2007: Centennial of the Helicopter?

[Dave_Jackson]

The American Helicopter Society's Current News;
2007: Centennial of the Helicopter?

Interesting article in the above.
Engineering Analysis of the 1907 Cornu Helicopter by Dr. Gordon Leishman


Is 2007 the Centennial?

[Graviman]

Good paper on Paul Cornu, Dave. Might give me a refresh from revision...

[slowrotor]

If the Cornu helicopter could not have flown according to Leishman, then who was first to fly?

Interesting article. Thanks Dave

[Fred Bound]

Second or first?

If Cornu has been discredited, then it might be Jacob Ellehammer in 1912.

But, then again......

[riff_raff]

I'm still laughing about the photo caption on page 3:

"The Cornu's rotary internal combustion engine worked fine, except under load!"

Frankly, I'm not really impressed by who did it first. I want to remember who was the first to do it right.

[Dave_Jackson]

Analysis of the 1907 Cornu Helicopter

Maybe it Did Fly, or, Maybe it Could Have Flown

Additional Information on Cornu's helicopter:

~ Clicking on the linked pages will double their size. ~

• Cornu's own technical description in 'History of the Helicopter', [ Page 21, Page 22, Page 23, Page 24 ]
• Article in the 2001 Fall-Winter edition of of American Helicopter Society's publication Vertiflite.
  The title is; Cornu's Helicopter ~ First in Flight? [ Page 54 & Page 54 ] by Dr. Gordon Leishman

Leishman's articles, in post #1 and above, look into the validity of Cornu's flight from the perspective of historical documentation and from the more interesting perspective of technical viability.

The Vertiflite article says "The engineering analysis of his concepts conducted in this paper show that Cornu's claim to successful piloted flight with a helicopter is extremely dubious". On page 12 it says ".. the results in Fig.22 show that the installed power required would have need to be about 40 hp".

The craft had an unusual blade planform and it had the advantage of twin-rotors. Out of curiosity, I therefore took the specifications from Leishman's articles and from Cornu's own technical description. The data was inserted into Prouty's Momentum Theory and into his Blade Element Theory. The pitch was set at an optimal 9º and the RPM of the rotors was increased until the thrust equaled the gross weight. Surpassingly, the power required in hover by MT is 21.52 hp and by BET it is 20.98 hp.

The motor produced 24 hp, therefore, it appears that the craft was capable of flying; if the belt was not slipping.

The interesting technical question then became, why does the knowledgeable and respected Professor Leishman feel that the craft would not fly while conventional MT and BET says that it would have?


Leishman's argument to show that Cornu's helicopter could not have flown with its 24 hp engine;

1/ He uses Momentum Theory, in which he shows that the Power Required (ideal) to hover is 14.7 hp.

2/ He then says; "we could expect the aerodynamic efficiency of the rotors to be no more than 50% (a figure of merit or 0.5) leading to a power required of about 30 hp."

3/ He then says; "Cornu also used an inefficient belt and pulley system to drive the rotors...." and ".... a conservative estimate of transmission losses, for Cornu to hover his machine free of the ground the installed power required would need to have been about 40 hp." This is an efficiency loss of 25%.


My counter argument to show that it could have flown;

1/ This is OK; with reservations.


2/ Leishman's figure of merit of 0.5 is exceptionally low; for the following reasons.

a/ Cornu's rotor blade places the lift out near the tip, in the region of higher velocities.

Prouty has mentioned the advantage of reverse taper.

Sikorsky's recent patent for its X2 blade has the widest chord out nearer the tip.

b/ There is nothing, that I can find, which suggests that Cornu's blades are particularly inefficient, particularly at their low air speeds.

c/ The lack of a tail rotor should improve the FOM by up to 10%.

d/ In a discussion about Ct/sigma on this forum last year it was shown that increasing the chord and decreasing the velocity over the blade will decrease the required power. Ref. Large Chord & Low Tip Speed

This suggests that the FOM should be well above 0.5.

3/ Leishman's power loss of 25% is exceptionally high; for the following reasons.

a/ The Cornu's family business was building bicycles. Therefore, they must have been very cognizant of bearing friction and transmission losses. In fact Cornu state; "all rotating parts, including the propellers, are installed on rollers".

b/ Outside sources state that flat belts are more efficient than V-belts. In addition, the use of flat belts was well known 100 years ago, since manufacturing plants powered their machinery by flat belts from overhead line-shafts.

This suggests that the power loss should be less than 10%.

Cornu may, or may not, have flown, however it appears to me that he could have.

Does anyone wish to expand on the pros and cons regarding the viability of flight?


Dave

[slowrotor]

The engine appears to be a V-8. It is possible the engine had more power than the designer stated. An analysis of the engine and the displacement, compression ratio and fuel used would be needed.
slowrotor

[Graviman]

Dave, i'm only just printing off the paper. I'm also planning to read Leishman's book this year. I'd be interested to know more about your rotor spreadsheet - like could i have a copy?

