Centrifugal compressor lift aircraft -- engineering help please

[Olas]

Hello, I have a design for a different type of aircraft. I wanted an actual engineer to give advice before I fork over for some parts to try and make a working model. Attached are some pics. some of the pics are as if it were a full size acft. But for the model I was just going to use two turbo compressor wheels and a drive belt from an electric motor and make the frame out of foam or carbon plates.
Function: the compressor wheels will work as a normal turbo compressor, although not trying to make 20psi of boost. more of just enough air flow to be ducted out the winglets through movable nozzles like a Harrier jet kinda.

the image with the two turbine engines you can ignore. it would just be the ultimate end goal. I am just focused on making an eletric RC version first. There is much more design detail I have but mostly just for flight controls and what not.

 

[anorlunda]

Please clarify. Are you trying to make a hovercraft, or something like a Harrier Jet?

 

[Olas]

Please clarify. Are you trying to make a hovercraft, or something like a Harrier Jet?

More like the Harrier. But oriented more towards constant ability to hover. I've looked into different ACFT weights and engine power and it just seemed like this would work. If the heavy Harrier can hover, at least temporarily using exhaust and bleed air, I figured that I could make it work using what is essentialy a lot of compressor bleed air.

 

[anorlunda]

Hint:

How many kw (or hp) do you think the motor needs?
How many kw (or hp) do you think a Harrier Pegasus engine makes?
What powers the motor?

 

[Olas]

Hint:

How many kw (or hp) do you think the motor needs?
How many kw (or hp) do you think a Harrier Pegasus engine makes?
What powers the motor?

Well the latest model of Harrier was 14,000 LBS empty and 31,000 LBS max With an engine producing 23,500 LBS of thrust. So i figure My model could follow a similar ratio of pwr to wght and just hack off some zero (to over simplify)
I should need very little torque to spin the compressors, but quite a bit of RPM. I have several motors/batteries/controllers picked out along with different sized belt drive gears to swap out for optimum rpm/torq.

 

[Nik_2213]

Hi !
Be wary of the Harrier's Pegasus rated thrust, as several techniques were used to briefly augment it including, IIRC, water injection...

FWIW, I used to know the guy who was lead designer on 'Tiger Team' that solved getting enough thrust to take the Pegasus design supersonic...
Needed a real-clever 'afterburner' system, apparently, though not at the back...
Didn't happen, you say ?
But it did, just in time for the entire project to be killed, victim of trans-Atlantic politics...

 

[jim hardy]

I should need very little torque to spin the compressors,

i think you should experiment with a vacuum cleaner to get a 'feel' for centrifugal pump behavior.

Cover the hose on your vacuum cleaner and note how the motor speeds up..
Then uncover it and listen to the motor slow back down.
That fascinated me as a child
but i didnt understand until Physics class that the impeller gets a lot harder to turn when airflow commences.

old jim

 

[Olas]

I guess what I am asking, is if anyone knows the math on what RPM, per a specific compressor wheel, would be required to create enough air flow in order to lift an rc craft that maybe weighs 8 or 10 lbs and how much power it would need. I am not trying to make boost pressure with them, just enough air flow for lift.

 

[anorlunda]

If you shop for drones in the range of 8-10 pounds, you can find the specs on the motors used and the power consumption.

Knowing the battery voltage, AH storage rating of the battery, and maximum flight time yields power.

Of course that ignores efficiency, but it does give you and order of magnitude guess.

 

[RonL]

More like the Harrier. But oriented more towards constant ability to hover. I've looked into different ACFT weights and engine power and it just seemed like this would work. If the heavy Harrier can hover, at least temporarily using exhaust and bleed air, I figured that I could make it work using what is essentialy a lot of compressor bleed air.

I ran across this a few weeks ago, I believe it might be fitting for your studies. :)

 

[Olas]

I ran across this a few weeks ago, I believe it might be fitting for your studies. :)

This is pretty cool. My idea is only semi similar, I don't want ducted fans per say

 

[anorlunda]

two turbo compressor wheels

I don't want ducted fans

What is the difference in your mind?

 

[jim hardy]

I guess what I am asking, is if anyone knows the math on what RPM, per a specific compressor wheel, would be required to create enough air flow in order to lift an rc craft that maybe weighs 8 or 10 lbs and how much power it would need. I am not trying to make boost pressure with them, just enough air flow for lift.

i think you have a couple separate things to calculate

how much air do you need to move through your compressor
and to what pressure should you raise it before handing it to your exhaust nozzle...?

Clearly you'll want at least 8 pounds of thrust to hover

and here's an introduction to thrust
https://www.grc.nasa.gov/www/k-12/airplane/thrsteq.html

since you're adding no fuel your exit and inlet mass flow (m'_e and m'₀) are the same

so you'll raise pressure then expand it through a nozzle back to atmospheric pressure but it'll leave with greater velocity.

Now here's a painless way to understand thrust with hardly any math
it is a thought experiment:
consider a very un-streamlined rocket , one that's a perfect cube.
It has some internal pressure P psi(provided by your centrifugal pump)
and one un-streamlined nozzle - in fact so un-streamlined it's just a hole in the bottom.

You can get a rough estimate of the thrust you'll make by choosing internal pressure and nozzle area
a 2 inch diameter nozzle would have area pi square inches
and to make 8 pounds thrust would take, lessee here Force = Pressure X Area
so Pressure = Force/Area
and 8 pounds of force divided by pi square inches = about 2.5 psi.

So the question then becomes how much air will 2.5psi push through a 2 inch orifice?

Bernoulli's equation will let you estimate velocity increase due to a given pressure drop,
but be aware it's only approximate because the air is compressible so an exact answer takes more calculation than this simple presentation at
http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

Density of air is so small you'll get surprising velocities with that much pressure.

From velocity and area you know volumetric flow rate
multiply that by density and you know mass flow rate
and the power your pump has to supply is one half second's mass flow X velocity^{^2}

I suppose one could derive a one-step formula or try one from a book
but i think it's better to figure things put from the basics...

old jim

 

[trurle]

View attachment 239939 View attachment 239940 Hello, I have a design for a different type of aircraft. I wanted an actual engineer to give advice before I fork over for some parts to try and make a working model. Attached are some pics. some of the pics are as if it were a full size acft. But for the model I was just going to use two turbo compressor wheels and a drive belt from an electric motor and make the frame out of foam or carbon plates.
Function: the compressor wheels will work as a normal turbo compressor, although not trying to make 20psi of boost. more of just enough air flow to be ducted out the winglets through movable nozzles like a Harrier jet kinda.

the image with the two turbine engines you can ignore. it would just be the ultimate end goal. I am just focused on making an eletric RC version first. There is much more design detail I have but mostly just for flight controls and what not.

I remember seeing the similar-concept light VTOL experimental craft in the Kagamihara museum.
http://www.city.kakamigahara.lg.jp/webmagagine/6874/24752/027127.html

Prototype in Kagamihara routed to wingtips only a part of the airflow though - ducting all the airflow will produce much more roll authority than needed, without easy method of pitch control. Rule of thumb is what 10% of airflow will be to winglets for roll control, and 90% blown down near center-of-mass to produce lift. Differential thrusting of 2 motors seated in tandem before and after center-of-mass do also provide pitch control.

 

[CWatters]

RC model Harriers have been built and flown but it's not easy. I suggest you contact people who have actually done it and know the scale of the problem. Might be videos on YouTube.