Vtol design: 1 vs 6 engines

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leviterande

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you also need to take into account that when you loose an engine you're not going to loose thrust from that engine but also the symmetric one on the other side. for example if you number engines 1-6 in clockwise order, if engine 2 cuts out you will have to shut down or reduce thrust in engine 5... unless, of course, you think flipping over would be fun... but i suspect not lol. also i saw no consideration in your weight estimates for for fuel... considering how light your craft is, it could make up a significant portion of your weight buildup.

fule can be 5kg for testing and if any engine/s is out all the other engines will compensate by lowering the rpms a little for perfect rebalance.

Mollers are too loud but he didnt use 32 inch props


Dana,
I read that you got 100kg out of a kawasaki 25hp what was the gearing do you remember perhaps?


kalle
 

Dana

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I read that you got 100kg out of a kawasaki 25hp what was the gearing do you remember perhaps?
100kg, no way. I never measured the thrust, not accurately at least, nor have I ever found a reliable quote on the engine's thrust. However, it did climb side by side with another 25hp machine with same pilot weight and same paraglider, other machine reliably rated at 145# thrust.

Gearing on mine was 3.4:1, engine rpm 12000. Other machine (Cors-Air M21Y engine) turned around 7500 rpm, gearing unknown.

-Dana

The most useful tool for dealing with management types is, of course, an automatic weapon.
 

leviterande

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100kg, no way. I never measured the thrust, not accurately at least, nor have I ever found a reliable quote on the engine's thrust. However, it did climb side by side with another 25hp machine with same pilot weight and same paraglider, other machine reliably rated at 145# thrust.

Gearing on mine was 3.4:1, engine rpm 12000. Other machine (Cors-Air M21Y engine) turned around 7500 rpm, gearing unknown.

-Dana

The most useful tool for dealing with management types is, of course, an automatic weapon.

hmm it must be my eyes played me a trick:lick:,

so you get 145lbs from that 25 at 3.4:1,..hmm the twin 210 19hp produces 128lb direct drive!..according to the seller I asked.. wow is he telling the truth? I think he may not.. but i dont know?

Kalle
 

leviterande

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I measured 5.5 lbs of static thrust per hp on a 10hp lawnmower with a handmade prop.

The problem with six engines is getting them all started.
I think six similar sized electric motors might be the answer. The endurance would be shorter, of course.
BB

truly not bad results at all, was it direct drive 27" prop I guess? or geared. do you think it is possible that that twin 19hp 210can produce 128# direct drive? if that is true then how come paramotors produce relatively low thrusts?

Kalle
 

Dan Thomas

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I think six similar sized electric motors might be the answer. The endurance would be shorter, of course.
BB
Electric motors are fine, until we decide what we're going to power them with. Batteries? A generator? Or a long extension cord? You end up with a LOT of weight, and very little range if it flies. As far as I know there's only one electric airplane flying, and that's an airplane, not a much less efficient VTOL craft.

Dan
 

leviterande

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Electric motors are fine, until we decide what we're going to power them with. Batteries? A generator? Or a long extension cord? You end up with a LOT of weight, and very little range if it flies. As far as I know there's only one electric airplane flying, and that's an airplane, not a much less efficient VTOL craft.

Dan

electric motors specially in VTOLS are totally out of question, they are perfect but the super batteries available will not even take off..

a generator wont work.. not only you have weight penality but additionaly it wont even take off becouze of the weight of the alternators.. A 45kw alternator is going to weight at least 80kg.. and dont forget the 45kw generator too;) plus fuel and controllers, cables.. and the motors themeselves.. HOnestly electric motors in vtol is an impossible dream .. unless we find a true super capacitor


Kalle
 

mstull

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I don't think the static thrust of an engine and prop on an airplane relates too well to a ducted fan. Without the duct, a very significant amount of thrust and efficiency is lost from the air near the tips of the prop blades circling back around the outside of the prop disc. Talking with prop makers, I learned that the outer several inches of each blade's thrust is lost this way. Since that is the fastest moving part of the blades, that have the most drag, it's a huge inefficiency for non-ducted props.

I wouldn't be surprised if the thrust to Hp ratio for ducted fans was double what airplanes can get, with an ideal prop turning the ideal prop RPM in an ideal duct.

I don't think any common aircraft props are particularly suitable for a ducted fan. I stumbled upon a prop maker that makes ducted fan blades some time ago. I don't remember the name. But you can probably do a search for ducted fan blades. As I recall, they were making them for a military application.

For ducted fans to be efficient, the prop tip clearance to the duct needs to be extremely small. If you mount the prop to the vibrating engine, the rubber engine mounts will allow too much motion, and force you to have a much larger, more inefficient prop tip clearance. Maybe the engine could be mounted off to the side, driving the prop through a long toothed belt, so the prop hub can be solidly fixed to the duct structure, isolating it from engine vibration. Otherwise an electric, turbine, or rotary engine might be an idea.

Don't be surprised if the ducted fan blades that exist are proprietary (sold only to the agency that paid for their development).

In answer to your original question, operating multiple engines would be a maintenance and tuning nightmare.
 

Dana

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Ducted fans are also horribly inefficient at low speeds. Thrust is relatively low; they really only make sense at high speeds (since thrust horsepower is thrust times velocity), or when the need to enclose the propeller is more important than efficiency considerations. In theory, Moller's Skycar concept seems to make sense (accept the inefficiency for the short VTOL phase and get reasonably good performance at high speed), but not that after 30 years of experimenting he still hasn't achieved free flight.

