VTOL - For your present and future personal mobility

Well I am not really qualified in this area, so you can take this for what you paid for it. I too have thought along similar lines although I have never taken it further that a bit of interested research.

I am assuming that a turbine or jet engine is not in the plans and the desire is more cost efective answers such as internal combustion and propellers.

I have found that a focused ducted fan with the air intake large and the air exhaust focused to a point or close column provided the best lift ratios. It would require 3 or more fans to balance as a a stool or table would. Basically the thust is multiplied by the reduction between the intake and exhaust ratios must as a reduction fitting on a water hose increases the distance the water is projected.

The success is based on the duct and fan blade shape. I concider 2 basic concepts as were promising:

1) a compressor as in a jet engine made of two blades, the intake blade has a lower pitch and larger diameter to collect a large amount of air and the second blade on the same shaft is of higher pitch and smaller diameter. The Duct is close to the blade tips to reduce loss and can be either a funnel shape or a bubble to form a chamber to absorb variations.

2) a single blade that spirals like a screw reducing diameter and increasing pitch to compensate. The Duct needs to follow very close to the blade it's entire length.

I am favoring the second concept. I have seen a few ideas that extended the duct beyond the intake blade and even wrapped over the ends of the blade to reduce slippage loss, but I feel it adds more weight that lift.

What seemes to be the key is the shape of the exhaust air collumn there are 2 shapes that I have seen perform well.

1) a cone shape where the thrust is focused to a point.

2) Collumn like a focused rod of air.

If the distance off the ground is to be constant, the cone shape permits the length of the duct to be reduced and thus the weight, in trade the variable height is sacrificed.

The Collumn shape will permit the height of flight to follow power appilied (non linear) but generally requires a longer duct to produce the shape, a nozzel may also be used.

My testing over the years has been linited to small models with little concern to real-life compensation in materials, stresses or weight. In my models the Collumn method produced significant increase in thust judged by the max speed of the model with the same engine compared to a standard propeller. but when applied vertically less power was required to hover with the Cone shape but sacrificed heavily in height variation with stability. I also noted that the Cone shape found it dificult to take off unless lifted to about 2/3 or 3/4 the normal flight height on a wire platform for take off, adding retractable landing gear was one answer but the additional weight more than exhausted the reduced power to hover.

Stability is in the ratio of the height of flight and the distance between the fans. The roughly trianglar shaped body with 18" per side was very stable at a height of about 6" but lost stability rapidly as it approached 18" high and crashed quickly when I rose above that height. I tried a rectangular shape and 4 fans and put a slight tilt to the thrust outward to the sides helped but required more power to acheive the same height. I also experimented with variable thrust angle to gain forward thrust but found it more efficient to add a 5th fan for the horizontal thrust. I later mounted this fan on a 360 degree rotating mount to steer and even brake.

The reduction in the diameter of the blade from that of a helocopter has to be compensated in the RPM of the Blades at the time seem to limit the design to an air cooled 2 stroke or that of a Wankel (rotory) motor of German design (Never tested).

The faster it was propelled horizontally the lower it flew and the more unstable it became. I tried adding wings for lift and a lifting body but the added weight was more than the limitations of my fans could take. I had some spectacular crash and tumbles without the lifting surfaces. I also tried using large very light built wings with dry cleaner plasic surfaces and filled the wings and body areas with helium to compensate for the added weight and was able to actually increase the lift more than the weight, (loosing one test set of wings to un-bridled lift)

I back burnered the project waiting for a new power source to be developed and there has been some new advancements in power to weight ratios in the outboard motor area using computer controls on the engine, I have it in the back of my head to look into as time permits.

basically I found the design to be similar in properties to a stool with the collumns of air acting as legs, very stable low and easy to fall off of when high. I placed the fans on extended arms to distance them and added stability to a point where the weight of the arms exceeded the earlier triangle or rectangle mounting. The flexing of the arms tended to angle the fans outward a degree or more, but as the fuel ran lower this reduced and the height increased and stability reduced with the weight.

