B280

OTHER: Intermeshing Configuration - Concerns

A collection of all objections to the intermeshing configuration, which are known to me; plus responses.

This Web site is dedicated to technically overcoming the causes of the valid objections.

Objection:

If the Flettner was so much better, why wasn't it developed further?

Response:

There were some restrictions on German industry after WW2 (no development of aircraft, no research & development on single wings, no missile technology,....).

Perhaps the primary helicopter companies at that time were too involved with their own ideas and were unaware of the Flettner's advanced features.

Prewitt Aircraft Company, a U.S. company with no reason for bias, evaluated and conducted twenty hours of flight-testing on one of the two remaining Nazi Flettner-282s. A number of comprehensive reports were created. "The performance of the helicopter was found to be very high, ...". Prewitt recommended "That all eligible and interested helicopter organizations be permitted to have their pilots fly this helicopter."

At a subsequent American Helicopter Society meeting, Flettner presented a movie on his FL-282. Much later, the chair of this meeting mentioned that Igor Sikorsky was taken aback by what he saw on the film. This would imply that Sikorsky had not taken advantage of Prewitt's recommendation. Perhaps none of the U.S. helicopter developers ever saw, evaluated, or flew the intermeshing FL-282.
In addition, the Flettner FL-282 was equipped with a rotor-governor. However, William Hunt, Igor Sikorsky's chief designer, writes in his book "I worked out a relatively simple adaptation of the forgoing [rotor-governor] for R-5, but could not generate sufficient interest to try it out. Even to this day, I believe I am correct in so stating, that neither has anyone else." This last phrase also implies that Sikorsky was unaware, or uninterested, in the more advanced FL-282.

Igor Sikorsky was a promoter; not a graduate engineer:

His formal education: Sikorsky, at the age of 14, attended the Russian Naval Academy in St. Petersburg, but he did not finish formal studies. For several months he attended an engineering school in Paris. Then a year and a half at Kiev's Polytechnic Institute. In 1914, Saint Petersburg Polytechnic Institute awarded him an honorary degree in engineering.

A very close relationship with the armed services; ".... AHS International Awards Program, .... since 1943 (did you know that Igor Sikorsky and Colonel H. Franklin Gregory were the first recipients of the AHS Honorary Fellow Award?) ....."

Egos & Propaganda ~ Sometimes misconceptions are born out of misrepresentations (politely called marketing).

On the Igor I. Sikorsky Historical Archives, Inc. web site is a page entitled 'His Aviation firsts'. This page lists a large number of declared 'firsts', including the following at the time of the Flettner;

"1943 R-4 First mass produced helicopter."

"1943 R-4 First helicopter to land on a ship .... "

"1945 R-4 is the only helicopter to serve in World War II "

All three are wrong.

September 1942 ~ The FL-282 landed on the M.S. Greif.
"The Kolibri "Humming Bird" was the first helicopter put into mass production" [10 BEFORE 1943; plus 14 during 1943 and 1944] "and the only helicopter to make any significant contributions in World War II."

In addition, the Focke Achgelis FL-223 was also mass produced [3 BEFORE 1943; plus 20 during 1943 and 1944] and it saw limited use in World War II.

Perhaps most western rotorcraft manufactures were intimidated by aerodynamic complexities of the rotor. Instead of compounding this problem by having two rotors, they took what they thought was the easier route, and countered the rotor torque with a tail rotor. In fact, Sikorsky's first helicopter had three tail rotors for control.

It has now been shown that the early western aerodynamists underestimated the thrust efficiency of twin rotor configurations.

Objection:

If the intermeshing concept is a good one then why are not existing helicopter companies producing them?

Response:

There are many technical reasons why the single rotor should never have become the predominant configuration. However, for a non-technical perspective, here are a couple of selections from the book by William Hunt, Igor's designer and project leader for the XR-4 and XR-5.

"By the mid 1930's Europe was more advanced in helicopter development than the USA. However, the European models that enjoyed initial success were all multi-rotor types: ....... Not one successful single-main-rotor helicopter existed anywhere. It was just the type of challenge that Igor Sikorsky needed to reactivate his thoughts regarding aircraft that did not need running takeoffs and landings."

