what is the best helicopter in autorotation
 

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

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

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.

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.

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.

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

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

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.

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!

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.

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.

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

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

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

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.

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.

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

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.

autorotation!
 

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:

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