How to Achieve 2500lbs+ Thrust: Ducted Fans, Turbofans, or Other Methods?

[jezym108]

hi
what is the easiest way of achieving 2500lbs+ of thrust, is it by using a ducted fan/propellors or turbofans?
first i also considered valveless pulse jet engines but i don't know if you can get enough thrust from them.

also are there better ways than the ones i mentioned?

help and advice will be appreciated

thanks

 

[Danger]

Welcome to PF, Jezym.
The easiest way would be a solid-fuel rocket. No moving parts.
What sort of project do you have in mind? 2,500 lbs is a hell of a lot of thrust to be playing around with if you don't know what you're doing. It could be hazardous.

 

[jezym108]

oh yeah
i completely forgot about the rocket but once the reaction starts you can't control it can you?
so I am basically looking for something that can be turned off and won't disintegrate the surrounding area

how much power would you need to produce 2500lbs of thrust with 70 inch blades in a ducted fan?
are there any formulas i can use to work that out?

i'm not doing it but the thrust is to generate lift for a VTOL aircraft
do you know how this could be done without helicopter rotors?

 

[Danger]

I don't. Fred Garvin and Astronuc are the resident experts in propulsion technology. Probably more so Fred in this case, since he works with jet engines daily. Astro's a bit more of a space engine guy.
There is also the possibility, though, of using a hybrid rocket. That is a solid fuel unit, so still fairly simple, but utilizes a throttleable liquid or gaseous oxydizer. You can therefore adjust the thrust as needed.
Cruise around the web a bit and look for the Mueller Sky Car as well. (I'm not sure that I spelled the name properly.) It was supposed to have hit the market by now... for over a million bucks... but apparently there have been some hang-ups. Anyhow, it uses 2 Wankel motors in each of 4 pods driving ducted fans. Maybe studying it can help you a bit.

 

[jezym108]

i thought about that as well
but for a vtol craft it wouldn't be safe would it because won't it just melt the tarmac

i thought that ducted fans are the best option because the thrust coming out from them would be cool enough for you to land on something soft like grass
thats why id like to know how about any equations where i can work out power needed if i know blade diameter and thrust

i don't think even a jet engines exhaust would get as a hot as a rocket's
i know rolls royce did an engine specifically for vtol:
http://en.wikipedia.org/wiki/Rolls-Royce_RB.108
and the size seems to be pretty good as well

 

[Danger]

You do seem to have a point about the thermal management. Direct jet-wash doesn't seem to be detrimental to tarmac over short periods; a rocket's power is a function of the exhaust speed/temperature, so they probably run them as hot as possible. I know that the old NERVA series nuclear rockets pumped the liquid hydrogen through a coil wrapped around the nozzle to keep it from melting (and to pre-heat the fuel).
Seriously, pal, I'm out of my depth here. I'm enjoying the conversation, but I'm afraid that I really can't be of any help to you. I don't even know if it would be advantageous to use variable-pitch blades in a ducted fan. Anything that I ever flew relied on wings for lift, not VTOL stuff.
I just realized that Russ Watters can probably help you out as well. While not in the field of aeronautics, he knows one hell of a lot about fans.

 

[jezym108]

well I am glad you have enjoyed the conversation
im going to contact frd garvin and russ watters
should i just search for them on the forums with the names you provided?

 

[Danger]

Yeah. Fred is off-line right now, but Russ is on. I just saw a new post by him up in the Physics section. If you're not in too much of a hurry, just hang around and wait; they'll show up here eventually. Otherwise, try Private Messaging them.

 

[jezym108]

ok danger
thanks for all your help

i'll give it till tomorrow and then PM them

 

[Danger]

Cool. I really have to log off now, anyhow. I'm at work, and so far haven't done any. Since there's only an hour and a half left in my shift, I probably should make some sort of attempt. I'll be back later today or tomorrow.