This is probably all old hat to you and a waste of time, but I offered what I found to be true. Hope if was of some help.

StarMind

"The reduction in the diameter of the blade from that of a helocopter(sic) has to be compensated in the RPM of the Blades at the time seem to limit the design to an air cooled 2 stroke or that of a Wankel(sic) (rotory)(sic) motor of German design (Never tested)."

Herein lies the problem. With any craft to hover with a 100% efficient engine you need 48 watts per kilogram of mass. When you take into account engine efficiencies and losses it's more like 150-200wKg^-1.

This was one of the big problems with the early helicopters as the power plants couldn't generate the power to weight ratio required. As a result you will find that nearly every helicopter today uses a gas turbine for its power plant. Somebody else may be able to give you an alternative but I have the sneaking suspicion that there is no alternative but to use a gas turbine, anything else will more than likely not be able to produce the power to weight ration required.

Having said that if you use a high bypass ratio twin axis fan jet you can easily split the output into three ducts, two from the low pressure fans and one from the turbine. If you engineer it correctly the thrust will be automatically balanced. This is in fact this is how the harrier works with the exception that it uses four ducts.

The next problem is where to get the propulsive thrust from, the harrier achieve this by vectoring the lifting thrust to be propulsive thrust and makes up the difference with lift from the wings. This isn't an option in your case so the only solution I can see is a separate propulsion engine. The second engine compounds the power to weight problem though because it doesn't contribute to the lift so it only counts on the dead weight side.

Unfortunately this VTOL thing is considerably more complex and tricky that most people think and this is reflected by the number of aircraft that have successfully met the challenge. To give you some idea how difficult it is remember that the harrier has been around in pretty much the same format for nearly 50 years and they are still building them.

Hi Masu

Where does the figure of 48-Watts/kg come from? I had thought that the power would depend on the mass of the air that you were moving - and so on the swept area. If you can make a rotor blade (or air intake) with adequate area, doesn't the power reduce?

As I see it an added difficulty here is the requirement to hover at low altitude with a modicum of safety, so we need either a method for providing lift that is not overly susceptible to the effects of gusts), or a predictive control system (using lidar to measure air speeds before the gust reaches you??) to stabilise a craft with appreciable windage.

Fyz

"Where does the figure of 48-Watts/kg come from? I had thought that the power would depend on the mass of the air that you were moving - and so on the swept area. If you can make a rotor blade (or air intake) with adequate area, doesn't the power reduce?"

Good point, my initial calculation was based on the energy that you would expend accelerating an equivalent mass of air by g. But yes after thinking again I believe you are correct and you can reduce the power you expend by increasing the mass of air that you deflect with you fan.

I think?

Thanks, I agree - and now I see where 48-W/kg comes from.

Looking a little at the detail of that: 9.8m/s is quite an air velocity when hovering 10-ft from the ground, and you need 12-sq-m of swept area just to support a couple with joint weight of 140-kg, which ignores the weight of the craft and the fuel - not to mention the picnic etc. that you'd need to make the jaunt worthwhile*. So I suppose you weren't being unduly pessimistic, even if we assume that air outside the radius of the rotors might add to the effective area

*But at least there's little purpose in taking a radio...

Might ground effects help at 10-ft altitude? With modern control, could you sit above the rotor? It might be quieter, and provide increased ground effect, and probably less windage. Just thoughts - I don't know anything about the topic

Fyz

Starmind, anything that has to do with VTOL is never a waste of time, and what you posted certainly isn't old hat! thanks for the post. It would be great if you could share the current status of your work on your project.

briefly, I'm working on a tandem seater, a la apache chopper. 1 propeller at the back(about the level of pax cabin top, and the forward prop, right at the front-lowest level of craft.

I planned on using hydro-static drive arrangement, though this would make the entire system highly flexible, it also adds crazy amount of extra weight.

