Vtol design: 1 vs 6 engines

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Dana

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Gear ratio is irrelevant to prop design; you just use the rpm at the prop (engine rpm divided by gear ratio). The HP doesn't change, except for minor gearing losses.

Javaprop is another good prop calculator, but you need to give it good information on blade area, blade airfoil, etc.

Building a direct lift aircraft that can lift its own weight is not all that difficult. Building one that is efficient requires a very large propeller... which means a helicopter. Building one that is controllable, well... aviation history is filled with attempts to build a direct lift machine... the Benson flying Jeep, various designs by Hiller, Moller's 30 years of failures, the millions recently spent on the SoloTrek, which disappeard without a whimper, and countless others.

Study the reasons why everybody else has failed... and then if you really have a new idea which solves the previous problems, calculating the required thrust will be a trivial matter.

-Dana

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leviterande

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correct Dana that is what I am exploring now , all the aspects including the most important: to get off the ground and calculating the required thrust

I really dont think vtols will be so expensive,, they should cost about the same as a good ultralight, it is the "idea" that makes it never the extreme complexity extreme money etc.. otherwise you woould see flying cars everday.

I have tried x-plane time ago and didnt get grip of it, well gonna go back and see thanx everybody

simplyput the main issues with VTOLS are that they have to be relatively inherently safe, low maintnecae, and low cost.. I know it is going to be expensive but I think not the aircrafts of today are cheap either

so by having an inherent relatively simple structure or design method for the whole concept you will make it cheap, low maintenaced and reliable.. what is that concept is the 640000$ question but I am almost certain that this is how the future of personal car flying will be

Kalle
 

leviterande

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Gear ratio is irrelevant to prop design; you just use the rpm at the prop (engine rpm divided by gear ratio). The HP doesn't change, except for minor gearing losses.
Dana, look at this


so disregarding belt drive losses you mean that I could gear a rotax 582 1:11 and be able to turn a 160" 550rpm?

(engine rpm:6500/11= 590)


Kalle
 

Dana

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Yes, that's the rpm you'd get from that gear ratio. Whether the horsepower or gear ratio is appropriate for your application is another story, I don't know.

I think you're mising the point. The main issue with VTOLs is controllability (how do you plan to control yours?), along with safety (what will you do when an engine fails at 50'?). Deal with those two problems first and everything else (lift, propeller size, even cost reduction) are easy to solve.

-Dana

"If I knew there were coming to my house a man with the fixed intention of doing me good I would run for my life." - Thoreau
 

leviterande

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""Whether the horsepower or gear ratio is appropriate for your application is another story, I don't know" thats why I am looking for a calculator that can do that.

anyway..

the control system depends on the concept, the one with one single propeller will be having a counter torque stators beneath that are fixed and have movable vanes. m the c.g will be at the thrust line so when the control vanes moves you move laterally and without "rolling"

the one with the two props wont need any counter stators and have smallvanes beneath the propwash to move laterally forward

finaly if i would choos to have 6-8 engines I dont need any stators or vanes.. but for the yaw only. Pitch and roll will be throttle controlled electronicaly by servos coupled to a gyro.

for safety all versons will have some kind of a wing above the crafts themeselvs.. also the final verson will have a huge airbag impact system that inflates instatnly at the moment of impact. so the. offcourse the first tests will be at maximum 10 feet to check everything


Kalle
 

Dan Thomas

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the control system depends on the concept, the one with one single propeller will be having a counter torque stators beneath that are fixed and have movable vanes. m the c.g will be at the thrust line so when the control vanes moves you move laterally and without "rolling."
finaly if i would choos to have 6-8 engines I dont need any stators or vanes.. but for the yaw only. Pitch and roll will be throttle controlled electronicaly by servos coupled to a gyro.
One rotor won't work at all. Gyroscopic forces have to be handled. Anytime you change the plane of rotation of the rotor, you will have a force applied to it 90 degrees in the direction of rotation. It won't be controllable.
The gyroscopic forces were one of the aspects that drove early helicopter designers crazy. If you study helicopters now, you will see that in order to get the rotor disc tilted forward to move forward, the maximum pitch is applied on the left side (in a North American machine, which has rotor rotation counterclockwise as seen from the top.). The minimum pitch is on the right so that the front is down. All of the rotor's movements are controlled like that: 90 degrees ahead. It's a happy accident, since in forward flight the advancing blade needs much less pitch than the retreating blade, but even then the rotor blades are usually free to move up and down to change the angle of attack to keep lift equal between left and right, and some (those with more than two blades) also allow the rotor blade to swing forward and back on a pivot in the hub.

