what is the best helicopter in autorotation

[zeeoo]

Hi Gents, in your opinion, what is the best helicopter in autorotation, and why ? (or the worse and why)
Thank you.

[slowrotor]

The Sud-Ouest Djinn had outstanding autorotation ability. I seem to recall reading that it really did'nt have a dead man curve if that is possible.
The heavy tip driven blades combined with a large disc on a light ship is the reason.
Empty weight 794lb (360kg)
rotor diameter 36ft

[Dave_Jackson]

The UltraSport 254 has a disk loading of 1.52 lbs/sq-ft. and a descent rate of 15 ft/sec. It is also claimed that the Ultrasport has enough inertia to autorotate to the ground then lift off and re-land.

Tip weight: 2 pounds.
T/K formula is 2.5 seconds.

Incidentally, it uses the VR7-B airfoil, which you are considering.

[Captain Pheremone]

A Mi 8 MTV - in the unlikely event of a dual engine failure - is great in training (High inertia blades) have not tried the real thing yet. Collective fully lowered (control RPM of course) you can autorotate at 120 km/hr, flare to level flight at very low forward speed and would impact the ground at less than 300'/min descent without raising collective.

Of course it helps to raise collective....!!! But you would walk away from the above scenario.

[NickLappos]

The big factors for a successful autorotaton design are clustered in three places:
1) Good rotor inertia, so the stored energy is high. Dave uses a T/K measurement, which I am sure is the total stored rotor energy (1/2 I omega squared) divided by the total power required to hover (energy/second). The quotient is the number of seconds of stored energy, so it is a measure of how much pitch pull there is at the bottom (although it is not the total number of seconds you have available, since you can't pull the rotor down to zero rpm!). Inertia also helps increase the reaction time available at the initial engine failure. It also helps keep the rotor rpm from wandering during maneuvers while in an autorotative descent.
Pilots love high inertia rotors for auto, and hate them for handling, since high inertia blades make the cyclic sluggish. The principle rotor flapping term is the flap inertia, the brother of the polar mement of inertia we are discussing here. Designers hate inertia, since the loads on the blade/head/mast/transmission feet are due to CF, so these parts get heavier when the blade does. It is safe to say that 1 kg of blade weight might be 3 kg of empty weight, the worst penalty of any part of the helicopter.
Experience has shown that T/K in the 2.5 second range is a very poor structural rotor system, but a nice autorotational aircraft. T/K of 1.0 is poor as an autorotational bird.

2) Low disk Loading helps greatly in two areas, the auto descent rate, and the cyclic flare effectiveness. Low descent rate means less vertical kinetic energy to absorb at the bottom, more time to judge the proper flare height, and more time to find a place to land. Better cyclic flare means more vertical g per longitudinal pitch rate (the rotor has more cyclic "bite") and the pilot can use less pitch angle, and less pitch angle rate to successfully burn off the flrward and vertical speed.

3) Tail wheel or tail bumper to allow landing in the flare, and protection of the tail. This is less important that the above, but far more important than the typical aerodynamic effects like airfoil and tip shapes.

All of the above are compormised down a bunch for twin Catagory A helos, where the probability of a total power failure is very small. They should be more important for a single piston aircraft, and a trainer, where the need to do a touchdown auto is much more normal.

[Shawn Coyle]

I'm partial to the R-44 after just over one hour...

[Vfrpilotpb]

FRom my point of being a PPL and now where near the hours you chaps must have, I feel the B206 and the R44 are very good , stable and give plenty of time to plan and think/look where you are going, even with a cab full of pax!

Peter R-B

[zeeoo]

Thanks Dave, Nick an others.

Nick, thanks for the detailed explanation.
I see the inertia reserve and low disk load is, as you confirm it, the most important factors.

But these factors, assuming the rotor is correctly designed to endorse CF efforts, have not the same efficiency if coupled to bad/uneffective blades.. am i wrong ?

