A booklet on helicopter history

I did not find the name of the author of the German text - Theodor Mohr - anywhere in the American text. Did I miss something? The text of course is a great source of information. The most striking passage to me is (p. 5): "Calculations revealed that reversing the direction of rotation should significantly improve directional stability.." Here is proof to the fact that theory brought about the major advancements in rotary wing flight.
 
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Hi Juergen,

I did not find the name of the author of the German text - Theodor Mohr - anywhere in the American text. Did I miss something?
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I bought the book years ago and then later found a copy of it on the Internet. It is such a good book that I copied the Internet version to my site, to assure its continued existence.

____________________________________________________________________________​

Apparently, Kaman also tried the outside-forward rotation and then went back to the breaststroke.

Being a little slow in the cognitive department, my proposed Intermeshing craft have also been designed with rotors turning outside-forward. This puts the advancing blade tips in clean air and theoretically results in a more equitable thrust distribution over the combined disk area.

Other reasons for going outside-forward are that the incorporation of 'absolutely' rigid rotors allows for a significantly smaller angle between the two masts. In fact, the outer advancing-blades are horizontal. This may overcome the directional stability problem and also minimize the torque-pitch coupling problem.

____________________________________________

The Future?

Perhaps the most perplexing problem of the Intermeshing configuration is driving and synchronization the two rotors. Anton came up with this beautifully simple idea for his next model, the FL339.

1509-C_Smaller.gif

This Worm & Gear means that only 3 gears are needed, and the complexity of an unusual angle between the shafts is not a problem. In addition, this concept will allow autorotation since ratios of 10:1 and less can back drive from gear to worm.




Dave
 
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Dave,

the worm drive looks very promising, a simple way of combining the high gear ratio you want and the need to synchronize the motion of the two rotors. There is one thing that always puzzled me: with both rotors inclined outward there seems to be a sort of "negative dihedral" in this design. Low speed (M < 0.3) airplanes with such a configuration would be highly unstable in roll, why is this different with a rotary wing aircraft?
 
Juergen,

There is one thing that always puzzled me: with both rotors inclined outward there seems to be a sort of "negative dihedral" in this design. Low speed (M < 0.3) airplanes with such a configuration would be highly unstable in roll, why is this different with a rotary wing aircraft?
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An interesting question, since dihedral is used to increase the lateral stability of an airplane.

One reason might be that the vertical distance between the rotorcraft's Center-of-Gravity and its Center-of-Thrust is much greater than that of an airplane. Another might be that both rotors have coning angles and the two may sum to result in a combined dihedral.


At 2.50 min. on this video it shows the craft being flown hands-off in slow flight.
WW2 German / Nazi Helicopters : FL185 Fw61 Fa223 FL265 FL282 - YouTube



Dave
 
kolibri282 said:
The most striking passage to me is (p. 5): "Calculations revealed that reversing the direction of rotation should significantly improve directional stability.." Here is proof to the fact that theory brought about the major advancements in rotary wing flight.
Click to expand...
Torque balance is incomplete if the two shafts aren’t parallel.

With outboard blades retreating, there is a nose-up residual that requires forward cyclic to balance. This places rotor thrust line aft of CG, improving angle of attack stability.

With outboard blades advancing, there is a nose-down residual that requires aft cyclic and locates rotor thrust line forward of CG, diminishing angle of attack stability.

It’s a bit like a dragster with lots of camber in the drive wheels; imagine the rotors are wheels in contact with the ground. With wheels propelling the dragster forward, drive torque lifts the nose.
 
ok help me understand one thing with the worm drive, How will it auto rotate?

I've never seen a worm being driven?
 
Model T Ford trucks had worm gear rear ends and they coasted to some extent. All depends upon worm ratio. Quadruple threaded (4 start) worms can be reverse driven but friction is always high. Takes a big oil sump and lots of oil to keep them cool.
 
There probably have been few, if any, reasons to drive a worm gear by its wheel gear. However, it is stated in manufacturer's manuals that ratios below 10:1 will not stop the wheel from driving the worm.

The efficiency of a worm gear ranges from 98% for the lowest ratios to 20% for the highest ratios.

In autorotation both of the wheels (rotors) will be driving the common worm and the only function of the worm will be that of maintaining synchronization.


Dave
 
Torque balance is incomplete if the two shafts aren’t parallel.....
Click to expand...
You have a unique way of concisely explaining involved problems, Chuck. I must admit that I had been off a bit on the wrong track. Considering roll stability the biggest problem with the inter meshing helo I referred the text to the roll axis, which is probably wrong. Dave is right that the geometry of helo and airplane is quite different but I feel that this is not the whole story. I am currently working out a problem in the dynamics of a steam turbine rotor in my job and hope to gain some insight in gyroscopic effects there. The solution to the lateral dynamics might be somewhere in that direction or at least I can perhaps rule that out at the end.
 
You may have heard the name Dick DeGraw, Juergen. Dick built and flies a synchopter powered by a pair of VW Beetle engines, one running forward and one running backwards (replaced the gear driven cam drive with a chain drive - cam, distributor, oil pump run the right way; only the crankshaft runs backwards – starters run either way, depending upon which way they’re facing).

