Ravivos

Hi.
I am currently planning a VTOL aircraft with a rotating engine.
the prop and engine will be mounted inside a shroud to generate a "prop fan"
the main idea is to build the shroud as tight as possible around the prop.
would it have any affect to the thrust (hopefully will generate more) or will it just straitning the propwash.

i am planning the entire aircraft using a CAD program.

Thank you for your answer.
Raviv.

BMatthews

Truthfully I don't think this is a good idea. The engine would need to be balanced for starters or it'll shake the whole works to pieces. Also vibration at that level will reduce the power output of the engine. And finally even if you did use a counterweight to accurately enough balance the engine the high RPM's needed to generate significant thrust will make the charge flow inside the engine into something that was never intended.

The rotating engine on things like the Cox helicopter work becuause the engine is attached to the rotor blade which only turns at lower RPM values where none of this is an issue.

So I'd say stick to a fixed engine and let the prop do the job it was intended for.

victorzamora

Do you mean that the engine spins in the opposite direction of the prop....because of torque rotating it the opposite way of the prop (Newton's laws there)? Or are you talking about a rotating engine such as what's on a V-22 Osprey? I think that in the second case, it would help.

I'm going out on a limb by saying this, but don't trust me . I have really no background or training on stuff like this (heck, I'm 18 about to go into college), but I have a pretty good gut feeling (uneducated guesses) about things. I'd bet that the duct would help, even if not much. It seems like by straightening the propwash, you'd decrease the amount of energy wasted on the prop blowing air outwards and in circles. If you could straighten the propwash, it'd increase efficiency a lot. The question is whether or not a reasonable duct can straighten propwash enough to make a difference!

Ravivos

Hi BMatthews.
I think you got it all wrong... or mybe it was me that didnt understand you.
The engine will not rotate at all, it is fixed to the airframe via the two pivot points.
thrust is generated by the prop, the only time that the engine will rotate is for turning the thrust vector 90 degrees downwards for vertical takeoff and landing.
There is no need to balance the engine because it is fixed.
the main question is the affect of the shroud on the propwash and thrust, i did some reading and found out the if the shroud is desigend and built properly, it will reduce the turbulence that caused by the prop tip and will cause a straight flow.
my question : is there a way to calculate (or even using some thumb rules) the affect of this shroud over the engine and prop performance.

victorzamora: it seems you get it right, and it seems you have a rather good "gut feeling"

p.s.
the lightning holes that i created in the engine mount will be coverd with balsa and microglass and will be sanded to a more aerodynamic shape in order to help straightning the airflow through the duct

BMatthews

Yes, I did misunderstand you. That's much better.

There was some work done on ducts and inlet lips for ducted fans years ago. I don't have it at my finger tips but the gist of it was that the prop generates a lot of turbulence that disturbs the air for a short distance in front of the prop. To ensure that this is kept contained in the duct you need to have a rather long duct such as a jet uses. If it's a ring shroud like you're showing then you need to use a rather prominent venturi to both smoothen the incoming airflow as well as contain this turbulence from the prop until it comes around the next time and can draw it in. The need for smootheing the airflow was also stressed for short ring shrouds. For a free open air shroud the lip needs to be something like seen on the Moller Skycar
or Bell X22 which both have rather rounded inlet lips. Even better from a low speed standpoint would be the extreme venturi like lips as used on a lot of hovercraft.

The same article had information on the need to keep the prop as close to the shroud walls as possible. As I recall he was describing the gap as a % of the tip chord of the fan blade and he noticed that things got better when the gap was down to less than 10% of the fan's tip chord. Much less and the gains were not as high and the problems of avoiding contact became greater. But if you look at real turbojets there's no doubt that they strive to limit the tip gap to a matter of only a few thousandths of an inch just for this reason. And it only took a gap of something like 1/16 inch to see a large drop in performance compared to a tight fit.

You may want to ask the guys in the Jet forum about this aspect. Some of them are still using or will remember their ducted fan days and will know more about what an acceptable gap is for performance and the effects of too large a gap. And more importantly what is the size of too large a gap.

TRCX

BMatthews has hit the nail on the head with regaurd to the ducts.
just one problem...
how are you planning to turn those ducts through 90 degrees. Theres alot of gyroscopic force in those rotating parts..
other wise it sounds like a good idea! goodluck!
chreers
kyle

Ravivos

the gyroscopic forces (precession) will not be a problem, the engine and shroud will be rotated by using an electric motor with a threaded axle , the axle will go through a thread in an arm that will rotate the entire duct 90 degrees.
the precession forces will then be balanced by the aircrafts control surfaces.

hi matthews, trcx, thank you for your answer.