"Another of Igor's talents was his ability to 'sell' his ideas to others." Apostrophes by William Hunt.

An example from; Vertika ~ Newsletter for the American Helicopter Museum & Education Center ~ Summer 2009

The result was ungainly, clumsy and hard to control, yet the VS-300 flew tethered. ....... he prepared a movie to demonstrate its capabilities. In order to mask the inherent clumsiness, he filmed the helicopter in slow motion.

Is William Hunt saying 'The single rotor became predominant because it was a challenge to do something different, then sell like hell'?

"The entirely unexpected and unfortunate delays suffered by the prime contractor, Platt-Le Page with its XR-1 ...... suddenly thrust the Sikorsky XR-4, YR-4, R-4 into the entirely unplanned limelight." These delays were, in part, due to the deteriorating cooperation between Nazi Germany and America.

For Bell the reason might be the speculative combination of an unsupported comment and a statement.

The unsupported comment from Lu Zuckerman's had to do with the slow-motion film of Sikorsky's VS300, which portrays the pilot's cyclic movements as being slow and effortless.

The web page by Charles Lumsden contains the following statement; "1938:-- [Young] Impressed by Igor Sikorsky's film, he concentrates on main rotor/antitorque tail rotor configurations.

Charles Kaman left United Technologies and started his company with only $10, 000.00. He felt that it would be impossible to go head-to-head with his much larger former employer, so he took some of the features of the intermeshing configuration and developed his craft for a niche market.

IMHO Kellett had it right. He wanted to develop a rigid 3-bladed intermeshing helicopter. Unfortunately, the death of his test pilot, due the failure of a control linkage, apparently ended his chances of getting the one million dollars that was needed to continue this development. His own death a year later would have brought final closure to his aspirations.

"I [Glidden Doman] committed office and shop space to him [Anton Flettner ".. at a relatively advanced age"] for the construction of a unique intermesher with no hinges. When the project did not go ahead I hired his top engineer and lost contact." ~ from the Spring 2001 AHS publication of Vertiflite.

Objection:

The intermeshing configuration is slower in forward flight then the single rotor configuration.

Response:

Charles Kaman developed a helicopter specifically with low disk loading. The initial market for this helicopter was to be crop dusting.
Unfortunately, this created the incorrect belief that the intermeshing helicopter (synchropter) is a slow speed configuration.

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The following is a picture of an enclosed Flettner FL-282. This picture was taken in 1941, a year before Sikorsky came out with the 'boxy' R-4.

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Below are the specifications of the two primary production helicopters of the early 40's. Both have comparable specifications, but the intermeshing FL-282 is credited with having the higher maximum forward speed.

First Production Helicopters	Flettner FL-282B	Sikorsky R-4B
Rotor Diameter:	39 ft. - 2.75 in.	38 ft
Total Disk Area:	1292 sq.ft.	1135 sq.ft.
Gross Weight:	2202 lb.	2581 lbs.
Disk Loading:	1.704 lb./sq.ft.	2.275 lb./sq.ft.
Horsepower:	160	200
Power Loading:	13.76 lb./hp	12.9 lb./hp.
Empty Weight::	1676 lbs	2141 lbs
Payload:	529 lbs	440 lbs
Payload/HP	3.78 lb/hp	2.2 lb/hp
Maximum speed:	93 mph	75 mph

Also, the intermeshing Kellett, which was built just after the Sikorsky R-4B, had a maximum speed of 104 mph.

In the years that followed, the Russians have produced fast coaxials. In addition, a side-by-side helicopter has held the rotorcraft speed record. The intermeshing configuration fits between these two.
For the future, the respected aerodynamicist Stepniewski's preference is an ABC Synchropter, with a cruise speed of up to 260 knots.

Charles Kaman left United Technologies (Sikorsky) when he was told that they "already had one chief engineer and did not need another". It appears that his $10,000.00 bankroll was insufficient to take on the established United Technologies head-to-head, so he went for a niche market.