Although the rotor will be operating near to stall, the FM of 0.5 is probably about right. The ideal blade taper planform for hover has chord inversely proportional to radius, which these aren't. X2 likely chose it's planform to avoid reverse flow aerofoil divergence, and will have been optimised for cruise not hover.

Besides the work on aerofoil section optimisation for laminar flow had only just been done by Ludwig Prandtl, so was not widely known. These blades are flat, so will transition at a low reynolds number - ie they are draggy.

The final difficulty in all of these designs seems to be controlablity. The small radius rotors is a hint at the future difficulty of providing sufficient structural stiffness in a rotor blade, only later overcome by Cierva...


Slowrotor, to estimate engine power it should be thought of as a pump. For this era, assume the valve timing was about right for the RPM chosen, but had no acoustic boost from exhaust or inlet. So you can get power from ideal volume flow, hence mass flow, hence power for ideal stoichiometric (14:1 by mass). Assume overall efficiency between 10% to 20% to get useable shaft horsepower.

http://en.wikipedia.org/wiki/Petrol#Energy_content


I'll read the paper over the next day or so to provide more useful feedback.

[Dave_Jackson]

For the fun of a technical argument.

slowrotor,

Wow!

There is a lot of information on the Web about his engines, and it is very impressive. Not only did he invent the V8 engine, he also used direct fuel injection. Of specific interest to aircraft is the statement "Its power-to-weight ratio was not surpassed for 25 years."


Mart,

Perhaps unfortunately, it not a simple spreadsheet. The algorithms are from pages 69-72 of Prouty's main book. These algorithms, plus more, were incorporated into a large Microsoft Access database and linked to many tables. I have previously looked at pulling a copy of it out of the database, however I never did because it would take many days of work.

I'm not ready to accept Leishman's 0.5 FoM. In addition, this FoM sits on top of the simplistic Momentum Theory (Actuator Disk).

You mention;
"The ideal blade taper planform for hover has chord inversely proportional to radius, which these aren't."

True but, this is the remark from Prouty, page 649. He is talking about aerodynamic performance in hover and vertical climb. "(Some preliminary studies indicate that perhaps inverse taper holds some promise in this regard.)"


Dave

[flyer43]

Marquis Raul Pateras Pescara

No competition for the first one to pilot a helicopter, but the website might well be a challenge to many of you as it is written entirely in French by Pescara's son.
There are some interesting technical notes and a number of pictures of his machine, which it is hard to believe actually flew. However, we all owe him very much as I believe he was possible the first to tackle the complex task of varying the pitch of the rotating blades and the direction of tilt of the whole rotor system.

http://www.pateras-pescara.net/helicopter.html

[Graviman]

Interesting Article Flyer43. Until Louis Breguet made his return, Marquis Raul Pateras Pescara certainly made the most convincing advances.

Dave, i think the Leishman calcs for helicopter performance stand, but i question the quoted 24 SHP of that V8 Antoinette engine. This site gives 50 SHP for a later version of the V8:
http://www.hydroretro.net/etudegh/antoinette.pdf

Wikipedia is not so useful here:
http://en.wikipedia.org/wiki/V8#History

This site talks about the 24CV Antoinette engine.
http://www.eurocopter.com/bourget/in...news.php?id=20

The problem here is that in france there used to be some strange system for "CV" that has an indirect relation to SHP:
http://fr.wikipedia.org/wiki/Cheval-vapeur

[Dave_Jackson]

Mart (Graviman)

Dave, i think the Leishman calcs for helicopter performance stand

I still have doubts.

1. Unquestionably, Leishman is a very knowledgeable Aerodynamicist. However, my previous experience in power transmission equipment probably exceeds his.
2. The Cornu rotor blades are very different from today's conventional blades. The Blade Element Theory, with its recognition of chord and root cutout, is much better than the simple Momentum Theory used by Leishman.
3. The actual HP of the Antoinette engine is an unknown. However, perhaps on the eastern shores of the Atlantic they don't use so much snake-oil in their engines.

Maybe it all has too do with;

French fries ~ Freedom fries
Fur flies

[Graviman]

Dave, i think then we both have reasonable doubt about the Cornu rotorcraft never getting airborn. I agree about belts having good possibility, and certainly Cornu paid them a lot of attention to avoid slipping - It was more an achilles heel.

The interesting part for me was Leishman's estimate of Ct/Sigma of 0.684. This seems to imply that the rotor was operating close, if not into, stall upon startup. At the very least the rotor was well into overpitching. Most likely Cornu had some successful moments, but the overpitching meant it was not repeatable enough to be caught on camera. A great shame. Pity too that he never continued development, with say lower disk loading.