Behind these mechanical factors, what are the aerodynamic factors ?
for example does a 3 bladed rotor autorotates better than a 2 bladed ? are the asymetrical airfoils better for autorotation ?
Is an enstrom or a ecureuil better than a B47 or a B206 in autorotation ?

I must say that by "good autorotation capabilities" , i mean a low Rate of descent and a good inertia reserve.

Thanks for helping me to understand.

[oldbeefer]

Remember my instructor in a Bell 47 doing an engine off, lifted to the hover, did a spot turn and then put it down again - gently. For me, that's a high inertia system!

[NickLappos]

zeeoo,

I think the airfoil effects could help, but if you designed for auto, would that rob powered flight payload? If so, the extra few percent rate of descent might not be worth the loss of efficiency.

The number of blades is not itself an issue, the solidity would be, I think. It gets harder to make a 2 bladed system as weight-efficient as a 3 or 4 bladed system, once the needed aerodynamic blade loading is achieved. Large 2 bladed helos are a nightmare for weight .

Also, vibration in forward flight favors 3 or 4 blades, since the "root shears" are very much driven by the number of blades. The texts have good discussions of this.

[zeeoo]

Nick,
your answer makes sense to me.
let me explain : i may be a litle too concerned by the "airfoil" thing since i've always read all the critical studies stating that the airfoil is "THE" holly grail. i think it has too uch importance in my mind.

I really prefer to know that the most important is the rotor as the "mechanical" system instead of a pure aerodynamic system.

My actual choice is : 2 blades or 3 blades ? the challenge is different, but i think it is worth a try if it makes a real improvement without restrictive problems.

I won't chase the few percent since i know that i can stay simpler and easier with a proven design.

for info the Max weight i talk about is 540 lb, single or 2 seat. I don't plan to design something bigger.
I just try to do correctly my homework instead of taking an existing empirical solution, meaning : buying an existing 2 seat rotor.

With humour, i'd say that designing a rotor with inertia is not a problem, nor to do a strong rotor.
the problem is the "light enough".

Thank you for your patient explanations.

[imabell]

sorry oldbeefer, not possible, that belongs in urban myth thread.:{

[Dave_Jackson]

zeeoo

Are you are proposing to develop a gyrocopter, a helicopter, or some form of hybrid?

The selection of; the number of blades, the airfoil, and the means of control, etc. will be very dependent upon this primary decision.
____________________

Prouty has a book entitled 'Military Helicopter Design Technology'. It is significant to note that the chapter 'Missions, Requirements and Desires' is at the beginning.


Dave

[Revolutionary]

How about the NHI 'Kolibri' (hummingbird), an unsuccessful Dutch design circa 1956? I've heard this helicopter had no H/V curve due to its tip-jet propulsion. It also had a less than twenty minute endurance on full tanks, so presumably one would need its autorotation capability at the end of most flights.

http://avia.russian.ee/vertigo/foto/nhi_kolibri-s.gif

[zeeoo]

Dave,
you know what i am developing.
i posted a 3D view here .
It is a simple gyrocopter, one seat. the mission of a such aircraft is simple to understand : low speed flight at human breath level, short take off with jump take off capabilities. the max weight is about 270 kg.
the "mission" concern is valid for all kind of commecial or military project, but this is not my case.

I also read some books and in mine : "L'HELICOPTERE theorie et pratique" from P.LEFORT and J.HAMANN, both engineers in the highest schools and both managers of the helicopter division at the french "Aerospatiale". They describe precisely the missions in the last chapters.

i tried to design a gyro to challenge the Xprize but , by respect for B Rutan, i stopped because he needs the price much more than me :D.

thanks.

[slowrotor]

In Prouty's book he says that a heavy helicopter will come down slower in autorotation. That seems contradictory... from the fact that a low disc loading is better. I assume he means that a particular helicopter will auto better at a higher weight because the parasite drag and power to the tail rotor is fixed. Therefor if more potential energy is available at altitude then the descent rate can be reduced. Still a bit confused.