I regularly encounter guys who are going to build a "simple" helicopter using just a big fixed-pitch propeller, tilting the whole engine to control movement. They haven't read the obitiuaries of the guys who tried that long ago.

As far as six small fans: Check out Moller. He still insists that such high disc loading will work. It might, too, at about one-quarter of a mile per gallon, and a loss of hearing after the first week of flight. And a loss of life if one engine coughs.

Dan
 

leviterande

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One rotor won't work at all. Gyroscopic forces have to be handled. Anytime you change the plane of rotation of the rotor, you will have a force applied to it 90 degrees in the direction of rotation. It won't be controllable.
The gyroscopic forces were one of the aspects that drove early helicopter designers crazy. If you study helicopters now, you will see that in order to get the rotor disc tilted forward to move forward, the maximum pitch is applied on the left side (in a North American machine, which has rotor rotation counterclockwise as seen from the top.). The minimum pitch is on the right so that the front is down. All of the rotor's movements are controlled like that: 90 degrees ahead. It's a happy accident, since in forward flight the advancing blade needs much less pitch than the retreating blade, but even then the rotor blades are usually free to move up and down to change the angle of attack to keep lift equal between left and right, and some (those with more than two blades) also allow the rotor blade to swing forward and back on a pivot in the hub.

I regularly encounter guys who are going to build a "simple" helicopter using just a big fixed-pitch propeller, tilting the whole engine to control movement. They haven't read the obitiuaries of the guys who tried that long ago.

As far as six small fans: Check out Moller. He still insists that such high disc loading will work. It might, too, at about one-quarter of a mile per gallon, and a loss of hearing after the first week of flight. And a loss of life if one engine coughs.

Dan

Hi Dan, thanx for your advice, I understand your point but I never intend to use a collective pitch nor a tail rotor system.

so I will not encounter pitch differntials between blades.. Also I will not tilt the propeller. the vanes that are to be situated under the propwash are to be located at the c.g which means that when you apply forward stick movement, what happens is that the vanes tilt backwards moving air backwards and imparting a forward laterall reaction. I still dont know how is it going to be with gyroscopic precession problems.

Cuz as you said with tilting big rotor,propeller you will have to apply 90 degreeds before moving into the desired direction. controll will be a nightmare that way. my idea is not to tilt anything as I mentioned before, the rotation plane will be the same during all manouvers and that thanx to the location of the control surfaces that are at the c.g.

The reason a prop tilt is a force applied a distance from the c.g

so gyroscopic forces should be eleiminated this way? my other concern is the fixed asymetrical airfoil stators. will they be enough to produce enough anti torque lift

in my small 20" models they work perfect but how is it in big models..


Kalle
 

Dan Thomas

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Hi Dan, thanx for your advice, I understand your point but I never intend to use a collective pitch nor a tail rotor system.

so I will not encounter pitch differntials between blades.. Also I will not tilt the propeller. the vanes that are to be situated under the propwash are to be located at the c.g which means that when you apply forward stick movement, what happens is that the vanes tilt backwards moving air backwards and imparting a forward laterall reaction. I still dont know how is it going to be with gyroscopic precession problems.

Cuz as you said with tilting big rotor,propeller you will have to apply 90 degreeds before moving into the desired direction. controll will be a nightmare that way. my idea is not to tilt anything as I mentioned before, the rotation plane will be the same during all manouvers and that thanx to the location of the control surfaces that are at the c.g.

The reason a prop tilt is a force applied a distance from the c.g

so gyroscopic forces should be eleiminated this way? Kalle
You'll still have trouble. What about banking turns? What about turbulence knocking it off level? Any tilt, in any direction, will cause a reaction at 90 degrees, and that reactive tilt will create another reaction at 90 degrees to it, and so on. It's the reason a spinning top will wobble around once it starts to fall off to one side. If you don't anticipate it and come up with ways to handle it, it will cause a failure of the whole idea. Even in fixed-pitch aircraft we have to deal with propeller precession: we find that the nose gets heavier in a right-hand steep turn, we find the nose pulling to the left when we raise the taildragger's tail, and so forth. When the prop is larger and heavier, the forces can be substantial.