Ravivos

Just a little update -
currently building the shroud, it is built to fit a 16" prop.
the shroud is been built from ply wood and balsa.
the inner face of the shroud is coverd with microglass fabric an epoxy.

I will update when i will have the results.

victorzamora

What motor are you going to use? I didn't even ask!

Ravivos

victor, your question kept me awake for a few nights.
when i designed the plane and shroud, i knew i need alot of thrust in order to perform as a VTOL.
i thought alot about a powerfull methanol engine like the YS120, but then decided to go gas.
eventually, i decided to go with the GF26I (crrc) that will spin a cut-down 17X6 prop at approximately 9300rpm, this will produce the thrust that is needed it order to lift an 11lbs plane verticaly off the ground.

victorzamora

That's an AWFULLY lot of rpm's on that gas engine!! I'm not sure if it's healthy for it. I'm not familiar with that engine, but still...that's what my Saito 100 is supposed to turn at.

Ravivos

I know, but the calculated thrust of this prop is just right for the design (i used Pe Rives thrust calculator),
i can always put a cut-down 18X6W at 8000 rpm for the same thrust... i will have to experimet a bit with various props for best performance.
one of the considerations of going gas is putting a much smaller (and lighter) gas tank, furthermore - i need a 100 percent of reliability, only a well tuned gas engine will give me it.
beside, with so many projects, i just cant afford running methanol ... [8D]

victorzamora

A 16x6W would probably be okay. It just seems like 9300rpm's is too much for that motor. You should probably go on the gas engines forum and ask about. It probably won't have the same static thrust as a 16x6W, but you might want to try a 16x10. It seems like ducted fan blades are really low diameter REALLY high pitch, and like 6 blades. I think that different props are going to be the center of your search.

Allfat

My Feedback: (1)

But ducted fans are usually used more for speed and less for pure lifting power.

Think of trying to hover a ducted fan jet. When 3D guys want to hover, they use high diameter, low pitch props. Since the OP is trying to do a VTOL project, and since VTOL is very similar to hovering a 3D plane, then I would venture to say that a high diameter low pitch prop would be the best for his application.

Edit: One added bonus of cutting down a longer prop is that you can ensure a perfect fit to the shroud.

victorzamora

Yeah, I just don't know what his primary concern is. If it's awesome vertical, then yes....large dia, low pitch it's gotta be. I was thinking high pitch, 16" dia to keep the revs down on the engine.

Another option is a M/A 16x8 3-blade.

victorzamora

I agree that shaving down the prop would have that same benefit.

BMatthews

But with this being a 16 inch shroud on an 11 lb model it's not so much a ducted fan as a propeller shroud to aid with lessening the tip loses and to help aid in diverting the airflow. Think of it as a really big and noisy sort of Hawker Harrier.


Ravivos, I'm looking forward to seeing the rest of the model and seeing how it all works out.

Are you planning on bleeding away parts of the air for reaction jets or will you use some other source of control for hovering guidance?

Ravivos

As Bruce understanded correctly, this is more of a prop fan than a ducted fan , it uses a conventional gas engine with a conventional prop inserted into a shroud to help straitning the propwash and maintain the ability to control the model in a hover mode.
because i use a big, tourqey engine, i need a large prop with the highes RPM i can get in order to achieve the highes thrust i can squeeze out of the engine.

the main concept is based on a design that i came across while searching for something else,
It is the "Vertigo" design that can rotate the engine shroud 90 degrees and facing the force vector downwards instead of backwards.
I really dont have any more information regurding this design and i am designing it all from scratch - larger engine, larger wing area to support the extra weight, larger tail surface to help moving the CG backwards (Higher Tail Volume Coefficient) ect... ect...
this is a very complex design and there are a lot of things needed to be calculated, one of the main problems i anticipate is the aircraft behavior at transition from vertical to horizontal vector,
i chose an airfoil that will produce good lift for high AoA without violently stalling and calculate the force needed to keep the plane in the air wile transition - it is actually the full thrust times the CLmax angle's sinus.
furthermore, the wing design needs to be so that the engine nacelle mounting point and thrust point will be exactly over the CG , so the wing have to be swept back, this might cause stability problems...

this is not the firts plane i am designing, and i am very familiar comfortable with the math involved.
to me, this is the essence of RC modeling - the design, problem solveing, new ideas, innovation ... these are the building blocks.

i found a single video of the concept, it is based on a small DF engine
[link=http://www.youtube.com/watch?v=bCJ70Fr1umA]vertigo vtol[/link]

thank you all for the tips and thoughts.

iron eagel

[/quote]

Are you planning on bleeding away parts of the air for reaction jets or will you use some other source of control for hovering guidance?

