Engineering Analysis of the 1907 Cornu Helicopter

[Rotor Rooter]

'2007: Centennial of the Helicopter?'

is the title of the current American Helicopter Society's Rotary Wing History Section.

The primary article is 'Engineering Analysis of the 1907 Cornu Helicopter'. It is written by Professor Leishman and it looks at the plausibility that Cornu was the first to fly a helicopter. This article looks into the validity of Cornu's flight from the perspective of historical documentation and from the more interesting perspective of technical viability.

The technical viability is the one of interest. The article's Abstract 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 article and from Cornu's own technical description in 'History of the Helicopter' [ Page 21, Page 22, Page 23, and Page 24]. 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 becomes 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?

Dave

[barnstorm2]

That is a very cool website.

Thanks!

[Rotor Rooter]

- the following addendum is submitted.

The above long paper by Leishman was preceded by his short 2-page article, on the very same subject, in the 2001 Fall-Winter edition of of American Helicopter Society's publication Vertiflite.
The title is; Cornu's Helicopter ~ First in Flight? and these are the two pages; Page 54 & Page 55.
~ Clicking on the pages will double their size. ~


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.

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

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

e/ In a discussion about Ct/sigma last year it was shown that increasing the chord and decreasing the velocity over the blade decreases 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.

This is the most important moment in Rotorcraft history. There must be one or more technically astute people who concur with Professor Leishman; and have the 'balls' to challenge.


Dave

[WaspAir]

It's been a while since I looked at any of the historical material, but I seem to recall that Cornu was running an Antoinette engine at pretty low rpm, and probably wasn't getting much more than half the rated hp out of that motor. That wouldn't be enough even with your more generous assumptions, would it?
If my memory about the numbers is correct, that would put the power available down into the same range as the 1903 Wright Flyer, which had extremely marginal performance as a fixed wing aircraft (even given remarkably good propeller efficiency when judged by modern standards and no-slip bicycle chain drive).

(If my memory is faulty about the engine type and/or rpm, please let me know.)

[Arnie Madsen]

It is well known Cornu liked lots of publicity and had many photographs taken of his machine.

Why is it then , the only picture of it "in flight" only show the rear wheels 6" off the ground in a type of momentary bounce.

With all the belt slip problems he was having , my guess is that the rotor closest to the driven portion of the belt received extra power & rpm while the belt slipped on the other rotor. This could have provided enough power to momentarily lift one end of the machine.

Also in the photo it shows the machine turning sideways on the platform , which could indicate more torque was being produced in one rotor than the other at that moment.

[barnstorm2]

Arnie,

On gyrowiki.com there is a newsreel from Wright Patt that has video footage of this machine flying.

.

[Rotor Rooter]

WaspAir and Arnie

You guys may be correct.
If he was unable to overcome the belt slip problem, its unlikely that he could hover, even in ground effect.
If he did lift 328 kg, as he says in the lower half of Page 22, then he probably could hover, even out of ground effect.

Who can one believe? Cornu or Leishman?

It sounds like the Chuck & Dennis Show on another thread.


barnstorm2

Were you able to get the video to run?

Dave

[Arnie Madsen]

Originally Posted by barnstorm2

Arnie,

On gyrowiki.com there is a newsreel from Wright Patt that has video footage of this machine flying.

.

For some reason I couldnt get the video to work today. I will try again later.

If Cornu was actually filmed in flight I wonder why the American Helicopter Association is now questioning it . Something does'nt add up here. Anyone know for sure ?

Thanks
Arnie

[barnstorm2]

Here is a link to the NASA. Site, it looks like it is indeed NOT the Cornu bur an early 4 rotor.

http://lisar.larc.nasa.gov/UTILS/inf...=LV-1998-00139

.

[joe nelson]

Tim,

There was a rotorcraft in the 1920's at Wright Field that looked like the Cornu aircraft. I think it was built by De Bothezat...not sure of the spelling. I have seen video of it hovering at about two feet and it appears to have no ability to do anything but hover.

[barnstorm2]

Originally Posted by joe nelson

Tim,

There was a rotorcraft in the 1920's at Wright Field that looked like the Cornu aircraft. I think it was built by De Bothezat...not sure of the spelling. I have seen video of it hovering at about two feet and it appears to have no ability to do anything but hover.

Yes, that is it!

[ferranrosello]

I don't understand why is so important if Cornu's helicopter flew or not. If it flew it was not in a controlled way, and he didn't endure his efforts until achieving a capable helicopter. This is a very different way of doing things than Wright brothers.

I believe that Cornu does not deserve the merit of the first helicopter flight. It is deserved by Breguet and Dorand. They made the first practical helicopter which achieved a proper flight in 1935-36.

Furthermore, I think than Cornu did not make a real contribution to helicopters development. Others have contributed much more, like Pescara or Cierva...

Ferran

[Rotor Rooter]

A slightly revised and expanded version of the previous 'Engineering Analysis of the 1907 Cornu Helicopter ' ~ by Gordon Leishman and Bradley Johnson has been published in the latest issue of The Journal of the American Helicopter Society (July 2009).

Perhaps the evaluation of Cornu's helicopter should have been done using a trike and not today's high aspect ratio helicopters.

This is an evaluation using a comparable Trike. IMHO, it suggests that Cornu's helicopter might have actually flown.

Dave

[Rotor Rooter]

For the interested.



For the very interested.

Leishman's 0.50 Figure of Merit;

His paper states; "The blades were also flat (i.e., they had no proper Airfoil section) ..."

Response; The pictures clearly show that both the rotor blades and the control surfaces were not flat. They were undercambered.

C. Beaty quote; "The early aviation experimenters began with flat plate wings but soon realized that curvature [undercamber] improved efficiency; influenced most likely by the observation of birds’ wings."

Response; The Side by side configuration is the optimal one for high Figure of Merit.

Leishman's 0.75 Transmission Efficiency;

His paper states; "... a transmission efficiency of η=0.75 ...".

Response; η=0.75 is far less than what flat-belt transmissions are capable of. Flat-belt transmissions can have up to 98% efficiency. This is one reason why they were used in Cornu's bicycle manufacturing facility and thousands of other plant at that time.

His paper states; "One issue was that the belt was twisted several times from the engine to the rotors."

Response; This is another of the flat-belt's attributes.

Cornu's Figure of Merit may, or may not, have been sufficient. The flat-belt transmission slippage may, or may not, have been solved. Modifications to the craft's structure may, or may not, have withstood the forces. The controls may, or may not, have provided stability. Not winning the D-A Grand Prix Prize of Aviation may, or may not, have been his reason for discontinuing the development of his side-by-side craft.

However, the technical arguments and the tenor in the paper suggest an attempt to justify a preconceived notion.

Dave

Edit:
CONCERN; The Cornu's blade pitch in the above comparison is 14.8-degrees. This raises the question of whether the rounded leading edge of Cornu's flattened tubing will or will not cause a leading edge stall.