A glider can increase its speed with added water ballast but the descent rate is not improved, actually increases the sink rate. (I think)

The Hiller Hornet could not be certified because of the high autorotation sink rate (about 3500fpm) and the reason was thought to be due to drag from the tip ram jets. The tremendous noise didn't help the design much either.

[zeeoo]

Hi Slow rotor,

the weight helps to regain speed but degrades the finesse.
the best glide ratio is designed at a certain speed/weight/centering.

Talking about tip jet helicopters, the diffrene between the Hornt and the Djinn was that the Djin used cold air , the nozzles were smaller with less drag.

are you interested in that kind of design?
have you data about the djinn and the hornet ?

thanks

[slowrotor]

zeeoo,
I have been researching all ideas that can be used to build a sport vertical takeoff aircraft.(not just helo's).
I am more concerned with simplicity, low cost and safety rather than performance.Looking for a safe way to fly low and very slow.
Tip jets were considered but I do not see any way to make jets work with a low cost piston engine.

If it was easy, everybody would be doing it!

P.S. There is a book at my local library, I think the title is somthing like "Whirlybirds" that has a chapter about the Hiller Hornet and tip ram jets.

[quadrirotor]

Dave could tell you that the Synchropter is ideal to autorotate after a power shut down as the nose goes down when there is no torque at the rotors!...all the other parameters are also very favorable for autorotating!...:ok:

[zeeoo]

Slow rotor,
off this topic, the djinn had a 11 m diameter rotor, 350 kg empty weight and was designed in the 60's.
just for numbers, do you know what would be the airflow needed to spin a 8 m diameter rotor at approx 350 rpm ?
I thik that atually, this could be done by using a centrifugal compressor with a 80 Hp engine.

Thanks

[slowrotor]

zeeoo,
Cold jets with a piston engine and added compressor?
How could a piston engine compete with a turbine that has a high power to weight ratio and a built in compressor?

A better plan might be installing turbines at the tip. Hiller was working on that also. R/C model people are building small turbojets, not sure if they have gotten to building turbofans, that could be interesting.
Electric turbofans is another idea, used in the R/C world. A small battery could provide a surge of power for a jump takeoff.

[zeeoo]

Slow rotor,
my numbers for the Djinn are an airflow of 1.1Kgs/sec.
A turbocharger like the garrett GT40 can provide 1kg/sec at 2.5 bars. Another example is the ROTREX superchargers, the bigger can provide 0.8 Kg at 2 bars, i asked their eng team and they estimate the power required to run 2 units at about 75 Kw.

BUT using cold jets for prerotation is different than for main power source. The airmass needed is far less since you don't power the rotor with it. BTW, for a gyro, the rotor doesn't acts at high AoA.
According to rough calculus,i have estimated that i can get 25 lb of thrust on each blade.

You can't compete with a gas turbine but the Artouste and Palouste are old generation turbines.

talking about RC turbines, i believe there's something to do, check this link : http://www.microjeteng.com/prop.html
but the price must be pretty high.

Thanks

Victor

[NickLappos]

zeeoo,

Try not to lose sight that power is power, so the rotor with tip jets eats power based on the thrust, disk loading and figure of merit. The tip jet nozzles will eat more power because they create high drag in the worst spot. The power losses due to fluid flow issues in the hub manifold and the blades (assuming cold cycle) are very high, perhaps 10 to 20% of the total power (vice 3% for mechanical transmissions). Losses at the tip nozzel for hot cycle are very high as well.

These factors generall cost so much power that the hover envelope of these machines is small compared with the conventonal helos, and the fuel flow in a hover is simply eye-watering.

The way to make some hay on this is like the rotodyne, where the powered tip is used for a few minutes at each end of the journey, of course.

[slowrotor]

zeeoo,
If your interested in just autogyro prespin for takeoff then you probably should consider very simple methods.
How about water jet at tip? Sounds silly, but I know of several models that used water ejected for takeoff.
Water is about 800 times the mass of air. And once it is ejected in your rotor spinup then the weight is gone but the rotor energy remains.

