Aerodynamics and aeroelasticity

[drgondog]

Chris - BTW an arcane branch of structures was first applied to your beloved bird, the Blackhawk. I can't remember his name but the guy Bell hired (a very smart Brit) to do 'survivavbilty' on the Bell competition for the AH-60 was whicked away by Hughes but not before I learned a little bit.

The spec called for crew survivability in an autorotate crash of 2G's (IIRC) and the problem was to UNWIND classic structures of desing to a higher load limit. Simply we had to design the bottom of the ship, (and by definition the bottom of the AH-60) to elastically deform (fail) 'outside-in' and absorb the energy of the crash - protecting the crew section while screwing up the bottom of the bird.

A sloppy 'cushion' of collapsing beer cans.

I say the 'first' only because it was totally unknown as an applied structures science at Hughes, Bell, Boeing, Kaman and Sikhorsky because we had to search hard to find John ?? (CRS). Obviously the art was known in UK but not applied to helicopers.. Maybe used elsewhere but first applied in states in airframe biz at Bell and Hughes in early 1970's.. I suspect even the auto business should have been studying it

 

[DerAdlerIstGelandet]

Interesting. You worked on that project?

 

[drgondog]

Interesting. You worked on that project?

I did - on the losing side. The ship we proposed to the Army was a much bigger Huey like Utility Tactical bird - but in my opinion we lost because our ship was still 'wedded' to the two blaed rotor and we could not meet the 'reduced internal g load' below the threshold - it still bounced your ass around at cruise to high cruise.

I was part of a three man team with the chief Aerodynamicist and head of the transmission group to solve 'another way'.. slipping back to mating structures with aeroelasticity - I designed a 'flat plate' interface between the Transmission and the fuselage attach points.

If you can visualize holding a long metal ruler or scale in your hands and bouncing (like a yo yo) it, you will notice a point on either side of the force (your oscillating hand motion) there is a location where the vertical deflection in both directions is zero. so when there is an 'up' force the ruler is convex up, and with down force, convex down. the points at the end are widely deflected at max range of motion up or down - but the 'nodes' don't move.

The challenge was to build a 'plate' analogous to the ruler and replace your oscillating hand with the rotor/transmission system.

So I designed a semi plate strong enough to take and distribute the rotot/transmission loads, bounce in the middle - but attach to fuselage at the four 'neutral nodes'. result was greatly improved ride because the fusleage was attached at the calm point while the rotor/transmission was bouncing around.

We actually built one for a Jet Ranger and it was called the 'Noda Magic Ride'

Big issues were pilot control 'feel' differences and had to do funky things on the drive shaft from engine to Transmission to be able to accomodate the deflections from a fixed engine mount to a bouncing transmission.

Great solution but not too practical.

 

[DerAdlerIstGelandet]

Basically you were working on a type of vibration absorber?

 

[drgondog]

Basically you were working on a type of vibration absorber?

How about 'isolating the vibration to the rotor pylon/platform' and based on harmonics of motion, letting that system flail away, but at the 4 nodes which attached fuselage to the Rotor/Pylon/Transmission system - it was at very low vibration.

think of a Pogo stick attached to, and bouncing the tuned plate. Where the plate had a natural zero deflection point ('Node') from the pogo stick bouncing up and down on it (the thin ruler analogy) and located far from the center of the plate (or thin ruler), it was smooth as glass.

If I could draw a picture from the side - of the plate - you would see one mode of the 'ruler' as convex up, and the other mode as convex down. Superimpose those two curves to the point where they 'cross' at two points - one at each outer 'third' of the two ruler pictures?

Where they cross is the zero deflection point of the two alternate deflections from a two motion vertical force (up then down) in the middle of the ruler. In the middle of the ruler is the maximum deflection up from the 'up force', then the maximum deflection down is from the 'down force'

The analogy from the helicopter is that lift from rotors - particularly two rotors is really not a constant 'single value.. the blades tend to unload a little in a two per rev beat..so its a vertical force, then a slightly lower vertical force, then a slightly higher vertical force, etc etc (the pogo stick analogy).. and your butt in the seat is going up and down to the same beat and you feel the G force variation as 'vibration'..

And equally the reason that fatigue is such a bitch in the helicopter Biz (segue back to Aeroelasticity).. IIRC we used about only 20,000 psi for 2024-t4 as the 'yield'/allowable stress for Limit Loads.



and that is what beats the shi# out of you in a high speed run in a Huey.. and also creates the 'wop wop wop' sound that is Sooo distinctive for Bell Ships

 

[DerAdlerIstGelandet]

Very interesting! Thanks for sharing that Bill! As a rotorhead I really enjoyed reading that.

 

[drgondog]

I think I have drained the swamp in making a few points about Aeroelasticity and the complexity of the field. I feel we have navigated into the Marcel Yawn zone

It is still one of the leading edge/bleeding edge blends of Theoretical and Applied Math, Control Theory, Computational Methods, Physics, Mechanics of Materials, Aerodynamics, Airframe Structures, and yes some 'art'.

Here is a AIAA paper that very well describes the convergence of the math, the mysteries, the unexpected complexities of manned flight and the giants that led the exploration teams.

I would suggest looking at Collar's Chart published in 1946 that for the first time linked the 'Triangle of Aerodynamic, Elastic and Intertial Forces" to the design and airframe issues of the day. When High Speed aero came into play, thermal effects came into play.

It wasn't until really powerful computers were blended with sophisticated computational methods at finite element level that the START of 'predictive' capabilities started to mature.

Mods - unless there are any more comments or questions you migh want to move this to the Technical Section?