 

[russ_watters]

Ducted fans are good for VTOL applications because of the high volume and low velocity. You can calculate pounds of thrust with Bernoulli's equation and power with Newton's laws.

For example, say you pick your rotor/fan diameter to be 10'. Via Bernoulli's equation, p= 1/2 rho V^2, 2500 lb of thrust requires a velocity of 261 ft/sec. And that equals 652,500 ft-lb/sec or 1,200 hp. Then assume a 50% efficient fan and call it 2,400 hp.

 

[jezym108]

thanks for that russ
so if this was to be done you would have to use a turboshaft to power the blades

but this would be safer than a VTOL craft with jet thrust because the air coming from the ducted fan will be cool compared to the hot gases from a jet wouldn't it?
like wouldn't a harrier jump jet burn the ground when its taking off vertically?

are there any ways the power figure can be reduced
ie. increasing blade diameter

also this is a stupid question but what does 10' mean is that 10 feet or 10 inches?

 

[Danger]

' means feet; " is for inches.

 

[jezym108]

i thought it was that but i just had to be sure

so danger do you agree that a turboshaft powering blades in a ducted fan is safer than normal jet thrust because of the hot gases coming out as thrust in a jet?

 

[russ_watters]

but this would be safer than a VTOL craft with jet thrust because the air coming from the ducted fan will be cool compared to the hot gases from a jet wouldn't it?
like wouldn't a harrier jump jet burn the ground when its taking off vertically?

With a high bypass turbofan, the velocity and the temperature are both lower than with, say, a turbojet. But still, I think the Harrier did damage some runways (not sure, though).

are there any ways the power figure can be reduced
ie. increasing blade diameter

Yes. With larger blades you get more lift with less power.

Basically, what you are now describing is a helicopter, though. (you wouldn't duct a fan that big)

 

[Danger]

so danger do you agree that a turboshaft powering blades in a ducted fan is safer than normal jet thrust because of the hot gases coming out as thrust in a jet?

Absolutely. Although you'll still have that jet exhaust coming out somewhere, it's certainly easier to manage with a turboshaft.
Keep in mind, however, that the power figures from Russ are based upon that 10' fan. There's no way in the world that you would be using something that size as a single unit. You'll want a few smaller ones working together. The fans on the Sky Car, for example, are maybe 2' at most. (I'm going by the pictures, rather than a spec sheet.) It requires only 1 Wankel motor per fan (the second is a backup in case of failure). That's another aspect that you'll have to address in your design. Anything that flies without wings has to have multiple redundancy. An aeroplane can glide to safety if an engine quits; a VTOL can't. I think that the Sky Car also contains a large parachute as a last resort. (Helicopter rotors actually are wings, so they can autogyrate down.)

 

[jezym108]

yeah i figured as much that the blades will be too big for a duct

for the shape of the craft we were considering a using something aerodynamic
i had an idea of doing something like this:
http://www.smartfish.ch/index.cfm/fuseaction/show/path/1-2.htm

so danger are you saying we should use around 2 smaller fans powered by the 1 turboshaft engine (there shouldn't be too much weight in the craft)

russ if i had 2 50 inch diameter blades in ducted fans how much power would i need to lift 2700lbs?
(i tried the equation but i don't get it can you break it down and explain it to me?)

 

[Danger]

That's a sweet-looking ride.
You really should stop asking my opinion about this stuff, because I don't know what I'm talking about. It's way over my head. In keeping with your basic ideas, my approach would be to run two turboshafts with cross-linked power trains to run at least 2 fans per side. That way, if either an engine or a fan packs it up you stay in the air. Again, though, this is just a hobby for me. Listen to the guys who know what they're talking about.