Dick said there is no problem with lateral or directional stability but he is very much aware of which way the outboard blade tips must go.

I suspect the article Dave posted was written by a journalist that may have been confused about stability issues.
 
Interesting discussion.

In fairness to the journalist, it was Dave who was confused about the stability issue, not knowing whether the subject was pitch stability or roll stability.

Chuck is absolutely correct when discussing the direction of rotor rotation. On all Intermeshing helicopters, to my knowledge, it is breaststroke (inside forward).

In addition to the torque-pitch advantage, another reason for going with breaststroke may be the improvement in the distribution of lift over the combined disk area. The rotor of a helicopter in forward flight has a lateral symmetry of moment, not a lateral symmetry of lift. The retreating blade generates its lift further out on the span.

The drawing at the top of this page Downwash - Intermeshing Disks in Forward Flight shows the distribution on an Intermeshing configuration and the thrust from the retreating blade is somewhat outboard of the thrust from the advancing blade.

________________

The above arguments are valid for Intermeshing helicopters with conventional rotors.

However, IMHO, an Intermeshing helicopter that incorporates the Advancing Blade Concept and Active Blade Twist will likely be best served by 'outside forward' rotation. The torque - pitch cross-coupling is significantly reduced because the angle between the masts is reduced from 24-degrees to 12-degrees. Also the Active Blade Twist will result in a better thrust distribution as shown in the second drawing on this page; Downwash - in Hover & Forward Flight


Dave
 
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Active Blade Twist will result in a better thrust distribution
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Dave my idea is that in a small, amateur built rotary wing aircraft active twist could best be realized using flaps in the rotor blades or tabs like Kaman uses them. Now since Kaman employs blade twist for flight control did they investigate using the tabs for active rotor blade control?
 
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Jergen,

The following is some information copied from web page http://www.unicopter.com/1375.html. It may have something of interest to you.


Dave

_______________

The Kaman flap at the tip. See below Servo Flap: ~ As of November 27, 2002, I have been unable to find any patent or Internet reference to the use of a device to control the pitch at the root and a separate device to control the pitch at the tip. Rick has mentioned that perhaps Kaman had a patent on this idea. Note that this idea entails a large amount of twist, from positive to negative.
1. Servo Flap Tip Method:

Performance of Swashplateless Ultralight Helicopter Rotor with Trailing-Edge Flaps for Primary Control ~ Have hard copy.
A Parametric Design Study for a Swashplateless Helicopter Rotor with Trailing-Edge Flaps ~ http://www.brazd.ru/books/b0005/book/58af127.pdf
There supposedly is a patent by Kaman on independent root and tip control.
Would the greater chord of a BERP type of tip provide leverage for pitching the blade?
I suspect that twisting the blade tip by servo-flap will have less authority than the torque tube method in overcoming the blade's resistance to twisting.
Related Patents:

Rick Andrew thinks that Kaman has a patent on a rotor blades control where a swashplate sets the root pitch and his servo-flap sets the 75% of span pitch. He is going to look up his copy of this and send it to me. I could not find anything relevant by doing a patent search on 'Kaman Aerospace' and by reviewing the Kaman folder. September 13, 2002

He came across the article. It was 'CTR Control System', on the Seasprite, in Aviation Week & Space Technology ~ Aug 5, 1968 ~ March 31, 2004

_________________________

Results of search on; ("active twist" OR "variable twist") AND blade AND (helicopter OR rotorcraft OR "rotary wing")

5,505,589 ~ Controllable variable twist rotor blade assembly ~ Have hard copy.

5,137,228 ~ Rotary wing with torsionally flexible blades ~ Have hard copy.

_________________________

Title: Individually Blade control by servo Flap and Blade root control - A Collaborative Research and development of programs. Author: Schimke, D. *) Author: Jaenker, P. * *) Author: Blaas, A.***) Author: Kube, R. Author: Schewe, G. Author: Kessler, CH * * * *) *)Eurocopter Germany Author: **)dasa Author: ***)zf aviation technology GmbH Author: ****)tu Braunschweig Title: Individually Blade control by servo Flap and Blade root control - A Collaborative Research and development of programs. Conference: 23rd European Rotorcraft forum, 16.-18 September 1997, Dresden Organ: Proceedings OF the 23rd European Rotorcraft forum Feature year: 1997 Key words: servo flap, rotor aerodynamics, individually blade control Language: English Type of publication: Proceeding (possibly in book form) Organizational unit: Institut for aeroelastic material
 
Jergen,

I spoke to Rick about the Kaman idea. He said that to his recollection the servo flaps were used for cyclic and collective flight control as usual.

The additional control at the root was only for collective. Its purpose was to optimize the craft to suit heavy loads or light loads.

Dave
 
Dave,
thank you for the information and the links, there is a lot of interesting stuff there which I have filed for the day when I get to designing the blades. I am sure we will have a lot to discuss then and I am glad there is someone on the list who also considers flaps/tabs a promising way of rotor control. I hope you'll continue to share your ideas.

PS: Ed of course will rally to our pennon...;-)
 
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