Dan
 

leviterande

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You'll still have trouble. What about banking turns? What about turbulence knocking it off level? Any tilt, in any direction, will cause a reaction at 90 degrees, and that reactive tilt will create another reaction at 90 degrees to it, and so on. It's the reason a spinning top will wobble around once it starts to fall off to one side. If you don't anticipate it and come up with ways to handle it, it will cause a failure of the whole idea. Even in fixed-pitch aircraft we have to deal with propeller precession: we find that the nose gets heavier in a right-hand steep turn, we find the nose pulling to the left when we raise the taildragger's tail, and so forth. When the prop is larger and heavier, the forces can be substantial.

Dan

I just wanted to say that any wind gusts will actually destroy the idea of any lateral movements so,.. well.. hmm.. the single propeller vtol idea is eliminated unless it is a rotor head system.

that leaves us to the tandem or coaxials

as I understand the coaxials are more inhernetly styable but less efficient then tandem. While reading about the Piasecki jeeps of the 60es they notised that the jeeps were "surisingly sensitive" to winds.. so they are more wind-sensitive then helicopters. the only possible reason could be that the more "thrust posts" or thrusters you have in a craft the more unstable it is? cuz the gyroscopic effects in the piasecki jeeps were cancelled


Kalle
 

leviterande

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Why do you say that's the only possible reason?

cuz from what i have read seen and tested indicate that, but i could be wrong offcourse..so why no tjust have look at some videos:

coaxial Hiller pawnee is very stable due to 3 reasons: central single thrust line, high c.g and kinetic control(he fires a a rifle!)
[video=youtube;dmSCyy0_OX4]http://www.youtube.com/watch?v=dmSCyy0_OX4&feature=related[/video]

piasecki jeep, I dont know if this verson have collective pitch
[video=youtube;P9i7WKdimPU]http://www.youtube.com/watch?v=P9i7WKdimPU[/video]



if you watch carefully you see that the V22 can be very dangerous at take offs , landings.. it wobbles alot
[video=youtube;dwKI9gy9AQM]http://www.youtube.com/watch?v=dwKI9gy9AQM[/video]

v22 crash
[video=youtube;VYeLishJ_Js]http://www.youtube.com/watch?v=VYeLishJ_Js&feature=related[/video]



istar is pretty stable but it has self correcting electronics
istar uav vtol - Google Bildsökning


only x-hawk video I have seen, Protoype
Urban Aeronautics

cypher: coaxial rotors
cypher vtol - Google Bildsökning

looks like the hiller but not even as stable as the original one.. i am convinced that thereason is that due to the bigger size the kinetic control is weak, and notice that the c.g is lower to
[video=youtube;vI-4ygOrgJ4]http://www.youtube.com/watch?v=vI-4ygOrgJ4&feature=related[/video]

[video=youtube;tR1QI4wUbPc]http://www.youtube.com/watch?v=tR1QI4wUbPc&feature=related[/video]

you tell me how jet engine can do this
[video=youtube;XJARrc40imk]http://www.youtube.com/watch?v=XJARrc40imk&feature=related[/video]

unstable springtail vtol: notice very low c.g
[video=youtube;UcdDvplWVf8]http://www.youtube.com/watch?v=UcdDvplWVf8&feature=related[/video]

kalle
 
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Dana

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To the best of my knowledge nobody has ever flown a man carrying ducted fan VTOL aircraft to any significant altitude, or any significant distance. The only successful VTOL aircraft I know of other than helicopters are the AV-8 Harrier and the V-22 Osprey.

-Dana

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leviterande

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Coaxials or tandem are good candidates


I have been thinking lately about using two 8ft contrarotating
propellers in tandem instead of many small ones..

so is it enough to use the rotax 592(65hp) to lift this machine that
weighs 200kg including me?

