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Downwash loss because of cabin


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#1 voyagerB

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Posted 28 May 2017 - 05:40

Are there any data regarding the helicopter rotor that pushes down on the cabin and tail rotor beam? Here's a picture of an Apache. You can clearly see that a large portion of the rotor actually sweeps over the fuselage pushing it downward. It's not called or considered 'downwash' (since something else is referred to by that). It's called vertical drag. Can be as much as 5%. Dependent on design even higher. 

 

Apache_Helicopter_MOD_45150281.jpg


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#2 avbug

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Posted 28 May 2017 - 09:25

Are you asking about lift loss in the area of the fuselage or "cabin", or are you asking about drag and opposition to lift by pressure from above?

 

Think about the portion of the disc which has the highest rotational velocity, lift, and moment.  Where does that occur?



#3 voyagerB

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Posted 28 May 2017 - 14:32

Are you asking about lift loss in the area of the fuselage or "cabin", or are you asking about drag and opposition to lift by pressure from above?

 

Think about the portion of the disc which has the highest rotational velocity, lift, and moment.  Where does that occur?

I realize that. What's the larger disk area minus the small one, take in account the area not occupied by the fuselage. Below: Robinson R22.

 

r22%2B%25281%2529.jpg



#4 adam32

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Posted 28 May 2017 - 15:29

What are you asking?

Make your rifle, your targets worst nightmare!

 

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#5 avbug

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Posted 28 May 2017 - 15:45

I realize that. 

 

 

You realize what? 

 

First, let's determine what you're asking.  It's unclear.



#6 Eric Hunt

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Posted 29 May 2017 - 20:16

The data you ask for will be in the test flight records of the thrust required to fly - because that thrust is the "lift" to overcome the weight, plus the extra bit to overcome the vertical drag.

 

Bear in mind that the drag vector, parallel to the relative airflow, has a component downwards, adding to the weight of the aircraft. Lift plus thrust = weight plus drag. In the hover, weight and drag are aligned, but as forward speed increases, they split, but still act downwards.



#7 chris pochari

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Posted 30 May 2017 - 01:22

if you're asking about lift than I did some searching about your question and the first thing that came up was your question so apparently this is not a commonly discussed issue. My guess would be that the air is diverted around the cabin/fuselage so the end result is less efficiency of lift. As the air has to pass around the cabin/fuselage some lift is lost. I'm not quite sure what percentage it but as you can see from this picture the fuselage takes up a pretty big area of Uh-60 https://tiananmenstr...-helicopter.jpg. One thing to remember is there is less induced flow at the inner part of the rotor than the tip.

It's only common sense to assume that the air above the fuselage is not creating as much lift than the free area. 


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#8 iChris

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Posted 30 May 2017 - 03:42

Are there any data regarding the helicopter rotor that pushes down on the cabin and tail rotor beam?  You can clearly see that a large portion of the rotor actually sweeps over the fuselage pushing it downward.

 

It's not called or considered 'downwash' (since something else is referred to by that). It's called vertical drag. Can be as much as 5%. Dependent on design even higher. 

 

That’s correct, the term is “Vertical Drag” i.e. the download penalty. We assume the thrust required by the main rotor is equal to the weight of the helicopter; however, there is an additional degree of power required due to the download, vertical drag on the helicopter fuselage due to the rotors induced slipstream velocity, especially during low-speed hovering and vertical flight.

 

The drag effect is often express as a fraction of the gross weight. Because of the download produced on the fuselage by the rotor wake, the required rotor thrust is 4% to 7% larger than the gross weight, producing a corresponding increase in required power, that’s what your 5% term relates to.

 

There’s data available along with a number of technical papers and references. Below are two equations that yield an approximation of the vertical drag. Also, four technical references.

 

Screen%20Shot%202017-05-30%20at%202.32.0

 

 Estimating Vertical Drag on Helicopter Fuselage during Hovering

 

 See pg. 37 - Rotary- Wing Aerodynamics Volume II - Performance Prediction of Helicopters

 

 Rotorcraft low-speed download drag definition and its reduction

 

Principles of Helicopter Aerodynamics 2nd Edition by Gordon J. Leishman D.Sc.(Eng.) Ph.D. F.R.Ae.S.


Edited by iChris, 30 May 2017 - 04:58.

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Regards,

Chris

#9 voyagerB

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Posted 30 May 2017 - 11:40

Purely looking at the percentage of 'rotor-swept' fuselage, the Apache will account for approx. 12.5%.

 

apache%2Bheli%2B%25282%2529.jpg



#10 Canadian47

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Posted 30 May 2017 - 20:04

http://13thdimension...he-skies-again/

Batcopter_1966.png


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#11 voyagerB

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Posted 30 May 2017 - 23:37

Holy crap, Batman!... What if we take away the bat wings? I like this design. The bulky fuselage the rotors are encountering is offset by the fuselage being shaped to produce lift during flight.

 

article-0-1F5E1E5500000578-74_964x578.jp



#12 ARM_Coder

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Posted 01 June 2017 - 15:35

Holy crap, Batman!... What if we take away the bat wings? I like this design. The bulky fuselage the rotors are encountering is offset by the fuselage being shaped to produce lift during flight.

 

article-0-1F5E1E5500000578-74_964x578.jp

 

I'm not aware of this project but it's quite plausible that during cruise it flies like an autogyro, powering the mains just during VTOL procedures.

 

So no vertical drag on the fuselage during cruise.

 

Anyway, I think the vertical fuselage drag tends to diminish with airspeed in almost any helicopter, as the airflow starts to lag behind, "seeing" more of the tailcone and less of the pod.

 

Cheers!


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