Do surfaces ahead of propellers decrease thrust?

[leviterande]

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

 

[russ_watters]

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.

 

[Nik_2213]

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...

 

[berkeman]

US XB-42 'Mixmaster'

 

[Klystron]

Possibly one of the weirdest looking push prop designs from WWII Japan,
Kyushu J7W rear mounted wings with canards
The rear engine and pusher prop were meant to be replaced by a jet engine.

 

[Klystron]

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.

 

[David Lewis]

... '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.

 

[Nik_2213]

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...

 

[leviterande]

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

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.

 

[David Lewis]

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.