I designed my craft based on split thrust philosophy,i.e.: imagine total weight of craft is 500kg at lift off, 1 set of thrust system would be set to produce 90-95 % of 300kg equivalent force. The remainder shall then be produced by the 2 thrust system. This way, the first system works independantly-no throttling what so ever, and you maneuver with the second system.

Ideally, I prefer roots/centrifugal type blowers compared a regular propeller. It's more compact and less intruding and clumsy, besides you do not want to chop too many branches when you're flying among the trees!

Enabling technologies! With turbo or mech. supercharging, we are able squeeze much more horses than it was possible, say a decade ago, 600-700 horses from a 2 litres engine is quite normal.

In any case, I'll be posting all variations/combinations in my upcoming website. Imagine flying you H-D V twin powered VTOL or a thruster combination of 10-15 tiny turbochargers!

"Enabling technologies! With turbo or mech. supercharging, we are able squeeze much more horses than it was possible, say a decade ago, 600-700 horses from a 2 litres engine is quite normal."

You will be pushing it to get this sort of power out of a 2 liter engine with the sort of reliability that you need. You would be lucky to get 300 Kw (400 Hp) out of a turbocharged 2 liter engine. Also keep in mind that you are going to need between 150 and 200 watts per kilo of craft just to hover. If you use a gas turbine half the mass of a 2 liter engine you can easily get 1.0 Mw and that makes your job considerably easier.

Also I have a word of engineering advice. You are mixing SI and imperial units and this is a recipe for catastrophe. Stick to one system of measure and since you are mostly using SI units stick with SI units and forger things like Hp. You are also confusing mass and force. You stated;

"I designed my craft based on split thrust philosophy,i.e.: imagine total weight of craft is 500kg at lift off, 1 set of thrust system would be set to produce 90-95 % of 300kg equivalent force."

Lift and force is measured in Newtons so before you try and work out the thrust convert the mass to weight by multiplying by g.

A small helium-filled balloon attached to a seat and a pedal-powered propeller on the back driven by the occupants' legs would work too. Consider it as aviation's answer to the pedalo.

"A small helium-filled balloon attached to a seat and a pedal-powered propeller on the back driven by the occupants' legs would work too."

I hate to tell you this but for a man and seat with a combined mass of 100 Kg that small balloon would need to be 6 m in diameter. Somehow I don't think a 6 m diameter balloon counts as small.

Larger than a Ferrari Boxter and much more graceful...

Presumably you are directing us to a parallel universe to find the Ferrari Boxter? Similarly to be sufficiently wind-free to hover at 10-feet under a balloon if there are any trees or steep banks around...

I was explaining project/plan in lay terms/language, however, your comments are definitely on the mark - I may end up confusing the reader or even myself. Thanks for the pointers, Masu.

For thrust production, I'm open to all types of thruster/powerplant combinations, and as a matter of practicle application - I have to! There are a number of small turo-fans/jets which would fit very well for my needs, question is cost, obviously! Williams WR24, KHD, and a few others. It has to be a multiple unit powerplant arrangement too. That's how I've planned this. I think my biggest nemesis yet would be fuel/flight duration.

For a given engine/powerplant thrust producing element(propeller/blower/compressor) combination, you will have x-kg nett thrust equivalent, i.e.:a 100 kw engine coupled to its matching propeller would provide x-newton thrust, and this would be in access of the engine/drive/propeller weight. It is this cumulative balance of nett thrusts that would make a VTOL a possibility. This is also why I need multiple engine set-up. 1 engine may not do(based on readily available powerplants,i.e.: rotax, lycoming, mcculloch, jetski engines and the like.)

Supercharger(centrifugal or screw) coupled to piston engine opens up yet another interesting possibility - pure jet!

"It has to be a multiple unit power plant arrangement too."

Not necessarily. Are you familiar with the multi shaft high bypass ratio fan jets like the Rolls Royce RB211? An single engine built along these likes can easily produce all the thrust required to lift the craft through multiple vents.

