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Author Topic: Spiral Flow Force Measure  (Read 777 times)

Offline Peter Germann

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Spiral Flow Force Measure
« on: July 26, 2019, 03:36:31 AM »
Attempting to quantify the roll moment caused by the spiral airflow of a 2-blade tractor propeller a static test arrangement was made:

Airstream generator:
Electric driven propeller 13 x 6 CCW, rotating at 9’000 RPM 50 mm (2 in) in front of the airplane spinner backplate. Pitch Speed: 87 Km/h (54 mph)

Test object:
Bob Hunt “Crossfire” with controls locked at neutral. AoA 0°. Pivoting around prop shaft.

Measure:
Force acting down on outer tip at 760 mm or 30 in from centerline.

Result of static measure:
Force: 60 Grams or 2.1 oz
Torque:  0.447 Nm or 3.95 lb.in
Peter Germann


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Offline pmackenzie

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Re: Spiral Flow Force Measure
« Reply #1 on: July 26, 2019, 07:00:03 AM »
Interesting experiment  :)
Did you try it with the controls deflected to see if that had any effect?
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Offline Peter Germann

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Re: Spiral Flow Force Measure
« Reply #2 on: July 26, 2019, 09:03:21 AM »
I did not dare to do so as I was afraid my entire setup would be blown apart by the quite impressive airstream...
Peter Germann

Online Brett Buck

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Re: Spiral Flow Force Measure
« Reply #3 on: July 26, 2019, 09:51:14 AM »
Attempting to quantify the roll moment caused by the spiral airflow of a 2-blade tractor propeller a static test arrangement was made:

Airstream generator:
Electric driven propeller 13 x 6 CCW, rotating at 9’000 RPM 50 mm (2 in) in front of the airplane spinner backplate. Pitch Speed: 87 Km/h (54 mph)

Test object:
Bob Hunt “Crossfire” with controls locked at neutral. AoA 0°. Pivoting around prop shaft.

Measure:
Force acting down on outer tip at 760 mm or 30 in from centerline.

Result of static measure:
Force: 60 Grams or 2.1 oz
Torque:  0.447 Nm or 3.95 lb.in

      Fascinating! If this test setup is representative, it means that there is nearly no net angular momentum in roll from the propulsion system, at least in steady level flight. That makes sense, you probably can't to more than straighten the flow with otherwise straight aerodynamic surfaces. This is generally consistent with most observation since CL started - otherwise everybody's airplane would have an otherwise inexplicable flap tweak or vertical CG offset to compensate for it.  Of course, on the ground, at launch, this is exactly what the "flow straightening" torque is, but the motor torque is vastly higher, which they knew in the 40s. 

   I'll have to give some thought to what it does during maneuvering - like, does the drastic increase in engine torque when the speed is reduced (but about the same spiral flow) cause it to roll to the left (conventional rotation) or right (reverse rotation). I think it almost certainly does do that. That may be why reverse-rotation airplanes do funny things in the top of the hourglass - yes, precession might weakly yaw it the direction you want (nose-out) but it also rolls right which you *do not want* and is a much more powerful force. For example, the thrust is only maybe 2.5 lb, the lift in a corner is about 50 lbs, so a given roll angle has 20x the effect of the same yaw angle. 

  This has opened up a new train of thought for me, very interesting experiment. Outstanding!

    Brett
   
p.s. thinking about this, something just dawned on me about Rabe rudders and what they are actually trying to accomplish...

   

Offline Peter Germann

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Re: Spiral Flow Force Measure
« Reply #4 on: July 27, 2019, 03:28:51 AM »
For example, the thrust is only maybe 2.5 lb, the lift in a corner is about 50 lbs, so a given roll angle has 20x the effect of the same yaw angle. 

For the purpose of further thinking we way want to find in-flight thrust. Using: http://nclra.org/Programs/LineRake.php  the internet returns something like 0.8 pounds which seems to be a bit low.
Peter Germann

Offline Wolfgang Nieuwkamp

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Re: Spiral Flow Force Measure
« Reply #5 on: July 27, 2019, 09:23:28 AM »
Thinking over Peter´s result:

Basically, there are five rolling moments during horizontal flight:

1. Motor torque
2. Aerodynamic moment, depending on angle of attack, wing warp, flap position, (Rabe-) rudder, trim tabs etc.
3. Centrifugal force times the vertical distance between the CG and the leadouts
4. Weight (including lines) times the distance to the lift vector
5.The spiralflow torque, determined by Peter for the first time IMHO.

Flying overhead, only moment "4" becomes zero, because there is no lift.

Motor torque can be measured by its current, but all other moments are difficult to measure during flight....

Regards,

Wolfgang




Online Brett Buck

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Re: Spiral Flow Force Measure
« Reply #6 on: July 27, 2019, 11:11:40 AM »
For the purpose of further thinking we way want to find in-flight thrust. Using: http://nclra.org/Programs/LineRake.php  the internet returns something like 0.8 pounds which seems to be a bit low.

     That's just for the lines, I think. It's more like 1.75-2 lb (~8 to 9 newtons) total. That's 0.25-0.3 hp, which is roughly consistent with prop efficiency around 40-50% for our engines, given the known in-flight shaft power. You can back it out from your level flight power, figure the motor/controller is 85% efficient, and the prop 50%, so 42.5% of your battery power gets transferred to the airplane. That sounds about consistent with your previously stated 480 watts, that gives you .27 hp in level flight, which is 1.9 lb/8.5 newtons

    Brett

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Re: Spiral Flow Force Measure
« Reply #7 on: July 27, 2019, 10:18:20 PM »
Boscombe Downs found the effect was rougly the same as the airstream affect on the same stationary propellor .

So if at 200 mph the airstream had a rotational force of ' X ' on a stationary propellor ,
The same propellor generating a airframe speed of 200 mph had aproximately that rotational , along its axis , force against the airframe .

Thus a few differant props would produce a few differant results .As in differant design of prop  varying resultant .

Offline Chris Wilson

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Re: Spiral Flow Force Measure
« Reply #8 on: September 04, 2019, 05:08:42 PM »
Attempting to quantify the roll moment caused by the spiral airflow of a 2-blade tractor propeller a static test arrangement was made:


Hi Peter,
              if torque from the air flow acting on the airframe is the only item under investigation then wouldnt it be more accurate to fix the engine to the ground and simply have the airframe pivot?

Engine torque pulse may cloud the results but in practice I cant see how one seperates the two.

Chris.
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Offline pmackenzie

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Re: Spiral Flow Force Measure
« Reply #9 on: September 04, 2019, 05:47:25 PM »
Hi Peter,
              if torque from the air flow acting on the airframe is the only item under investigation then wouldnt it be more accurate to fix the engine to the ground and simply have the airframe pivot?



Look at his picture again, that is exactly what he did :)
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Offline Chris Wilson

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Re: Spiral Flow Force Measure
« Reply #10 on: September 04, 2019, 09:25:50 PM »
Look at his picture again, that is exactly what he did :)
Ok, thanks Pat.
Didn't pick that up before.
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 Nothing in life is so exhilarating as to be shot at without result.  It's not enough that we do our best; sometimes we have to do what's required


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