# Ducted Fans

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#### Unclematt

##### Well-Known Member
Do any of you know an average RPM a ducted fan of a 1 meter diameter might experience in VTOL use? I know thats vague, but a rough range would be fine.

#### KC135DELTA

##### Well-Known Member
Do any of you know an average RPM a ducted fan of a 1 meter diameter might experience in VTOL use? I know thats vague, but a rough range would be fine.
What kind of shaft hp are we talking about? Fixed blade pitch? If you have an engineer as a friend he can probably help you figure out the formulas listed above to a simple guess and check calculator.

#### Unclematt

##### Well-Known Member
What kind of shaft hp are we talking about? Fixed blade pitch? If you have an engineer as a friend he can probably help you figure out the formulas listed above to a simple guess and check calculator.
Let me ask this another way: what is the highest RPM you know of for a ducted fan of any size?

#### Lucrum

##### Well-Known Member
Log Member
Let me ask this another way: what is the highest RPM you know of for a ducted fan of any size?
Smaller diameter electric RC ducted fans can approach 50,000 RPM.

Fan speeds on smaller turbofan engine can be 10,000+ RPM.

Generally speaking for us lowly home builders it will be restricted by diameter and the desire to keep tip speeds well below mach 1.0. I think .70 to maybe .80 is common depending on materials.

#### Topaz

##### Super Moderator
Staff member
Log Member
Touching on an earlier topic within this thread, there is now video of the CC02 flying.

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#### Bob Mears

##### Well-Known Member
That thing needs a rotary engine in it just so it dont sound like a tractor!

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#### HumanPoweredDesigner

##### Well-Known Member
I'm no expert but just getting my novice opinion in on ducted fans:

They work best around props with very high disc loading, which probably have high coefficients of thrust. The savings comes largely from the reduction of tip losses. The skin drag and weight do not usually make up for this on lightly loaded discs, but are not as big a penalty on slow moving hover craft where the thrust is large compared to the skin drag of the duct...I mean shroud. I know even less about ducts, and was actually refering more to a shroud.

The tolerances must be exact, so usually the diameter is small, to keep weight down, and strength up, compared to thrust and vibrational forces. If you make it too light, it may get chopped up. And if it is not close enough, it will not have nearly as much benefit, and the benefits might not make up for the penalties.

#### orion

##### Well-Known Member
It is important to delineate between ducted fans and shrouded props since there are some significant differences in function and purpose.

#### Monty

##### Well-Known Member
I have looked at this problem extensively. The bottom line is that there is no way a ducted fan of equal installed power will ever match a prop in the GA flight regime.

A carefully and properly designed ducted fan can theoretically match a prop in cruise, but the propulsion efficiency will be horrible everywhere else. The ducted fan will require 2 to 3X the installed power of a prop plane of equivalent climb and takeoff performance. The cruise speed of the ducted fan will be higher due to the greater installed power. The airframe, fan, inlet, exit, and engine will have to be designed together just like a jet airframe. There is no technical reason to do this other than to get a jet fighter "look". The un-ducted fan will out perform the ducted fan. The complexity is high, and it can use a higher rpm auto engine without a redrive, but a CS prop with a reduction drive is still going to outperform the other two.

There is an iron clad relationship between the rotor loading and airframe speed dictated by physics. This is why helicopters use large rotors and large mass flow/hp and pure jets are on the other extreme. There is also an iron clad relationship between fuel burn and airframe speed. You simply can't cheat mother nature.

I'm not saying it can't be made to work, just that you are going to pay a significant price. There may be specific design trades where this price is acceptable, but don't expect miracles.

#### QuixoteDouglas

##### Member
Well have you read my thread on M0.7~0.85? In order to make a bucted fan work you need a very high power density out of the powerplant. Not imposible but you need an out side of the box solution. One of my life goals is to build a powerplant that is 1,000hp out off 1.5L that would quite effectively drive a ducted fan airplane to as fast as you wanted to go give or take 25% of the speed of sound.
Correct you are my friend----- 1000hp is necessary---

But----- not impossible any more.. One thing you forgot to stipulate is How heavy that power plant would be?

My current design utilizes a High output Aluminum race engine that has about a 500lbs attributed to it... along wih the paxton Supercharger (extra 40lbs) we will be getting 600+ Hp out of the single unit (engine actually rated 600HP on regular aspiration)

I have chose this powerplant so that we can achieve 400-450 at cruise RPM.

I will be using a twin engine, twin fan assembly and engineering this aircraft to come in under 3200 lbs-

Details to be listed once the Website is up...

Cheers

#### Noah

##### Member
A 1.5L I6 spinning 10,000rpm making 525ft lbs of torque would do the trick. The 2JZ and S54 both go to 9000 rpm and make 300ish torque with 3L Turbo to 2 atmospheres or 14psi and you've got a winner. I6s are mechanically balanced and thus bulletproof. If you want a cheaper route just get a Chevy LS v8 (9326 or 1). It will weigh about the same be limited to 350-500hp but be cheap. You'd have to have 40psi+ of boost to make 1000hp with 1.5L.