 

[jezym108]

hey
i'm just running ideas by you and you seem to be coming up with some good ideas
and I am just glad people are replying to my questions

so run two turboshafts running 4 ducted fans? with that the engines alone will weigh about 500kg
(also i don't know if there will be enough space for 4 ducted fans) so what about 2 turboshafts powering 2 ducted fans?

i think overall the aircraft will have 3 engines, 2 for lift and one for moving forward, i wonder how much power will be needed to go 350mph+
(it would be great if fuel economy was over 10mpg)
know how this could be done?

i love the smartfish design it looks amazing

 

[Danger]

so run two turboshafts running 4 ducted fans? with that the engines alone will weigh about 500kg
(also i don't know if there will be enough space for 4 ducted fans) so what about 2 turboshafts powering 2 ducted fans?

If the safetly factor isn't of great significance, then just go with the one engine and a couple of fans. The reasoning for the set-up that I suggested is that if one engine quits, the other one can still drive both fans. The two fans per side aspect was to ensure symemtrical lift even if you happen to shell a blade on one of them. If this is purely a design exercise that's never going to get off of the ground, then you don't need any safety factors at all. That would be like putting airbags in a concept car; what's the point? Of course, that opens up a whole new world of possibilities, such as electrically-driven fans tapping into an isotope power supply.
Maybe it would be a good idea to walk us through exactly what this thing is supposed to be. I've got the basic concept, but I don't know what the rules are.

 

[RonL]

hey
i'm just running ideas by you and you seem to be coming up with some good ideas
and I am just glad people are replying to my questions

so run two turboshafts running 4 ducted fans? with that the engines alone will weigh about 500kg
(also i don't know if there will be enough space for 4 ducted fans) so what about 2 turboshafts powering 2 ducted fans?

i think overall the aircraft will have 3 engines, 2 for lift and one for moving forward, i wonder how much power will be needed to go 350mph+
(it would be great if fuel economy was over 10mpg)
know how this could be done?

i love the smartfish design it looks amazing

That is a beautiful plane, and it seems you have some study in front of you. the internet is now full of sites that give all the info you need. Not too long ago i was at a CAF hanger, and outside was a F-86 jet for sale, the pilot i was with said it uses around 150 gallons of fuel per hour, if that is anywhere close to correct, you will need deep pockets to feed your passion. Be it jet or piston engine, to fly that fast will =$$$$$$$$$ Hope you have fun

 

[jezym108]

well at the moment its just a group of us brainstorming about how this could be done, but if it turns out it is possible i don't see why we won't be able to start actually doing it apart from the financial problems that will arise
i would think a prototype alone would cost in the region of £100,000 (approx $195,000) depending on how cheaply we can get the big parts like the engines from, do you think a jet engine company will let you lease an engine from them?

well danger the idea is that it is an aircraft for 2-4 people which can achieve VTOL without using big helicopter rotors (and without burning the surrounding area) and go over 350mph, also regarding size i would think it should be able to fit in a area of 7 metres x 10 metres
and have fuel economy of 10mpg of above

i agree with you ronL
i calculated to get 10mpg you would have the engine going through 50 gallons in an hour when the plane is flying at 500mph
(also weren't older engine less efficient than modern engines?)

and i think with the aerodynamics of the smartfish this is possible, because i think they are proposing that their plane will hit over 500mph with 1000lbs of thrust with the weight at 1000kg

 

[FredGarvin]

2500 Lbf? That's right in our range. We deal with start up aircraft manufacturers all the time. Easy? No. But, it's a lot better than a rocket motor...and safer.

Your cost estimate is way off though. That $200k might get you a fuselage. It won't get you a single engine though, not to mention the control system. While leasing is a viable option for certified aircraft, you would be very hard pressed to let anyone lease you an engine for an experimental aircraft. You would have to buy them.

I certainly hope that you have a good idea of the preliminary analyses required to put an engine in an aircraft. Do you have any idea of what the inlet distortion is like? You want to go VSTOL, can you guarantee that the engine will have sufficient airflow in all modes of flight? Aircraft inlet design is extremely difficult when you need to combine both low speed and high speed aero effects. Distortion is a killer for compressors. That's just one consideration. My point being that you will need an obscene amount of money to get something like this to the first flight stage. Just an FYI.

