Do surfaces ahead of propellers decrease thrust?

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Discussion Overview

The discussion revolves around the effects of surfaces positioned ahead of propellers on thrust generation, particularly in the context of airships and multirotors. Participants explore experimental observations, theoretical implications, and comparisons between different types of aircraft, including blimps and quadcopters.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant questions how propellers on airships can generate thrust when their intakes are "somewhat blocked" by the airship's structure, citing personal experiments with a multirotor that failed to lift when obstructed by a foam sheet.
  • Another participant suggests that the difference in performance between airships and quadcopters may stem from the buoyancy of airships, which allows them to rely less on thrust for vertical lift compared to quadcopters.
  • Some participants note that 'puller' (tractor) props are generally more efficient in clean air compared to 'pusher' props in disturbed airflow, with implications for design constraints in aircraft.
  • One participant proposes modifications to the experimental setup, such as using U-shaped blocking surfaces to allow airflow to the propellers, and suggests various testing methods to better simulate airship conditions.
  • There is a mention of boundary layers and their effects on propeller efficiency, with some participants discussing the depth of boundary layers in relation to propeller diameters.
  • A participant indicates ongoing experimentation with flying models and expresses surprise at conflicting results, hinting at a forthcoming presentation of findings.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the impact of surfaces ahead of propellers on thrust, with no consensus reached on the underlying mechanisms or outcomes. The discussion remains unresolved with differing interpretations of experimental results and theoretical principles.

Contextual Notes

Some participants highlight limitations in existing data and the need for further experimentation to clarify the relationship between surface proximity, thrust generation, and aircraft design. There are references to specific aircraft designs and their operational contexts that may influence the discussion.

leviterande
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blimp.png

Hi!
I have had this question for ages, nearly impossible to find anything on the web. My experiments confused me even further! Its concerning the blockage effects of surfaces ahead of a propeller. For example let's consider the usual dual vectoring propellers on the sides of airshipcars like in the skyship 600 for instance.The gap between propeller disc and underside of blimp is 1diameter.

In vertical flight the propeller's intake are faced by the underside of the blimp body itself. How are the propellers still able to generate thrust when the propellers' intakes are "somewhat blocked" due the very close surface of the blimp?

As An experiment I attached a foam sheet about 1.5 diameters above a big 28 lbs 8-rotored octo- multirotor in hopes to see that the multirotor would still lift. At full throttle the multirotor didnt even move or lift. Obviously it lost almost all of its thrust due to the the sheet blocking the airflow above.. even though the foam sheet was high above the props, about 1.5 diameters.

Can someone help me on what could be going on here? does having a surface above a propeller decrease its thrust? Why did my experiment showed different results than the blimp. Are there other factors involved? if so.. what is the relation between distance, disc loading, surface shape, other factors etc and thrust decrease? Thanks!
Best Regards
 
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leviterande said:
For example let's consider the usual dual vectoring propellers on the sides of airshipcars like in the skyship 600 for instance.The gap between propeller disc and underside of blimp is 1diameter.

In vertical flight...
Do they take off vertically? I didn't think the did. Do you have any videos of blimps taking off that show it?
How are the propellers still able to generate thrust when the propellers' intakes are "somewhat blocked" due the very close surface of the blimp?

As An experiment I attached a foam sheet about 1.5 diameters above a big 28 lbs 8-rotored octo- multirotor in hopes to see that the multirotor would still lift. At full throttle the multirotor didnt even move or lift. Obviously it lost almost all of its thrust due to the the sheet blocking the airflow above.. even though the foam sheet was high above the props, about 1.5 diameters.

Can someone help me on what could be going on here?
Well, assuming you are correct about how blimps use their fans, the obvious answer would be in the difference between a blimp and a quadcopter. Blimps are close to neutrally buoyant and use their fans primarily for forward motion. Quadcopters need a large portion of their thrust just to get off the ground, so losing a fraction of the thrust could prevent liftoff.

Try doing your testing with the quadcopter sitting on a small scale.
 
IIRC, has been well established that 'puller' props, working in the 'clean' air ahead of fuselage or engine nacelle, are usually more efficient than 'pusher' props in 'disturbed' air-flow...

Design constraints may mean there's no-where else to put such a prop, so you got to live with it.
IMHO, this is less critical at the low air-speeds of air-ships, motor-gliders etc...

On the prototype US XB-42 'Mixmaster', IIRC, its 'pusher' layout allowed the fuselage to be very, very sleek, off-setting that efficiency loss and setting speed records. Soon trumped by jets, of course, of course...
 
Nik_2213 said:
US XB-42 'Mixmaster'
1568501582836.png
 
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For the OP's experiments (if you are still experimenting):

Consider bending the blocking foam surface in U shapes that allow some air flow to the props. The flat surface in your diagrams apparently 'stalls' your propellors. Airships permits air flow along surfaces to the (horizontally mounted) props.

To simulate a blimp shape or dirigible nacelles consider rigidly mounting an oblate spheroid like a 'Nerf' football or or lightweight (American) football with the engine-props in different positions depending on the test. The 28 pound octo might be excessive depending on your simulation. Consider the above aircraft designs with rear mounted engine and prop(s). Try passive tests vertically applying lift to a tether attached to a measured weight on a scale, as Russ suggests. The scale reading should decrease as lift increases.

For horizontal testing consider running the tether through 90 degrees, perhaps through a pulley, in order to use the same scale and weight. NASA uses strain gauges to test tethered scale models. A light purpose strain gauge may not be expensive. Or you can rig a facsimile from an old scale. Glue small threads or ribbons on surfaces to indicate air flow and direction.
 
Nik_2213 said:
... 'puller' [tractor] props, working in the 'clean' air ahead of fuselage or engine nacelle, are usually more efficient than 'pusher' props in 'disturbed' air-flow...
There is an efficiency increase (tractor or pusher) when the propeller operates inside the boundary layer because the boundary layer moves more slowly than the free stream.
 
My sloppy phrasing: I should have said 'turbulent' rather than 'disturbed'.

FWIW, I'd have thought boundary layers are generally much shallower than prop diameters...
 
Hi, I have done some more extensive experimentation on the subject with various flying models. I am going to present the results once I am finished soon. Data on such configurations are pretty much non existent on google. Let me just say I am a little surprised by my contradicting results but I think I have a conclusion. I experiment a lot, most of the things I look for arent found easily in NACA reports or such

russ_watters said:
Do they take off vertically? I didn't think the did?
Yes ofcourse but fans only lift a small portion of the all up weight due to buoyancy. Still they need to produce thrust in this position.
 
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Nik_2213 said:
I'd have thought boundary layers are generally much shallower than prop diameters...
True except for airships. On the Hindenburg, Akron and Macon, for examples, the propellers were immersed in the boundary layer, which yielded a significant increase in propulsive efficiency.
 
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