Homebuilt Coaxial Helicopter (RELOADED)

quadrirotor

André MARTIN
Be serious, Dave!...And in the coaxial topic!...
Here you have a new variation of the Sikorsky project which converges to the GYROCOAX, as you could have a vectorizing tail...It seems the vectorizing tail is a solution for simplification!...
 

Attachments

Rotor Rooter

Dave Jackson
Andre, OK but if we are going to get serious I've got to vote against the Coaxial configuration.


For homebuilt rotorcraft; the thrust-to-power ratio is a serious concern, particularly when using 2-stroke engines. The coaxial has a significantly inferior thrust-to-power ratio in comparison to bilateral twin main rotor configurations.


For fast future craft; I suspect that the coaxial, such as Sikorsky's X2 may have a terminal problem. The earlier Sikorsky S-69 ABC was unable to achieve the top speed objective, due to excessive vibration. It had provisions for vibration damping devices but for some reason they never added them.

The following is copied from PPRuNe. It may explain the reason for this problem and an inability for the coaxial configuration to excape it.
"Consider the side of the craft coaxial ABC] where the upper blades are advancing and the lower blades are retreating, during cruise. The use of ABC means that the upper (advancing) blades are providing most of the thrust. The lower (retreating) blades are meeting the airflow with a sharp leading edge.

We know that a sharp leading edge can only operate within a small range of AOA. We also know that there is an amount of time required for the sectors of an airfoil to recover from a stall. In addition, we know that the lower blades on this side of the craft are passing in and out of the thrust of the upper blades 8 times per rotor revolution.

IMHO, it is reasonable to assume that segments of the lower retreating blades will passing in and out of stall, at rates of up to 8 times per RRPM."


The bilateral twin main rotor configurations should not be subjected to this specific concern.


Dave
 

quadrirotor

André MARTIN
Andre, OK but if we are going to get serious I've got to vote against the Coaxial configuration.


For homebuilt rotorcraft; the thrust-to-power ratio is a serious concern, particularly when using 2-stroke engines (not mandatory). The coaxial has a significantly inferior thrust-to-power ratio in comparison to bilateral twin main rotor configurations. For private purposes, at 100 mph, it's not a problem...

For fast future craft (that's out of concern!!!); I suspect that the coaxial, such as Sikorsky's X2 may have a terminal problem (AMEN!). The earlier Sikorsky S-69 ABC was unable to achieve the top speed objective, due to excessive vibration. It had provisions for vibration damping devices but for some reason they never added them.

The following is copied from PPRuNe. It may explain the reason for this problem and an inability for the coaxial configuration to excape it.
"Consider the side of the craft coaxial ABC] where the upper blades are advancing and the lower blades are retreating, during cruise. The use of ABC means that the upper (advancing) blades are providing most of the thrust. The lower (retreating) blades are meeting the airflow with a sharp leading edge.(????)

We know that a sharp leading edge can only operate within a small range of AOA. We also know that there is an amount of time required for the sectors of an airfoil to recover from a stall. In addition, we know that the lower blades on this side of the craft are passing in and out of the thrust of the upper blades 8 times per rotor revolution (FALSE, at cruise, no blade passes in the downwash of the upper blades!!!!).

IMHO, it is reasonable to assume that segments of the lower retreating blades will passing in and out of stall, at rates of up to 8 times per RRPM."


The bilateral twin main rotor configurations should not be subjected to this specific concern.(for homebuilding purposes: too complex, too large, not compact enough, too much dead weight, too much airframe blown during take off, etc...and above all: i don't like side by side rotors, except synchropters...)

Dave
Once again, this forum is mainly peacefull homebuilding oriented, not high speed warrior rotorcraft... You certainly saw the video on the GYRODYNE, this craft with two tandem seats: it's all we need!!!:violin: :der:
The best, of course, would be the ROTORFLY...But a little expensive for the moment!...
 
Last edited:
I agree, we should be able to use the Gyrodyne drive and control system on a modern fuselage with two seats. This should be easy to do.

Maybe we should get together to design and build a few. :D
 

Rotor Rooter

Dave Jackson
André

Homebuilt coaxial only.
The "high speed warrior rotorcraft" can be discussed; if someone wishes to.