If you go with multiple engines you end up going backwards as two engines that each produce 50 Kw invariably weigh more than one engine that produces 100 Kw. The other thing is that with two engines you halve the mean time between failures. This is because you have twice as may things that can break. It is a common misconception that twin engine aircraft are safer that than single engine aircraft. They aren't because, like your craft, most can't fly for long with an engine down and two engines are twice as likely to have a failure than one.

The other advantage of a single power plant is that it self balances the thrust going to the multiple lifting jets. If you have separate engines supplying the thrust then you need some complex control to regulate and balance the system, an imbalance can be disastrous.

Personally I would stick to a single power plant if at all possible, it makes everything so much simpler.

Information on Rolls Royce RB211 is abundant in the web(it's also highly popular for industrial application-due mainly to its rugged engineering.), however, I'm doubtful of the number of amateurs who could actually afford her, Masu!

Using readily available engines obviously would make the craft rather bulky/clumsy, then again, you do have something to work on - within one's pocket depth limit too!

I tend to agree on the possible difficulties in maintaining the craft in stable hover or flight(all directions). The trick is to maintain the craft in constant motion while hovering within a prescribed locus(small one of course).

"Information on Rolls Royce RB211 is abundant in the web(it's also highly popular for industrial application-due mainly to its rugged engineering.), however, I'm doubtful of the number of amateurs who could actually afford her,"

I wasn't suggesting that you used an RB211 but was enquiring if you understood the concept of the triple coaxial system that Rolls Royce use in the RB211. The majority of people don't understand that it is actually three engines with three separate drive shafts sharing a common axis.

I don't know if anybody has managed to miniaturize the concept yet but there are some pretty small gas turbines out there now and it might be worth looking into. The great things about gas turbines is their reliability and phenomenal power to weight ratio.

Most discussion appears to revolve around power plants. What you should be looking to as well is for methods of increased lift from compact lifting surfaces and that involves increasing the flow over these surfaces, rather than wasting energy in direct reaction engines. I have an answer, but like you wanted to patent it, or at least prove it experimentally.

Well duikerbok, without you providing further details, we have only the option of what is readily available and practicle enough for immediate application.

That would leave us with 1. pure jet, 2. turbo-jet/turbo-fan, 3. engine driven fan (by turboshaft/piston/wankel engine, 4. propeller,5. roots/centrifugal/gear supercharger-driven by engine in (3), and 6. rocket motor(I'm not going there!).

For those who would like to try/test the feasibilty of any of these arrangements, you could take the power supply element out of the VTOL picture - umbilicals. Be it hydraulic/electrical/compressed air, supply this thru' the umbilical to you VTOL craft and see/test/feel if the arrangement that you have chosen works or suits you best.

Hoses and such are readily available and they're quite affordable too. With a 100m length, you could pretty much carry out all tests that's needed for you to decide/commit on the engine-on-board VTOL. You could even ferry your girlfriend/wife/partner (to win votes, basically!)

...got a little excited there, I guess, Masu. Well, RB211 is the first three spool(that what such arrangements are called) aero-gas turbine that I was fascinated by, basically each spool is mechanically independant and which performance is linked thermodynamically.This way, the first spool(closest to the combustors) could ensure optimum air supply to the combustors within its op. range.

Actually, such application, as proposed by yourself is interesting if not challenging, but my preference would be to couple 2 turboshafts(1 if big enough) to axial/centri. blowers, i.e.: 1 to 'neutralize' or 'near neutralize' the lift-off weight of VTOL and the 2nd for general maneuvring. It would be my dream set-up for a VTOL, mainly due to the turboshaft/blower combo cost. It would also be the most compact and lightest arrangement-and most appealing!

Hello,

New to this forum, but excited about VTOL's. Have developed subscale VTOL model that has achieved aft end vtol lift (albeit partial) with multiple engines. Quite simply it's basic rocket science: mass flow x velocity=thrust...Now that I've stated the obvious...don't forget weight and balance. Yes, weight (distributed weight) typicallybane to any aircraft designer is needed in VTOL....it gives the stability w/small sacrifice in lift performance.

