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I would like to know if creating a strong back pressure within a propeller duct will enable an aircraft to attain a higher altitude.

Please reference the conceptual drawing below.

This is showing a ducted propeller with the addition of a centrifugal blower/compressor. This hybrid propeller would consist of four parts sandwiched together and then welded in order to make one piece that rotates as one unit.

The purpose of the compressor is to create a strong back pressure within the duct which should enable the propeller to produce more thrust/lift and should enable the aircraft, particularly a helicopter or drone, to attain a higher-than-normal altitude. I believe that the merging of the airstream from the propellers with the airstream coming out of the compressor is what will create the strong back pressure throughout the duct.

I think one key application for this type of 'pressurized' ducted propeller would be high-altitude rescues such as rescuing stranded mountain climbers, or reaching an airliner that went down up in the mountains above the reach of most helicopters.

Will creating a strong back pressure within a ducted propeller enable an aircraft to attain a higher altitude?

enter image description here

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  • $\begingroup$ Helicopters are capable of reaching remarkable altitudes (see here). If your device works at all I doubt it would have sufficient effect to overcome the extra weight. $\endgroup$
    – CatchAsCatchCan
    Commented May 13, 2019 at 4:56
  • $\begingroup$ Is the blower in the way of the propeller air stream? $\endgroup$
    – Koyovis
    Commented May 13, 2019 at 6:32
  • $\begingroup$ @Redd Herring, I thought about the extra weight issue yet I am wondering if the increased thrust would overcome that. I am not an aeronautical engineer so I do not know how to calculate this to determine if it would or not. $\endgroup$
    – user36220
    Commented May 13, 2019 at 10:41
  • $\begingroup$ @Koyovis, yes it is. The two airstreams would compress one another as they exit the duct which as far as I can see should increase the overall static air pressure within the duct and cause the propellers to produce more thrust/lift. $\endgroup$
    – user36220
    Commented May 13, 2019 at 10:46
  • $\begingroup$ It's not pressure you want to increase. See some discussion in aviation.stackexchange.com/q/51934/524. $\endgroup$
    – Jan Hudec
    Commented May 14, 2019 at 5:50

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A very creative thought and project - a centrifugal compressor could be used as a means of propulsion if the exhaust stream is properly nozzled. So what you're basically doing is switching on another engine to produce more thrust, but:

  • The extra engine is in the way of the propeller, blocking its outlet flow.
  • Propellers and rotors are very efficient thrust generators on their own.
  • Creating static pressure is not the most effective way of creating thrust, better to convert it into air flow velocity.

Anything that is in the way of air flow creates back pressure and/or friction. The propeller flow is blocked, and the compressor flow runs into the side wall and does not accelerate any air backwards.

The way the air from the centrifugal compressor is normally deflected is depicted below (from an old uni book).

enter image description here

Air flow needs to flow and be guided gently. If it hits an obstruction, it pushes against it and creates negative thrust, like a person in a sailboat pushing against the mast.

It won't fly, I'm afraid...

Starting an extra engine might help in reaching places that are hard to get to. A simple propeller or turbofan is best - but when the extra propulsion is off, it is dead weight. Best to crank up the engines that are there already.

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  • $\begingroup$ @ Koyovis, I was thinking that the propeller airflow would bend the compressor airflow downward so that the compressor airflow would not run into the side wall and these two airflows would then exit the duct, yet I don't know whether or not this would occur. $\endgroup$
    – user36220
    Commented May 13, 2019 at 12:55
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    $\begingroup$ Yes to a an extent it will, but it needs a lot more space. Or a properly shaped duct that streams all the flow in the way that is most beneficial. I've added a drawing of a diffusor, normally mounted around a centrifugal compressor. $\endgroup$
    – Koyovis
    Commented May 13, 2019 at 13:09
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    $\begingroup$ The propeller would bend the airflow downwards, as it is creating a pressure difference. It is just not very efficient because it is a 90degree corner. But this could be fixed with a more sophisticated design. The problem still remains, that your compressor does not get enough air, especially not with such a small duct as depictied in your design $\endgroup$
    – Lumis
    Commented May 13, 2019 at 13:11
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    $\begingroup$ @Lumis There is a proper intake shape in the OP picture, so the free stream air would be compressed into it. The compressor only has to suck in the air itself when at zero airspeed. $\endgroup$
    – Koyovis
    Commented May 13, 2019 at 13:24
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What you are suggesting is effectively trading efficiency for higher altitudes.

This concept does indeed look creative, but does have a fatal flaw

Let's assume we try to hover with a drone/helicopter: In order to hover, lift generated by the propeller must be equal to the mass of your drone:

enter image description here

(m-Mass;g-gravimetric constant; ρ-air density; S-blade area;v-local blade speed)

This very simplified formula is the basis for the blade element theory, that is used to calculate the lift for a propeller

With higher altitidues the air density decreases. As blade area, gravimetric constant and mass remain the same, either lift coefficient or velocity (in this case RPM) have to be increased.

For an altitude of 5500m the necessary lift/RPM is already twice as high, for 11000m even four times.

enter image description here (Source:https://en.wikipedia.org/wiki/International_Standard_Atmosphere)

Your approach of using a would work, assuming you can increase the air density with you compressor. Unfortunately, another problem arises: where does your air come from? To provide a sufficient air density, the air intake has to be designed appropriately. Not only do you need a compressor, but in addition a suction pump is required to guarantee sufficient air intake. This is also the problem of conventional helicopters. Due to their stationary flight requirement, at a certain altitude there is not enough air to generate lift. Please note, that in this analysis engine heat problems (not enough cooling air) are omitted!

There are examples of startospheric aircraft using propellers. These aircraft are not helicopters, that are limited by their maximum possible RPM, but airplances, that can make up the necessary blade velocity by using their forward flight components in addition to the rotational component. Their propeller diameter is smaller than comparable aircraft while the blade area remains the same. This leads to multi-blade propellers with a larger relative chord lenght.

enter image description here (Source: https://www.intelligentliving.co/stratosphere-sunpower-plane/)

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  • $\begingroup$ @ Lumis, I am thinking that as long as the aircraft is in motion, air will be compressed into the air intake pipe of the centrifugal blower/compressor, and this will enable the aircraft to achieve higher altitude flight. This pressurized propeller duct will thus probably perform better as a propulsion device for an airplane than for a helicopter or drone. $\endgroup$
    – user36220
    Commented May 13, 2019 at 13:15
  • $\begingroup$ That's a given, as it recieves more air. $\endgroup$
    – Lumis
    Commented May 13, 2019 at 13:18

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