The importance of Power Loading

The following sketch, from this web page, show two extreme configurations, that of the coaxial and the side-by-side. Prouty's calculations show that 50% more horsepower is required to hover a coaxial helicopter than side-by-side helicopter with the same specifications.



There are two basic solutions to overcome this difference;
  1. Install a much larger engine, power-train and rotors etc. in the coaxial. However, this will add even more weight, which in turn requires an even large engine, power-train and rotors etc. 50% more.
  2. Use a very light and less reliable engine and power-train etc. in the coaxial. However, this will involve risk.
How much risk does a person want to take?
~ the Robinson uses a 4-stroke Lycoming engine, which is then derated.
~ the Original Helicycle and the Mini-500 used 2-stroke Rotax engines, which were then given tuned exhausts to extract more power out of these lightweight engines.


You mentioned;
too complex, too large, not compact enough, too much dead weight, too much airframe blown during take off, etc.
A side-by-side helicopter with an empty weight that is nearly half that of a comparable coaxial means that many of your concerns are, or can easily be, eliminated.


Dave
 
Last edited:

quadrirotor

André MARTIN
Sorry Dave, this thread deals with homebuilt coaxial helicopters...I opened this thread for you:
http://www.rotaryforum.com/forum/showthread.php?t=14667

You could expose the advantages of the side-by-side rotors...
Most of the time you bring complications, never simplications or a technological solution...

Many people seems aside of their shoes!... But, I think that most of the time, they are dysinformation agents!...

That's what we need, with a pretty aerodynamic shape! The Wagner Skytrac or the GYROCOAX!...
 

Attachments

Last edited:

Rotor Rooter

Dave Jackson
André
Sorry Dave, this thread deals with homebuilt coaxial helicopters
I believe that my remarks and information are directed at the coaxial configuration. They are not the promotion of any specific configuration.

My 'mode de opération' is to look for the technical shortcomings of ALL configurations and be aware of them early, at at the stage of the 'notepad' not the 'helipad'.


Dave
 

Bruno

Member
Yes, an amazing device built by a certain Dr. Schoffmann of Germany. (Hope I have not misspelled his name.) He rarely manages to get out of ground effect, but I do find it amazing that his device flies at all.

Here's a picture.
 

Attachments

Sita

Guest
"out of ground effect" is roughly 1 1/2 times rotor diameter.With only 2.13 meters diameter,Mr. Schöffman flies regularly out of ground effect.
I have seen him reaching a height of about 10 meters last year in Wels (Austria).


Cita
 

Bruno

Member
"out of ground effect" is roughly 1 1/2 times rotor diameter.With only 2.13 meters diameter,Mr. Schöffman flies regularly out of ground effect.
I have seen him reaching a height of about 10 meters last year in Wels (Austria).


Cita
Cita, I respect you because of your impressive background, and do not mean to contradict you. However, all the photos and videos I have seen show Mr. Schöffman hovering just a few feet above ground -- less than his own body height. Do you have better pictures or videos?
 

Sita

Guest
Bruno,

Thanks for the flowers!:D

"Herr Schöffman" would be flying "out of ground effect" theoretically speaking when his feet would be roughly 1.2 m above the ground.
Even this short clip shows Mr. Schöffman a few times higher than this.
I agree though that even the 30+ hp he's using seems marginal with the extreme small rotor diameter and I guess that the 10m altitude I saw last year is probably the best he can do under near perfect conditions.
I had the chance to examine his little helo and the simplicity in both design and construction is beyond believe.
Although very sencitive (spelling?) I'm surprised at how well he's able to control the craft,after all 2.13 m diameter is VERY small,even for a UL helo.

Cita
 

Bruno

Member
Bruno,

Thanks for the flowers!:D

"Herr Schöffman" would be flying "out of ground effect" theoretically speaking when his feet would be roughly 1.2 m above the ground.
Even this short clip shows Mr. Schöffman a few times higher than this.
I agree though that even the 30+ hp he's using seems marginal with the extreme small rotor diameter and I guess that the 10m altitude I saw last year is probably the best he can do under near perfect conditions.
I had the chance to examine his little helo and the simplicity in both design and construction is beyond believe.
Although very sencitive (spelling?) I'm surprised at how well he's able to control the craft,after all 2.13 m diameter is VERY small,even for a UL helo.

Cita
Having observed it at close range, do you think it could be improved to a notable degree?
 
Top