[/quote]
Bruce,

This idea is very interesting I know that a Canadian design years ago did something like that (Avro?).
How would you go about doing something like that? Bleeding air for reaction jets...
I have a df design that could use some help on roll control as it is a attempt at STOL. My oddball design if you recall...

Regards
Paul

Ravivos

Hi paul.
although i am not Bruce, i can give you my opinion regurding you df, i thought about it but decided going with a simpler solution.
the main idea is to use some of the high pressure air in the duct and direct it to the wingtips.
in a duct design, you need to "scoop" air from the duct and tunnel it to special openings in the wingtips, then you have the ability to control rolling (or yawing, depend on were you direct the airstream) by simply closing or opening a hatch at the output port.
the hatch is controld by a servo , one for each wing.
if you really want to do it right, ithe input scoop, tunnel cross section area and output ports needs to be calculated for ideal performance.

Allfat

My Feedback: (1)

Yeah, I was responding to Victor's post about how ducted fans use many small blades with high pitch. He suggested a 16x10, and he even admitted that it would have less static thrust, which Ravivos needs as much static thrust as he can get to pull a plane vertically off of the ground. We are on the same page, but maybe I worded that post incorrectly.

Basically I was saying that the ducted fan idea is not really what he is going for, more like a 3D type application, but instead of the whole plane being vertical and hanging from the prop, the plane is horizontal and the wings are vertical hanging from the prop. Essentially it is the same concept. Now, based on that concept, all the 3D guys use high diameter low pitch props for hovering, and Ravivos should do the same to get a similar effect.

iron eagel

Ravivos,
Hi! Thanks I thought that was basicly how it was done but was just guessing about the process. Sorry for diverting from your original thread but Bruces question about attitude control in hover or low speed flight of your aircraft really go me thinking about a concern I had with a design I have been working on for the past couple of years. The thought of using vectored thrust is STOL or VTOL craft is something I have been toying with for some time, and I have been following your post from the original, as I think we have a comon interest. Check out: Oddball design 101 in scatch builing and design thread...
Regards
Paul

CrateCruncher

My Feedback: (1)

Ravivos, before viewing the video I would have thought you'd have all kinds of stability issues with only one thrust source. I would guess the stability and control arise from those perpendicular thrust planes directly aft of the prop on the gimbal. Are there any accelerometers (aka heli gyro's) called for in the design?

BTW, RealFlight has a Harrier jet with VTOL directional thrust you might find helpful in training for the maiden. This is a very cool project. Where can I get a copy of the plan?

Ravivos

Hi Paul, mybe you can elaborate on a few aspects of your design, as you said - we do have a common interest and i would gladly try giving my own thought for your design and probably get a few ideas from it

CrateCruncher
there are many ways to control the airplane in a hover mode, i am planning on using a stability system by FMA, its called Co-Pilot and works as four IR detectors that "see" the temperature difference between the sky and the earth, it is then trying to level the plane on two axis (pitch and roll) according to the horizon,
for yawing, i plan on using a gyro.
thank you for your kind words, i really dont have any plans for this because i am designing it as i go,
you can continue keep track for this project and i will be more than happy to share all the information and design aspects so you can build one yourself.
as for now, i finished designing the engine nacelle, next section will be the center section fof the fuselage and then the wings (parallel design to the tail as these two connected).
if all goes well, i can estime that i will finish it by the end of this year... on the other hand - you can never know [8D]

victorzamora

Yeah, the reason I was suggesting a 16x10 wasn't for added thrust...it was to keep the loading on the engine high enough to keep it from over-revving. And, actually, a 2-blade APC prop has to turn at 9700rpm to put out as much thrust as the 16x10 turning 8400. A 16x6W would have to turn to 9100rpm to be able to put out the same thrust. I don't think that that much rpm is healthy for the engine.

The 16x10 at 8400 puts out 15.4lbs of thrust. A 16x8 MAS 3-blade puts out 15.25lbs of thrust. On an 11lb plane, that's just under 1.5:1. It will be by no means have amazing vertical, but still....enough for VTOL.