I am counting on losses offcourse and that is the real real problem
cuz you will have two belt drives and a "gear" to turn both
propellers.. the loss will be 15%?= so 55hp is left.

accounting that I will hover with 90% of the available power(or how
much do you heliguys use when you hover?) I will have a usefull 49hp
to hover wit.


so each 8ft 2 blade low pitch propeller will be fed with 25 hp


disc loading´50 sqft
power/area 25/50= 0,5

8,6859 x 0,5^(-0,3107) x 25hp = 270lbs for each prop!

so total thrust is 600lbs/240kg using 90% of the engine so is it possible to use a 65 hp.. cuz if
we really have 600lbs of thrust for hover, the craft`s strong
airframe: chassi, frame, props, shafts, gears, belts, engine mounts, controls, cockcpit, landing gear, bearings, must weight in at 80-90kg/190lbs at max without the engine, pilot or fuel

can one make a complete airframe excluding the engine and pilot about 80kg? or am I dreaming?
My biggest concern is the high level of stress on the both shafts that will be stretched from the central engine by belt drives


thanx
Kalle
 

CNCRouterman

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My experience with helicopters is pretty thin, but I recall my pilot saying that the power setting for hover/take off was the same as level flight cruise. For us, hovering was being just high enough to see over the trees or intervening hill so I could spot for the artillery. It was a blast :roll:, however, as it was part of our general familiarization training, not core training, it was only a one day event.
 

leviterande

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Eric, how funny is it that i happened to just read about the power levels of cruizing vs hover for small helicopters like a R22.. yeah I suppose they will consume huge power even in cruize-mode so it is really wise to make a very efficient system.


After some research into the matter I have lately been thinking about designing the most efficient of all, a 30ft coaxial peripheral rotor system. do you think that the coaxial system is better then a tandem. the reason I vote for coaxials is that gear losses and weight of transmission kill any efficiency with a tandem design


Regards
Kalle
 

Holden

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The key to lift is to get the L/D ratio as high as possible. That occurs at around 6-7 degrees. The blade then needs to be optimized to 0-20 mph at max RPM (.6-.8 mach at tips) speed and max power. No off the shelf prop used on an airplane will do what you want to do by at least 1/2. You need more of a helicopter blade, not a airplane blade. The difference in lift, like I say, can be 2xtimes.
 

leviterande

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The key to lift is to get the L/D ratio as high as possible. That occurs at around 6-7 degrees. The blade then needs to be optimized to 0-20 mph at max RPM (.6-.8 mach at tips) speed and max power. No off the shelf prop used on an airplane will do what you want to do by at least 1/2. You need more of a helicopter blade, not a airplane blade. The difference in lift, like I say, can be 2xtimes.

I didnt get you, 0-20mph? I know that the tip speed should be around 0,7 mach but that isnt 20mph?


one very diffecult issue I find is who to listen to, what source is a good one, for example I found one page saying that the coaxial is way more efficient and stable then a tandem since you get rid of the weight and transmission losses, and I agree on that.. then I found another page that says the opposite claiming that in order to have a coaxial you will need 50% more horsepower!!! to lift the same weight! .. I understand tha aswell but who wins in the end..

A tandem design will produce more thrust without the slightest of a doubt then a coaxial. but a tandem will need more transmission gears, parts and weight decreasing the lift capacity. so in my own view if both will lift the same capacity a coaxial is a lot simpler then a tandem to build or what do you think..


Kalle
 

Holden

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My background is Masters Mechanical Engineer, R&D in aircraft, windtunnel, RC SCALE modeling, cars, engines...20 years trying to get it right:) I tried to get a kitplane company going, but stopped before I went bankrupt...still working on ideas to get another go...which is similar to your problem (vtol) by the way...

0-20 mph means the prop is twisted for that speed range of incoming flow at 0-20 mph. If you go from the center to the tip, each inch will be at the right AOA to the relative wind. If the wind coming in is 5 mph vs 100 mph the twist for equal lift is different.

Make an excel program, and model it. Then you can see for yourself. I made one years ago. You will see the total trust change as you twist each section (say an inch) from inboard to the tip, and the twist will be different to get the real AOA the same depending on the incoming speed and RPM.

Basically you are making a prop that is optimized for very slow speed, which is what a Helicopter blade is doing for the most part. A heli needs max lift on takeoff...straight up. That flow is near zero-20 mph down.

Airplane props are set in twist for some speed, say, a cruise at 100 mph, and as you deviate from that speed and rpm your efficiency goes down.

Take a Cessna 172 (going from memory here), if you model it by taking a digital level out to the airplane and measuring the pitch along the prop, you will see it is best at some speed, say, 100 mph or so, and most of the prop is stalled on takeoff (0-20), making such a prop very poor for vertical lift.