MAYBE you could do 40psi with 100LL but you probably need ethanol, a more complex chemically doped hydrocarbon or an oxidizer like no2. Ethanol is truly the best cheap fuel but its been under prohibition since we realized other countries can produce it cheaper than us. (example: Brazil doesn't have to bomb the middle east, but they do still have economic agricultural slavery like California has with illegal immigrants) HOWEVER a swimming pool sized tub of algae 4 inches deep could create enough hydrogen to power a normal v6 car indefinitely at the minimal cost of P,N,K aka fertilizer or a few cents on the dollar compared to fossil and bio fuels. Hydrogen is the ideal fuel. Watch out for genetically modified bacteria in the 21st century.... but you don't need GMO algae to make good hydrogen just like you don't need GMO to make good food.

Edit: This is assuming you want a piston engine and I have no idea why you would. H Ford originally made electric and ethanol cars and used hemp for many parts. Later, after lawsuits, he wrote his autobiography and talked non-stop about cotton because its the number 1 export in America. The piston engine was a cheap way to force car customers back to the dealer and limit their fuel choice. Same reason diesel is taxed twice that of gasoline to keep people buying diesels inherently keeping them from waste oil. Fords #1 friend is British Petroleum and GMs #1 (secret) friend is (or was before obama/bp/gm) Ford. I'd suggest using a ramjet for high speed cruise (below and above supersonic) and a ducted electric prop for getting up there and generating electricity once you're there. This solution would weigh the same but could theoretically last millions of hours without ANY new parts, use almost any fuel, and cost $10k instead of$30+k for the 1.5l assuming you did all the work yourself.

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#### Starman

##### Well-Known Member
I'd suggest using a ramjet for high speed cruise and a ducted electric prop for getting up there and generating electricity once you're there. This solution ... use almost any fuel, ...
Now there's a most excellent idea I haven't thought of yet, but where does the almost no fuel part come from?

That does sound like a good idea, do you think it could be practical and efficient?

#### Lucrum

##### Well-Known Member
Log Member
That does sound like a good idea, do you think it could be practical and efficient?
I wouldn't think so.

#### Starman

##### Well-Known Member
I wouldn't think so.
I now think I envisioned something that Noah didn't mean. I envisioned putting the electric prop (actually fan in this case) inside the ramjet intake, because you could use it to light the ramjet fire at low speeds and turn it into a turbojet, and it's all internal. Does that seem possibly practical or efficient? =)

OK, definitely not fuel efficient, but otherwise practical?

#### Dana

##### Super Moderator
Staff member
I seem to recall seeing something about a hybrid ramjet/turbojet for high altitude hypersonic flight. Take off and climb to altitude, then variable geometry to bypass the turbine and compressor. I don't know if it was ever tried or just something that was proposed, though.

-Dana

Life is a sexually transmitted disease.

##### Well-Known Member
I now think I envisioned something that Noah didn't mean. I envisioned putting the electric prop (actually fan in this case) inside the ramjet intake, because you could use it to light the ramjet fire at low speeds and turn it into a turbojet, and it's all internal. Does that seem possibly practical or efficient? =)

OK, definitely not fuel efficient, but otherwise practical?
I don't see it working in real life. We're talking huge amounts of power, a couple hundred G's at the tip and putting a prop+electric motor there...

The ramjet has two problems. Noise (comparable to a rocket) and fuel consumption. Both might be acceptable for T/O and landing only.

#### Noah

##### Member
Almost ANY fuel. Not fuel efficient until 500mph+. A turbine will be more efficient below 500mph but a ramjet can be as efficient above that speed. Its very easy to tune a ramjet for efficiency unlike a turbine.

Ramjets cost 1/1000th of a turbine. Use a motor to get up to ramjet speed and you're good to go.

By any fuel I mean:
Whiskey
Wood gas
Ethanol
Gasoline
100ll
etc.

Building them together would make it as efficient as a turbine at low speeds, but it would still require power to turn the fan at those low speeds. If you have the power to turn the fan, theres no point in using what is essentially an afterburner on your fan. It would be cool for low passes though!

Having them separate is cake because Ramjets are just tubes with a flameholder and can bolt anywhere outside the plane.

Electric fan would be perfect because you can have a lightweight 15 minutes of battery and get up to altitude before using the ramjets and fuel to recharge the batteries and maintain cruise.

Cliff notes:
Electric/Engine fan to get to altitude.
Low cost/reliable/MULTI FUEL ramjet to maintain cruise.
Could recharge electric fan like a RAT Ram air turbine - Wikipedia, the free encyclopedia

This is the cheapest and most reliable solution for high speed cruise short of replacing the fan unit altogether and using disposable rockets to get to ram/scram jet altitude like NASA does.

#### Starman

##### Well-Known Member
I prefer whiskey for fuel myself, but keep in mind your fuel supply will last about 20 seconds at most, and your batteries will get drained in about 40 seconds. Have you checked how it will glide when the power is off?