Stanley Hiller was also confronted with the same problem. From the Smithsonian ~ "He realized that competing with Sikorsky, and other firms that specialized in single rotor helicopters, was nearly impossible. They already enjoyed a head start in rotorcraft marketing, and they were far richer in material and financial assets than Hiller's fledgling company."

Objection:

The Kaman synchropter has a mushy feel to it.

Response:

This is due to Kaman's servo-flap control system with its large aspect ratio and flexible blades. It is not the result of the intermeshing configuration.

By Charles Kaman from [KOEY p.35-36] ~ "So here I was hung up on the realization that flaps mounted on the trailing edge of the rotor blade simply didn't produce lift and control as theory said they should,.... One Sunday morning several weeks later I was staring at the rotor when suddenly the answer came to me. I was in effect using the servo flaps as an aileron on a wing, while presuming that the wing, or in this case the rotor blade, would be stiff. By depressing the aileron or lowering it, it would increase the lift. That is how airplanes bank in turns, and how this rotor should work. But it didn't! It was obvious, therefore, that my rotor blade was not rigid. ... Well the answer was quit clear: Why fight the problem? Make the main blade so that it will deflect,"

Obviously, the utilization of 'Absolutely' Rigid Rotors and the elimination of servo flaps overcomes this problem.

Objection:

Low-speed lateral instability: "The twin lateral configuration had inherent handling difficulties that the Germans had hidden well - specifically that in crosswinds, or while maneuvering, one rotor would move through effective transitional lift while the other had not, causing a low-speed lateral instability.- [Journal of the American Helicopter Society, p. 28 & referencing [HOTH]~ In an excellent article by Roger Conner]

Response:

This comment appears to be based on [Source ~ HOTH p.130]. The craft being discussed is the Side-by-side SE 3000. This apparent difficulty will reduce as the center distance between the rotors is reduced. In other words, it will be less of a concern for the Interleaving configuration, a significantly reduced concern for the Intermeshing configuration, and of no concern to the Coaxial configuration. For more on this see; UniCopter_Stability.html - PPRuNe

Tilt-rotor Configuration: Do the V22 and other tilt-rotors experience this phenomenon? Perhaps yes, because their rotors are Side-by-side. I.e. large lateral separation.

Objection:

The intermeshing helicopter is slow in forward flight because of its high drag.

Response:

	Profile drag:	If the single rotor helicopter has the same number of blades then the profile drag will be the same.
	Induced drag:	If the single rotor helicopter has the same number of blades then the induced drag will be the same.
	Parasite drag:	Assuming similar fuselages, the parasite drag should be similiar. The difference is that the intermeshing has two rotor hubs, whereas the single rotor has one larger hub plus the drag of the side force on the vertical stabilizer, which is required to offset the torque of the single rotor.

Specific notes:

The SynchroLite is in the FAR 103 Ultralight category. Forward speed is not a concern, since it is regulated to a maximum forward velocity of only 63-mph.

The UniCopter's rotors are located in close proximity to the fuselage. This is due to the ARR/ABC concept. It is known that decreasing the distance between rotor and fuselage, plus fairing them together, results in a decrease of parasitic drag.

A high forward speed was claimed for the intermeshing SNCAC NC.2001 "Abeille". Outside Site

Objection:

Addition cost of second rotor and related components.

Response:

With a few exceptions, all helicopters have two rotors. However, there is a greater assortment of different gears and parts in a main+tail rotor helicopter then there is in an intermeshing helicopter. The commonality of parts in intermeshing helicopters results in the advantage of large production runs of identical parts. Also, the lower power required for twin main rotor configurations allows for the parts to be slightly smaller..

Objection:

Greater maintenance and risk because of extra transmission, rotor and flight controls components.

Response:

This may be partially true. However, the single rotor helicopter may actually have more gear-sets than the intermeshing configuration. It's just that some of them are smaller, because they are related to the tail rotor. The fact that they are smaller does not necessarily make them any less critical.