I tend to disagree at least from a functional aspect about single engine VTOL. Having experience with military aircraft, its about safety redundancy that will ultimately flow to general aviation projects as the recent posts suggest.

Harrier, as venerable aged warrior VTOL it is, is not the most efficient aircraft but a very functional rugged VTOL. Hosts of issues have plagued the AV-8B, the Hawker, and all other variants. It has the highest crash and fatality percentage of all military aircraft...which is why we used to call it the Widowmaker...Pegasus is a good motor but any failure and you cant exactly go to #2 as backup.

Good luck with your VTOL...it can work.....but you have to lay the ducting correctly and be able to control the mass flow of thousands of lbs thrust (or ounces / ft lbs depending on scale ) with teeny little actuators to aid in stability. Lots of moving parts in VTOL that have to synchonize...not just the powerplant.

"I tend to disagree at least from a functional aspect about single engine VTOL. Having experience with military aircraft, its about safety redundancy that will ultimately flow to general aviation projects as the recent posts suggest. "

Lets look at this for a moment. No matter what you do two engines are twice as likely to fail as one engine, that's just simple statistics. Now if you have two engines working at anything over 50% of their maximum capacity each supplying half the lift and have an engine failure you are stuffed. Because of the weight restrictions I doubt you would be able to build a VTOL aircraft that only used 50% of each engines capacity so that means failure of either engine will result in a crash.

The upshot is two engines means twice as many failures and twice as many engine failure related crashes.

The amount of excess capacity required to make an engine failure non fatal decreases with the number of engines but the likelihood of an engine failure increases. Put simply I think you will find it difficult to build a multi engine VTOL aircraft as reliable as a single engine one.

I think we are discussing the same point from 2 different ends.

1. The success of any multiple engine VTOL (say X planes, X-14B, x-22, etc..just look 'em up or Yak Forgers, JSF F-35, Dassault Balsac, and host of other experimental and real world vtols) is based on high thrust to weigh ratios, including Harrier (varies from 2:1 to 8:1 depending on payload). Obvious statement: to realize vertical lift require a t/w ratio greater than 1. You never exceed the thrust rating, because the rated thrust is ample or exceeds the min. needed to achieve vertical lift.

2. To achieve efficiency in VTOL is to reduce airframe weight and maximize powerplant thrust. Now the Harrier in example generates abou 22 thousand lbs thrust and without payload has a t/w ratio of approx 8:1. The JSF is a multiple system VTOL (the direct lift fan has a redundancy feature , it can lift the entire JSF in the event the PW supercruise motor fails in Vertical Takeoff or Landing mode.) Thus with both powerplants running, thrust is distributed (directly and through porting) and fuel efficiency is realized. It is an example of safety redundancy in a multiple engine a/c. Although not an efficient one, since the Direct Lift fan is really dead weight in forward flight.

I seriously question your stats about failure rates in multiple powerplants. While I do know from experience multiple subsystem aircraft incur high maintenance (failures), the tradeoff is safety and prolonged flight time than from single engine operation. Not all maint. in multiple systems are engine related but can be induced because of multiple engines and failure frequencies are not as high as you state.

Here's an example

1. Single engine failure-plane crashes, pilots die. Asset and human loss. Unless it glides very well....

2. Dual engine failure-plane lands or is able to make controlled crash. Crew and plane live. Depsite damages to plane (loss of engine, hard landing). It will cost less$$ to fix and repair a broken plane than to fix and repair a crater in the ground with smithereens of the plane scattered...not to mention loss of human life.

You can't sell to a public or gain FAA approval on VTOL without some power redundancy because of the experimental nature of VTOL.

Back to t/w ratings. Welcome to the 21st century, aluminum and steel (heavy metals) are losing the throne. Carbon fiber & composites in aersopace manufacturing reduces weight anywhere from 20-80% depending on structure.