-Dana

Give a man a fish and you feed him for a day. Teach him to use the Net and he won't bother you for weeks.
 

leviterande

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I don't think the static thrust of an engine and prop on an airplane relates too well to a ducted fan. Without the duct, a very significant amount of thrust and efficiency is lost from the air near the tips of the prop blades circling back around the outside of the prop disc. Talking with prop makers, I learned that the outer several inches of each blade's thrust is lost this way. Since that is the fastest moving part of the blades, that have the most drag, it's a huge inefficiency for non-ducted props.

I wouldn't be surprised if the thrust to Hp ratio for ducted fans was double what airplanes can get, with an ideal prop turning the ideal prop RPM in an ideal duct.

I don't think any common aircraft props are particularly suitable for a ducted fan. I stumbled upon a prop maker that makes ducted fan blades some time ago. I don't remember the name. But you can probably do a search for ducted fan blades. As I recall, they were making them for a military application.

For ducted fans to be efficient, the prop tip clearance to the duct needs to be extremely small. If you mount the prop to the vibrating engine, the rubber engine mounts will allow too much motion, and force you to have a much larger, more inefficient prop tip clearance. Maybe the engine could be mounted off to the side, driving the prop through a long toothed belt, so the prop hub can be solidly fixed to the duct structure, isolating it from engine vibration. Otherwise an electric, turbine, or rotary engine might be an idea.

Don't be surprised if the ducted fan blades that exist are proprietary (sold only to the agency that paid for their development).

In answer to your original question, operating multiple engines would be a maintenance and tuning nightmare.
I agree with you but what about shrouded propeller with a minimum clearence and a huge intake lip. as I read a large intake lip will increaase your thrust without problem to 40% more and even to 60%

another interesting aproach is
Hartzell props, the q-tips propellers


BTW, I really still want to know if these can produce the claimed 128# of thrust, did anybody test engines in this reange?
SV-210cc Twin R/C Airplane Engine
Kalle
 

leviterande

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been thinking more into the matter, knowing there are billions of designs for a vtol.. a tandem propeller might be a good candidate just like the old good piasecki vtol jeep. an engine would in this case drive the both of the propellers , even if it is more complex then having each engine driving its own prop, the balance and stability will be much better with the two props rotating at the exact same rpm, eliminating gyroscopic preccesion and torque problems.

I will have to find a way to couple the enigne to the props. beltdrives should be used since they transfer the energy with minimum losses. the props should be around 2.2m running at 1200 rpms

thrust will as a rough guess be 240 kg and horsepower required is between 40-60? more? since the gear ratio is around 6:1 I want to try to make the whole thing as efficient as a heli as possible without having too large rotors. The center of gravity should fairly high.

taking account with "belt drive losses" how much hp do we really need to get thrust above 200kg with this configuration.. I tried many static thrust calculators and they seem to be way wrong


Kalle
 

BBerson

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The thrust per horsepower depends on the disc loading. Some helicopter books have a chart that will give the answer. Or you can calculate the answer from the power formula.

For example:
Disc loading of 16 lb/ft = power loading of about 7 lbs/hp.
Disc loading of 8 lb/ft = power loading of about 11 lbs/hp.
This assumes the efficiency is about 80% ( varies from 50 to 90%)

BB
 

leviterande

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The thrust per horsepower depends on the disc loading. Some helicopter books have a chart that will give the answer. Or you can calculate the answer from the power formula.

For example:
Disc loading of 16 lb/ft = power loading of about 7 lbs/hp.
Disc loading of 8 lb/ft = power loading of about 11 lbs/hp.
This assumes the efficiency is about 80% ( varies from 50 to 90%)

BB
I dont know but something seems to be really wrong here, but maybe cuz I am sleepy now.. my calculation showed that the disc loading will be for "each prop" around 5,8lb/ft!!!

diamter 2,2meter=7ft
r=3 ft
Disc area r2* pi=38 ft

half weight of the gross weight: 90kg but lets say 100kg=222lbs
222lb/38ft= 5,8lbs/ft?! so does that mean I will have around 13lb per hp hmm?


Kalle
 

BBerson

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so does that mean I will have around 13lb per hp hmm?


Kalle
Yes, I calculate about 12lb per hp.
It depends on how well the prop is designed and also the tip speed. Helicopter designers use the words "figure of merit" for this. I was using a figure of merit of .8

You need extra thrust for climb, higher altitude or a balky engine etc. Maybe double would be good.

You are on the proper course with low disc loading.
BB
 

leviterande

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I just cant find any calculator where you put in the engines horsepower, torque and, rpm and gear ratio to see the thrust, do you know any? this is the closest I found but I doubt its accuracy.. it matcehd however my smaller models

Static Thrust Calculator - STRC



so in theory if we have a 92" prop turning at 1150rpm we have 306# thrust. 2 rotors=612 which may be sufficient for a 440lbs craft the required horsepower is 37 so the needed horsepower is 74 hp my question is if we have an engine at 74 hp how much do we need to gear down to make the 92" prop turn 1200rpm?

if anyone knows a calculator where you can input the gearing data that would be highly appreciated


thanx
Kalle
 

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BBerson

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The X-Plane flight simulator can design helicopter rotors and props.

The model airplane prop calculator you posted does not account for the prop or rotor blade chord width. You must choose a blade chord that matches the available power at the chosen tip speed and blade pitch angle.
BB
 

leviterande

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The X-Plane flight simulator can design helicopter rotors and props.

The model airplane prop calculator you posted does not account for the prop or rotor blade chord width. You must choose a blade chord that matches the available power at the chosen tip speed and blade pitch angle.
BB
do you know of any such calculator and with gear ratio option too? maybe x-plane has such a calculator


Kalle
 

BBerson

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X-Plane has gear ratio. You can design any type of prop or rotor.
And you can fly the VTOL on the computer!
X-Plane uses blade element theory to calculate aero forces, it's not perfect, but should get you in the ballpark. I use an old versionthat it runs better without all the memory needs of a new version.
BB
 
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