A picture of Flettner FL-282's compact transmission, rotor hubs and flight-controls;

Objection:

Weak directional stability and control.

Response:

The stability concern has probably been solved on the K-Max by placing the "main" rudder further aft behind the rotor disk, similar to conventional single rotor helicopters. The K-Max also has additional controllable rudders to assist, particularly during autorotation.

The control concern has validity because all coaxial and all intermeshing (to my knowledge) helicopters have used 'differential collective' for yaw control. This control reverses in autorotation. 'Opposed longitudinal cyclic' has been used on intermeshing helicopters, but its authority is limited.

There are a number of potential solutions to this problem, which do not entail 'differential collective'. They are covered on various pages on this site, such as; UniCopter - Directional

Objection:

Risk of transmission failure and subsequent loss of rotor synchronization.

Response:

This is an advantage. There are hundreds of potential failure points in a helicopter. The interconnecting final reduction is a short-coupled pair of gearsets, which are intentionally over designed.

"The H43 Husky is the only aircraft in the U.S. military to complete it's service life with no loss of life do to mechanical failure!"

"I spend just as much time worrying about this [loss of synchronization] as I do about the main rotor system flying off any tail-rotored aircraft." ~ Pilot with over 9000 totals hours, of which 3000 are K-Max and Husky.

Objection:

The risk of blade to blade contact.

Information:

re: Flettner

50 pilots were being trained to fly the Flettner FL-282. This included blind and bad weather flying, to which some accidents were attributed. One fatal accident on Nov 10, 1944 occurred when blind flying in a cloud and it is assumed that the pilot dived his machine then pulled back so violently on the controls that the rotor blades were forced into each other or the tail. Subsequently the diving speed was limited to 109 m.p.h.

"All attempts to get blade interference in flight have been unsuccessful." from US evaluation of Flettner FL-282. [Source ~ ETF p.25]

re: Kellett

The initial flights of the XR-8 showed that there was insufficient directional control. To solve this, the rotors were redesigned to give opposed longitudinal cyclic. The two rotors have three blades each and this additional flapping caused the blades of opposing rotors to occasional make contact with each other. They then produced and tested 2-blade rotors, like the Flettner, but this resulted in severe vibration. A rigid rotor system was then proposed but proved untenable because of the significant reengineering required and the unavailability of composite construction in those days.

The XR-10 was then build incorporating the lessons learned from the XR-8 but there was blade contact during an autorotation. On a subsequent test, the control system failed, killing the test pilot.

Response:

The use of only two blades per rotor has never been shown to result in blade to blade contact.

The combination of three blades per rotor and completely rigid rotors [UniCopter] will negate any chance of blade to blade contact. DESIGN: UniCopter - Rotor - Disk - Blade-to-blade Clearance

The inclusion of a fixed azimuth hub spring on the three blade CVJ+HS rotor [SynchroLite] should eliminate any chance of incorrect tip closure.

Related to this subject is the fact that the intermeshing helicopter does not need a tail rotor [CAA PAPER 2003/1 ~ Helicopter Tail Rotor Failures], which is required on a single rotor helicopter to counteract the main rotor's torque. Therefore, the intermeshing helicopter should not be subjected to the fatality rate experienced by single rotor helicopters resulting from a blade incursion into the tail boom.

Objection:

A disadvantage of synchropters is the low rotor ground clearance on the sides of the helicopter.

Response:
Flettner FL-282 "Taxiing the helicopter under its own power was strictly forbidden."

This is a valid objection for current intermeshing helicopters. The blades could be damage by contact with the ground. In addition, they could be a hazard to people on the ground, which approach the helicopter from the side. At heliports these two concerns could be overcome, but at unprotected sites this would be a concern.

The movement is toward extremely rigid rotors. Sikorsky's Advancing Blade Concept (ABC) is an example of this. The masts of the UniCopter-ABC are only 9º from vertical. The rotors have a precone of 4º and the blades are 'absolutely' rigid. This means that the blades at the side are only 5º from horizontal, plus they do not teeter, flap or have any droop.