So the multiple engine VTOL has :

1. An all composite frame. Reduced weight. I should remind you that Rutan builds planes that weigh only 900lbs dry (no fuel or engine)...all composite. (That's Burt Rutan of Scaled Composites in case you didn't know)

2.Small gas turbines, specifically turbofan (all have dry t/w ratings well over 1). The multiple engine VTOL or any VTOL benefits best from a high bypass ducted fan or turbofan. You can buy one from Williams International , Pratt & Whitney, or even GE for $500K-$800K with thrust ratings at approx 2 to 3 thousand lbs (the engine itself weighs between 300-800lbs). Add 2 or 3 of these little motors and now you have a decent payload capability plus fuel. Add porting, reaction jets and a thourough composite airframe built around weight and balance and the motors and you have a personal VTOL. (multiple engines)-that can go CTOL if any engine breaks.....and will compete in the VLJ category....the one I'm working on.

By the way my stats are all verifiable. You can google them.

nice analyses and input guest,..copy on structural material-carbon/composite, etc., my personal preference would be carbon composite, Ti and Mg.

I've provided some detail/philosophy w.r.t approach on VTOL design/powerplant option,etc at http://www.indramamg-vtol.com , ..work-in-progress!

TOW I'm working on is 500-600kg, i.e.: pilot + girlfriend, FUEL, fuselage, engines+thrusters and control system.

I was also looking at using WR24-6, KHD T-117 or similar mini turbojets, hopefully they're cheaper than the ones you proposed!(way over my present budget!). Intend to use these to 'neutralize' VTOL craft TOW, and use another set of engine(s)-likely to be 2 stroke piston engines driving roots blowers or Procharger type centrifugal superchargers, for geberal maneuvring and forward flight.

Wish you good luck on your VLJ

Not all maint. in multiple systems are engine related but can be induced because of multiple engines and failure frequencies are not as high as you state.

If a single engine has a probability of failure of x then two have a probability of 2x and so on. That's a fundamental rule of statistics and there is no way to get around it.

For a multi engine system to be able to accommodate the failure of s single engine the remaining engines must have enough excess capacity to make up for the loss. So for a multi engine system to be able to accommodate the failure of a single engine the normal output of each engine must be

Two engine system 50%

Three engine system 66%

Four engine system 75%

The question is can you afford to run an engine at these greatly reduced capacities and still get the thing of the ground. If you cant and need to operate at over these power settings then a single engine system will be more reliable.

Another hassle with a VTOL system is thrust distribution. If you have four engines, one in each corner, and loose an engine you are a goner regardless of how much surplus thrust you have.

This doesn't mean it's impossible but it dose make things far more complex and complexity means an increase if the probability of failures.

Another thing to consider is propulsive thrust. If you have a separate engine that is used to supply the thrust needed to move laterally then it is dead weight. If you intend to use the same engines then there needs to be some way to make up for the loss of lift as thrust is vectored from lifting to propulsion. The harrier dose this with wings but you could use a portion of an engines thrust. Keep in mind however that gas turbines are notorious for throttle lag and there can be a considerable delay between adjusting the throttle and getting the power.

I have been a glider pilot for over 25 years and have see a great change in materials. Glass Reinforced Plastic GRP or fiberglass was a huge step forward from metal which in turn was light years ahead of wood and canvas. Carbon fiber is a phenomenal material but it has some serious draw backs. The biggest is that it is very hard to repair and in a lot of instances a single ding can mean the complete replacement of a sub structure. As for Ti and Mg if you are not experienced in the use of these materials and have a bottomless money pit then forget about them completely. They are unbelievably difficult materials to work with and require extremely complex and expensive procedures and tooling. Personally I would stick with GRP wherever possible as it is reasonably easy to work with, is well established and relatively cheap. Just remember though there are different grades of glass fiber and you can't use the stuff from the local hardware store in aircraft.

The incredibly small number of and lack of variation in design of VTOL aircraft is testament to how difficult it is but it's not impossible. Having said all that and probably put you off going anywhere near a VTOL aircraft for life I still think it is a great project and hope you succeed. I think the best technique is to attack the problem in the same way the Wright brothers did. Look at the problem as a series of simple interrelated problems that can each be solve separately

I wish you luck and hope you have fun along the way because it certainly is a admirable project.