Could you share some of your simulation results? I'm interested in seeing why you dissagree with Leishman's findings.

[Dave_Jackson]

Mart,

At the bottom of this page are the forms that you are asking for. Helicopter - Outside - Coaxial - Cornu, Paul, 1907 - First flight

The interesting part for me was Leishman's estimate of Ct/Sigma of 0.684

Yes it is interesting. I will look into it, also.
_________________

Leishman's latest book The Helicopter: Thinking Forward, Looking Back was ordered last month.

Last night, I came across a review of this book. You will find this review interesting. http://www.rusi.org/downloads/assets...e_0207_RDS.pdf


Dave

[Dave_Jackson]

Mart,

Leishman's estimate of Ct/Sigma of 0.684 is based on a rotor speed of 85 rpm. My calculations are based on a rotor speed of 218 rpm, which is obtainable if the engine and belt work to expectations.

Dave

[Graviman]

Dave,

I had a reread of Prouty Blade element theory with tip correction last night. I'm a bit confused because your spreadsheet looks like just momentum theory with the assumption of 0.7 FM. I don't understand how this proves Leishman's 0.5 FM wrong?

I'll overlook the Lycoming O-235 producing 105.2HP too.


Delta3,

I may not get the chance to read Leishman as thoroughly as he deserves this year, since i am supposed to be getting up to speed on differential geometry for my Physics final year project! I'll have the book on standby though so i can refer to any pages or equations you might reference.

That book Dave points out looks interesting too. Clearly X2 has already answered any concerns about speed limitations. It might be a useful engineer's guide for general performance calculations / parameters.

[Dave_Jackson]

Mart,

I'm a bit confused because your spreadsheet looks like just momentum theory with the assumption of 0.7 FM. I don't understand how this proves Leishman's 0.5 FM wrong?

There are four forms, One is Momentum Theory. The FORM: Flight Hovering says at the top 'Combined Momentum & Blade Element Theory w/ Empirical Corrections'

Yes you are correct in that the Figure of Merit only applies to the FORM: Momentum Theory. If a FM of .5 had been used the HP by Momentum Theory would be 15 hp * 2 rotors = 30 hp. However, I have been questioning Leishman's low FM of 0.5, particularly when Momentum Theory includes the 10% power loss of a tail rotor, which does not exist on the Cornu.

Discussing the Blade Element Theory;
Due to Cornu's large cutout and wide chord, I felt that his helicopter's aerodynamic performance might also be considered as four small hang-glider wings. In addition, Cornu's the mechanical driving of the '4 wings' would be more efficient than the aerodynamic transmission from the propeller to the wings on a hang glider.

This assumption of mine may be wrong. It appears that the top speed of a modern empowered hang-glider is 90 fps, and the speed of a sailplane is somewhere around 150 fps. whereas at 0.75R on Cornu's 'wings' (blades) the speed at 24 hp would be 180 fps. Of course, a hovering Cornu does not need to consider the parasitic drag of the pilot and engine etc.

Another piece of information is the ongoing work on the Ultralight Side-by-side Electrotor. This page shows the power calculations using the same Prouty algorithms. It is showing that it will require about 22 hp to lift 550 lbs.

All very interesting.

Dave

[Graviman]

Dave,

So basically your model of combined blade element and momentum method predicts an FM of 0.7. This depends on the Cl vs AOA and Cd vs AOA graphs used (not shown). The general wisdom is that Cl/Cd for a flat aerofoil is lower than for a fat aerofoil. FM of 0.5 still feels reasonable to me.

The root cutout may not have a significant effect, as you say. This may be why Sikorsky/Schweizer opted to use this to avoid reverse flow divergence. I would not have thought that it would increase FM, however.

The book looks interesting, and i'll likely get a copy for reference. I think X2 has already answered Leishman's concerns about high speed rotorcraft development though. I note, with interest on another thread, that BERP IV allows the Merlin EH101 to get to 199Kts - this is worth watching...

[slowrotor]

Dave,
You could put Cornu's design into the X-Plane helicopter design simulator and see if it flies and how much power is required.
X-Plane uses blade element theory.

[Dave_Jackson]

Mart, you say

So basically your model of combined blade element and momentum method predicts an FM of 0.7.

It's not MY model. It is the 21 steps directly from pages 69 to 72 in Prouty's book, as previously mentioned. You may wish to read throught the 21 steps.

This depends on the Cl vs AOA and Cd vs AOA graphs used (not shown).

I selected the 8-H-12 profile, as shown on the forms.

The general wisdom is that Cl/Cd for a flat aerofoil is lower than for a fat aerofoil.

? The Cornu airfoil is cambered.


slowrotor,
Thanks for the thought of using X-plane.

Blade Element Theory is a standard method; perhaps with some users tweaking it a little. One concern might be that it was developed for larger helicopters and it may not scale down well to the very small helicopters.

Dave