Possible partial solutions: See Personnel on Ground

An unsatisfactory reply could be;- 1/ there is no tail rotor to contend with, and 2/ the single main rotor on very small recreational helicopters, such as the Helicycle, are also at head height.

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A larger intermeshing helicopter that incorporates the Advancing Blade Concept will not have this problem.

Two total solutions and one partial solution are given here; Rotor Concepts ~ Reducing the Obliquity

Objection:

Additional time required balancing two rotors.

Response:

Valid for the SynchroLite. The UniCopter might not require much or any balancing.

Objection:

Excessive vibration.

Response:

Problem;

This is a valid objection and it may be the primary objection. Helicopters with twin rotors will be subjected to an increased rotor induced vibration due to the aerodynamic rotor-to-rotor interaction. Higher forward speeds will necessitate higher disk loading, unless a propeller is incorporated, and a higher disk loading may intensify the vibration.

The 2-blade offset flapping hinged rotors of the Flettner may well have added to the rotor-induced vibration. See OTHER: Aerodynamics - Vibration - Rotor Induced - Analysis of Flettner FL-282 ~ Intermeshing

The note in Kaman patent 2,455,866, which was for a servo-flap on the leading edge only, seems to support this. See; OTHER: Helicopter - Outside - Intermeshing - Kaman - H-43 Huskie ~ Patents

Solutions:

The 3-blade Kellett rotors apparently produced less vibration then the 2-blade rotors. This is because the blades will cross 9 times per revolution instead of 4 times. The slightly smaller disks will rotate faster. These increases will reduce the amplitude and take the frequency above 50 Hz, which is quite a bit above the objectionable 15 Hz.

The UniCopter is to have a pusher propeller and one of the advantages of this is a reduction in the thrust of the rotors during forward flight.

Independent Root and Pitch Control plus 2P Higher Harmonic Control on the root will provide a better distribution of thrust.

Much research is presently being conducted toward reducing vibration in helicopters and those helicopters with aerodynamically interactive rotors (tandem, coaxial, intermeshing) will realizes the greatest improvements

See: OTHER: Aerodynamics - Vibration - Rotor Induced

See: SynchroLite ~ Vibration Analysis - Rotor Induced

See: UniCopter ~ Vibration.html

Objection:

Fatal accidents:

Flettner: During Prewitt testing. ~ "I believe that the Flettner crashed on one of the test hops, killing the pilot."

Kellett: One crash, which resulted in the death of the test pilot. However, it was not related to the fact that the craft was an intermeshing one.

Kaman: There are two instances of Kaman Huskies having serious accidents, one fatal, due to the failure of the transmission.

Response:

Do not know if this is above or below the industry norm. In addition, I have been unable to find anything that supports the Flettner remark. The Prewitt report of the testing does not mention any crash. In fact it states that no maintenance was required throughout the testing program.

Quote from a helicopter pilot with 9,000 hours on 10 different craft and 3 different rotor configurations; "The intermeshing system is very reliable. The H43 Husky is the only aircraft in the U.S. military to complete its service life with no loss of life due to mechanical failure!"

Objection:

The difficulty in using more than two blades on each rotor because of the blade strike problem.

Response:

If rigid rotors are used, as proposed in the UniCopter, then there is ample blade to blade clearance. In fact, Kellett wanted to build a newer version of his three-blade per rotor intermeshing helicopter, using rigid blades. Unfortunately, he was unable to raise the million dollars he wanted, because of the fatal crash of his previous version.

Objection:

There is a torque - pitch coupling, which causes the nose to lower or rise (depending on direction of rotation of the rotors) when the power is lost.

Response:

The basis for this assumption are;

a logical assessment of the configuration &
one of the Huskie models had a long 'stovepipe' exhaust, which pointed down at the tail.

Solutions:

The proposed craft on this site have reduced obliqueness and this will reduce the cross-coupling.

For additional possibilities see: 0842. OTHER: Flight Dynamics - General Cross-Coupling - Torque ~ Pitch

Last Revised: October 26, 2010