In the ideal world you would have a variable twisting prop that would always maintain max L/D. There are several ways to do that, which I have looked into, but have not done it yet.

As for coaxial, I would think a counter rotating coaxial would be best, just like several helicopters have. That is what I am going to do, but mine is an airplane of sorts with VTOL.

I hope I did not offend you in this long explanation. Ask more questions if you like...
 

leviterande

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My background is Masters Mechanical Engineer, R&D in aircraft, windtunnel, RC SCALE modeling, cars, engines...20 years trying to get it right:) I tried to get a kitplane company going, but stopped before I went bankrupt...still working on ideas to get another go...which is similar to your problem (vtol) by the way...

0-20 mph means the prop is twisted for that speed range of incoming flow at 0-20 mph. If you go from the center to the tip, each inch will be at the right AOA to the relative wind. If the wind coming in is 5 mph vs 100 mph the twist for equal lift is different.

Make an excel program, and model it. Then you can see for yourself. I made one years ago. You will see the total trust change as you twist each section (say an inch) from inboard to the tip, and the twist will be different to get the real AOA the same depending on the incoming speed and RPM.

Basically you are making a prop that is optimized for very slow speed, which is what a Helicopter blade is doing for the most part. A heli needs max lift on takeoff...straight up. That flow is near zero-20 mph down.

Airplane props are set in twist for some speed, say, a cruise at 100 mph, and as you deviate from that speed and rpm your efficiency goes down.

Take a Cessna 172 (going from memory here), if you model it by taking a digital level out to the airplane and measuring the pitch along the prop, you will see it is best at some speed, say, 100 mph or so, and most of the prop is stalled on takeoff (0-20), making such a prop very poor for vertical lift.

In the ideal world you would have a variable twisting prop that would always maintain max L/D. There are several ways to do that, which I have looked into, but have not done it yet.

As for coaxial, I would think a counter rotating coaxial would be best, just like several helicopters have. That is what I am going to do, but mine is an airplane of sorts with VTOL.

I hope I did not offend you in this long explanation. Ask more questions if you like...

Ok so by 20mph you meant the speed of airflow entering the prop? not the tip speed itself right?

Regards
Kalle
 

Holden

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Kalle,

Yes.

Another note: as an airplane goes faster you will find that the best thrust with a fixed twist is to slow the rpm and increase pitch, which is opposite to what an engine wants and what is common in industry. This requires an extra large prop (low power density) Depending on set point, this means you want a two speed transmission with variable pitch with a fixed twist. A constant speed prop does not have two speeds between the crank and prop (such as a planetary gear would yield) and as you slow the prop the engine power goes way down. I found that 1800 rpm (8 foot prop, 10 inch chord!) on takeoff gave best at around 1200 rpm in sport plane cruise (138 mph).

In a typical Cessna with 2700 rpm peak power, that would be 1750 rpm in the cruise! A Lycoming would be out of the green at such low RPM. Also, remember as you go faster the RPM must decrease to maintain max Mach number, and if you go really fast 200-250 kts the slower prop becomes more important, making it even more important to have a low power loading and high L/D.

Most props are High power loading to avoid hitting a wall if the engine quits, which mean if you want a good prop, it must pitch to AOA and not be constant speed. When you put in power the pitch angle changes to some ideal, say 6 degrees, and is regulated by a trailing flat, similar to what is found on the Kaman twin blade helicopter. So, to get a real VTOL, you have to think out of the box, because nobody has gone there...and for good reasons.

When you get a correct prop, you realize that a typical piston engine will destroy that baby in short order (talk to Helicopter transmission designers using piston engines!), or that prop will be as heavy as the airplane itself. The power pulses will kill the prop.

Also, to make it quiet in the extreme so that thousands can commute to work...you need to run the prop around <.4 Mach... As you can see, nobody does it this way, and nobody has the lift that is possible...

You realize that engines won't work, props all need to be new, and on and on...and that is just the firewall forward...

You are in for some serious R&D... I would go get a Master's degree in aero engineering to undertake such a thing. Since nobody will fund it, you have to have you own money and cash flow, and at least 10-20k hours of work time...lots of fun... That is what I am doing...so it is possible...

Enough said?
 
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