[zeeoo]

Slow rotor :
your system has interest, but the way to produce high pressure water jets may be heavier than high pressure air, the water leaks may quickly degrade the rotor mechanism.. air leaks don't.... BTW this is a one or two-use system and i am not for that.....

Nick,
very interesting.. let me try to defend my idea...
(1) yes, i feel the tip nozzles induce great losses..that's why i think in using wing-nozzles : http://zeeoo.free.fr/gyro/nozzle.jpg
the drag and little pitch up moment may be neglectable, BTW, the air-jet doesnt act where the tip-vortex is, and this vortex decreases the efficiency of the tip jet, right?
The use of a squared nozzle is made to avoid the air spinning at the output, this effet can be improved by the use of a nozzle grid (like quiteners in wind tunnels).

(2) You took the example of the rotodyne : YES if think in using the tip jets only for pre rotation and a residual in-flight power..
Let's say that i would share the power by 35 hp to the comp and 50 to the prop (that is enough for FW flight).
The weight of a supercharger w/oil cooling circuit is about 12 lbs.

it may be a little diffiult to do, but, if it works, that adds a great safety feature and allows a higher fw speed.

Thanks for your kind and interesting contribution.

Victor

[Dave_Jackson]

quadrirotor,

"Synchropter is ideal to autorotate after a power shut down as the nose goes down when there is no torque at the rotors!"

True. This rotation about the craft's lateral axis is due to the angle of the masts off the vertical and the breaststroke (inside forward) rotation of the rotors. Kaman may have found it a little 'too' ideal. :eek: On an early model of the Huskie they directed the exhaust downward in an apparent attempt to reduce this pitching on the loss of power.

http://www.unicopter.com/Temporary/Exhaust.gif

This is one of a number of reasons for increasing the rigidity of intermeshing rotors. Rigid rotors will allow for a reduction in the angle between the two masts and this reduces the pitch component of the torque of the main rotors.

_____________________

zeeoo,

The reason for questioning your project is that I believe your 'gyrocopter' will not be marketable in the US Sport plane/pilot category, should you desire to do so. A collective control, for a jump takeoff, or helicopter style cyclic control, which is necessary for a rotor with more than two blades, will not be accepted.


Dave

Editied to remove the idiotic comments on electric rotorcraft.

[zeeoo]

Dave_Jackson,

I'v been told about this limitation. But i don't plan to market something at the moment. XWe have not this limitation in france and europe.
BTW, a rumour says that Ermie Boyette (dominator )and Dick Degraw are preparing a kind of 3 bladed rotor with jump take off capabilities... i bet they will find the right category... i bet the 3 bladed rotors offer is prone to increase.

BTW some 3 bladed gyros (lioré and la cierva) didn't have a helicopter-like cyclic control but a "teetering" 3 bladed one with an offset gimbal.....

http://rvtpanet.club.fr/Articles/Livres_historiques/C30_tete_rotor.jpg

thanks

[bugdevheli]

Nick Lappos says that pilots hate high inertia rotor systems because of the slow reaction times. Whilst I am sure he is correct. I am still convinced that for the new generation of vlh machines, (especially when considering what level of experience the novice would be pilots may have), the necessity for excellent autorotational performance outweighs the downside of slow responce time. Observe Slowrotors comments. He wants low and slow. Zeoo I think seeks something similar. These single place machines are in my view what todays quad bikes are. A nice toy for not going very far in on a sunday afternoon.

[zeeoo]

Bug,
good comparison, may i ad that the quad market is very healthy and that their usage is far beyond the simple recreationnal : all-terrain patrols, beach watch and rescue, wildlife watch, small transportation, utility vehicle, rough terrain reckon, embedded vehicle , it is simple to operate, simple to maintain and repair, a lot of peole can purchase it and even use it on the road..
I is not a land rover, but it can replace it where it can do the same job at less cost..
i like the comparison.
thanks

Victor

[Dave_Jackson]

zeeoo,

It will be interesting to see how they do it. It could result in a safer gyrocopter. Hopefully, it might also be a first step in eventually getting helicopters included in the Sport Plane/pilot category.
_____________________

"BTW some 3 bladed gyros (lioré and la cierva) didn't have a helicopter-like cyclic control but a "teetering" 3 bladed one with an offset gimbal....."