 

[jezym108]

ok i understand
how much will an engine producing about 2500lb of thrust cost?
are there any engines similar to this:
http://en.wikipedia.org/wiki/Rolls-Royce_RB.108

also how hot will the exhaust gases be?

would you also say that using jet thrust is a better option than a turboshaft powering 2 ducted fans?

 

[FredGarvin]

Two ducted fans would more than likely be heavier than a turbofan with ducts like a Harrier does. However, it could be an easier set up to make work.

Cost? Well, I can't really say. I just don't know. Not cheap.

 

[jezym108]

can you just give an approximate figure for an engine with 2500lbf
are we looking at something over $40,000

for a jet thrust setup is the best way to copy the harrier design and have 4 ducts on the bottom of the craft?

also why do you say that it would be easier to have a ducted fan setup than a jet thrust setup i though it would be the opposite

 

[FredGarvin]

can you just give an approximate figure for an engine with 2500lbf
are we looking at something over $40,000

For a modern engine, you'd be looking at, most likely, something in the range of 8 to 10 times that. Perhaps you could find something used?

for a jet thrust setup is the best way to copy the harrier design and have 4 ducts on the bottom of the craft?

Well, I can't tell you what's "best" for your design. All I am saying is that a single source with ducting is going to be lighter and more efficient than a power turbine connected to a transmission connected to a fan that is connected to a duct.

also why do you say that it would be easier to have a ducted fan setup than a jet thrust setup i though it would be the opposite

I think it would be easier in that, as I said above, the mechanical layout would be easier from an airframer's perspective because you are not too terribly worried about aerodynamic effects on your engine like you are for a turbojet or fan. Like I mentioned before, if you use a turbofan, you have to be extremely careful, in all modes of flight, to ensure the inlet air going to the engines experiences as little distortion as possible. You still need to worry about that with a power turbine, but probably not to the same extreme. Also, you're looking at a ducting system that requires some very intricate engineering using a turbofan because of the way they need to move plus being able to handle high temperatures. A ducted fan set up wouldn't have quite as many headaches in that realm. Just like anything in engineering, you need to do a study to examine the tradeoffs involved with each method and look at them against your particular project. Each has their pros and cons. Just because a method is good for someone else doesn't mean it's good for you.

 

[jezym108]

thanks for the info fred

i agree with you that a single lift source is better because it saves the weight

a jet thrust maybe more complicated but the ducted fan setup will have increased weight and weight is the biggest problem for VTOL aircrafts

i was talking to a couple people about vtol and one came up with the idea of using compressed air/hydrogen peroxide tip jets as this would be cheaper than having jet thrust (also no torque effect)

one of the problems is that there will be a lot of drag during forward flight unless you can somehow 'hide' the rotors
2 ways that i thought of solving this was to actually turn one of the rotors (2 rotor craft) 180 degrees so won't the rotors just act like small secondary wings?
also i thought of some kind of extending rotors like an extending aerial and i did some research on it and i came across this site:
http://www.popularmechanics.com/technology/industry/4224761.html

so basically i can use both my solutions to the drag problems on the rotors from the video, first after transitioning into forward flight the rotors will be brought to a complete hault and the length of the rotors would have gone down and then on of the rotors will be turned by 180 degrees to act like an additional wing.
what do you think?
these rotors could be powered by a peroxide tip jet or from a secondary engine.

would this work?

 

[jezym108]

hey how come no one's replied

 

[RonL]

hey how come no one's replied

If you have not seen the program "battle of the x planes", go to PBS.org, a very comprehensive look at what two major companies went through to provide a successful design for the dept. of defense.

The design you would like to discuss is beyond the interest, or ability of all but a few on this, or any forum. Getting the information, and money together at the same time will be your challenge. I am very interested, but feel like any information i might have would be of little value in this design.

Do continue on, as there is a great future in design engineering, and things you learn from this will carry over into many other areas.

Ron