the aero drag of the air frame (at constant velocity) and is a function of of the frontal area of the air frame. Larger frontal area, more drag.
A plane with 100 sq.in. of frontal area may have a maximum velocity of 25 mph. Use the same power setup (including the prop) on an aircraft that has half the frontal area. The top speed will be higher as there is less drag. Since drag must equal thrust at constant velocity the inflight thrust is now lower than with the 100 sq.in. frontal area ship. The difference between static thrust and inflight thrust is what allows your airplane to accelerate to its top speed. The mass of the airplane will determine how long it takes to reach that top speed. Heavier ships accelerate slower, need longer take off runs. The only accurate method of determining inflight thrust (aero drag) is to measure the constant velocity of the plane in level flight, zero wind (radar gun, etc,) and the frontal area. Knowing the ambient air temperature and relative humidity, the dynamic pressure due to velocity can be calculated, hence the the drag is known. This value is the inflight thrust. 
Jun 29, 2002, 10:16 PM  
Walled Lake, MI, USA
Joined Feb 2000
11,182 Posts

Dick, most people probably haven't seen the kind of numbers that I'm getting because I'm doing some pretty odd combinations. Basically, I was looking to overpower a lightweight (~9 oz.), high wing area (>500 sq. in.) IFO. The combination that I looked at was a high Kv Johnson 250 (EMPS MM1 in PCalc) on 7 AAA NiMH 720mAh cells in a 6.6:1 GWS 280size gearbox with a 10x4 APC prop.
The numbers that PCalc spit out are: 5.819 motor amps (close to the limit on the AAA NiMH cells), 21.4 mph pitch speed (the IFO can get away with a much lower pitch speed than most aircraft), 7:25 fullthrottle duration, 12.5 oz. static thrust and 12 oz. inflight thrust. As I said, I'm only telling you what PCalc told me. I'd be very interested if anyone with a technical background has any thoughts on these numbers and what they mean in real life. 

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