PS I would suggest investing in an ejector seat for use in the prototype. As an instructor of mine once said it's better to have it and not need it than to need it and not have it.

Yes. I see your statistics, however, generic and boilerplate. I believe them...I was looking for perhaps something a bit more specific...i.e failure rate in multi-system a/c example-747 as compared to single engine a/c in a crisis scenario...i suppose i could find those via FAA...and prove your metrics right, but I will err on safety redundancy and take the hit.

Regarding, lift loss, during hover, my currrent config is a triposter setting..similar to the stool design mentioned in prior posts. 2 fwd ducted fans with articulating thrust deflectors (range from 0 degrees to -90 degrees) vector thrust for main lift. #3 DF is primarily for pitch axis control and being modified for yaw control through bleed air - reaction jet type thrust distribution. #3 thrust deflectors have a range of +30 degrees to -90 degrees. CG is equidistant (obviously) between the 3 motors, to compensate for fwd payload shift or cg shift, which is challenging during transition but can be overcome by simple puffers or reaction jets in the wings and fwd. fuselage.

Engines are controlled electronically, in real-time FADEC in scale model through ESC's. so that each lift motor adjusts to the lowest minimum rating....if #1 is having a tough day at 90% , #2 is trimmed electronically without pilot input to match #1. # 3 is controlled independently but can be synced to #1 and #2. #3 can drive the ship forward, control pitch and yaw in static hover (fixed and transitioning altitude). All 3 motors due to the unique propulsion system / config provide forward thrust in flight and can also do so in hover mode to escape ground effect (ballooning) . Should #1 or #2 fail in hover #3 can sub to minimize crash impact via thrust deflection. Any failure if flight and a/c becomes CTOL very easily. Yes turbine IO lag is a real consideration, which is why #3 must be there, ready for any sudden axis change through transition to fwd flight.

experimental version is fixed wing, with high lift surface area...approx 2-3x that of Harrier to reduce stall speed. wings provide only 40-50% of total lift surface area...workload is distributed (in Airframe & Powerplant)...that is the efficiency model of this a/c...that i'm trying to prove.

thanks for your input

"I was looking for perhaps something a bit more specific...i.e failure rate in multi-system a/c example-747 as compared to single engine a/c in a crisis scenario...i suppose i could find those via FAA...and prove your metrics right"

There is a site called the Aviation Safety Network that has a database that lists all the airliner accidents going back to 1942 with the causes and a whole host of other information. If it hasn't got exactly the data you are looking for he may be able to generate a special report for you. The site is run by Harro Ranter on a completely voluntary basis and the only funding for the site comes from the occasional donation from users. He dose a good job so if you find the site useful it would be a nice gesture to sling him a few dollars.

Coping with an engine failure is going to be a serious problem due to the many problems already mentioned. If you can figure out how to build some sort of power transmission that can take the output of all the engines and distribute it to the lifting fans it would make things simpler. For example if you use the engines to generate electricity then run the fans with electric motors you don't have the instability problems when you loose power from one engine. Efficiency is going to be a problem though as any transmission system is going to chew up power.

Something else I thought of is have you toyed with the autogiro concept. Once you get the thing off the ground and begin transiting to forward flight you can use the forward motion to turn the rotor and create lift. In the event of an engine failure the forward motion of the craft and rotation of the rotor means you don't fall out of the sky like a rock.

VTOL is and incredibly difficult thing to do and the fact that to date the Harrier is the only aircraft that has been commercially successful VTOL is not a good omen. None the less it a worthy goal and I hoe you succeed, so good luck.

http://world.honda.com/AircraftEngines/

Does anyone know how much this baby costs?

Fit this to your VTOL, and you can bring your entire family for a picnic!

It has the word "aircraft" in the description and that usually means mind bogglingly expensive.