Interesting. I assume from your comment and sketch that the blades have a very small offset hinge and then the hub is mounted on a gyrocopter style of gimbal. Perhaps this is why it has been mentioned that the larger gyrocopters required excessively large cyclic control forces.

I suspect that a minimal offset will result in acceptable cyclic control input forces, whereas a larger offset will result in crisper maneuvering. Pick one. The 'Sunday driver' will require both and this probably necessitates a conventional helicopter flight-control system.

To add to the subject, the following is the spider control used on the Ciarva W-11 rotor.
http://www.unicopter.com/Temporary/Cierva_W-11.gif


Dave

[quadrirotor]

Three blades, no collective, small offset, no delta3...for homebuilt.

http://www.rotaryforum.com/forum/attachment.php?attachmentid=1481&stc=1&thumb=1

[zeeoo]

a pic is better with some details

[quadrirotor]

you must ask Doléac!

[zeeoo]

I think doleac didnt do any tri-bladed, the airframe is a averso guepard but i may be wrong.

[belly tank]

shaun coyle said.....I'm partial to the R-44 after just over one hour...

he he...youd be suprised shaun they auto quite well.....but id rather auto a jet banger any day...

did my check on a 407 a couple of weeks ago...what a piece of grunt but it falls like a lead balloon!!

im assuming with a professional of your experience the r44 would be a snack........one day i will get to the HAI and meet all you legends i cant wait!!

[quadrirotor]

Le tripale:
-moyeu rotor réalisé par Joseph Gisquet (gyroclub Bois de la Pierre).
-pales réalisées par Xavier Averso.
-essais réalisés par Xavier.
-rapport Vol moteur #142 février 1998.

[zeeoo]

From Xavier Averso himself :
this rotor was not for a gyro use, it was a special order fom a customer.
he took 3 shortened blades (2.80m) he put on a tri-bladed hub some guys were doing, this hub had no lead-lag hinges.
results : long take off (small diameter) , high command efforts, better high speed and finesse (lower disk pitch).
he stopped the test to not damage the blades.

[offshoreigor]

Gentlemen, IMHO the best helicopter in an autorotation is the one you walk away from. Practice, practice, practice.

:ok: OffshoreIgor :ok:

[moosp]

FWIW

I renewed a local requirement of a 90 day recency on the R44 today after having flown and autorotated R22, B47G and EC120 for the previous three months. It included auotorotations to the hover.

Yep, the R44 is my favourite of the four. Shallower angle, benign rotor speed control in turning descent, plenty of rotor energy left at the end of the flare and all at a pace that gave spare time for thinking.

Maybe Frank is right in saying that the R44 would make a great ab initio training machine.

Now, if we can get the cost per hour down...

[rumline]

Best autos?
Well it can't pull a limp dick out of a lard bucket or keep it's doors closed properly .....but the Enstrom 480 autos like a dream!...Lots of inertia, great controllability, easy to maintain Rrpm. I guess one could say the same for a '47 but the Enstrom gets the nod in range.
My thots anyways

[CareBear]

Bell 214B - awsome machine - basically a stretched 212 airframe with a chinook engine and a couple of chinook blades !!!

Touchdown, lift to the hover, spot turn and land - no problem!!

[Darren999]

Bell47's are fantastic!! Very easy to Autorotate, in both vertical decent, and extending the glide, no drama's at all..

[wg13_dummy]

Agree on the Bell 47. 40 kts, no idea on the ROD (no VSI!), very forgiving and quite a leisurely trip down. As opposed to the Lynx! Autos like a supertanker and has the range of a house brick!

[zeeoo]

Does that mean that, for the most of you, the "old heavy" two bladed tetering rotors are better in autorotation ?
Thanks
Victor