S97 Raider
Join Date: Oct 2002
Location: 1 Dunghill Mansions, Putney
Posts: 1,797
Likes: 0
Received 1 Like
on
1 Post
Originally Posted by SansAnhedral
Well for JHL, the program office actually had settled on a (quad) tiltrotor solution only
- Bell's QTR (with 55 ft diameter rotors)
- A conventional tiltrotor (CTR) that Bell had additionally been funded to study in parallel with QTR (with 80 ft diameter rotors)
- Karem's OSTR (specifically the TR75, with 75 ft diameter rotors). As you may recall, the Army's "representative" JHL configuration used for wargaming purposes from 2007 on – the High Efficiency Tilt Rotor (HETR) – was very reminiscent of the OSTR.
I/C
Not quite true. Prior to the JHL effort being folded into the JFTL AoA (which included various fixed-wing STOL and V/STOL designs) in late 2010, the concept definition work had been narrowed down to three candidate configurations:
- Bell's QTR (with 55 ft diameter rotors)
- A conventional tiltrotor (CTR) that Bell had additionally been funded to study in parallel with QTR (with 80 ft diameter rotors)
- Karem's OSTR (specifically the TR75, with 75 ft diameter rotors). As you may recall, the Army's "representative" JHL configuration used for wargaming purposes from 2007 on – the High Efficiency Tilt Rotor (HETR) – was very reminiscent of the OSTR.
TR75 relies on some very dubious tuning claims, in addition to extremely high risk, high frequency individual blade control.
Loads, radius, airfoil thickness, blade stiffness, and tip deflection all change so that the blades are strained the same. I don't think that new materials are required as gross weights increase, though I can certainly imagine that specific pitch bearing joint technologies work well at certain size ranges and may practically constrain the hub design at some point.
@etudiant
We don't really have a firm grasp even on conventional rotor systems engineering
(consider this lupine one confused).
[QUOTE=Lonewolf_50;9755171]What about a bearingless rotor?
A 'bearingless rotor' is rarely totally bearingless, but 'reduced bearing count rotor' doesn't sound as sexy. Same comment applies... there are specific bearing types that are well suited to particular sized components based on loads and motions. It's a matter of seeing which ones package reasonably into your specific hub needs. As far as the flexbeam portion of a bearingless design, my comment on strain levels being relatively constant with rotor diameter still applies. In practice, of course, there are plenty of other design considerations that make a bearingless rotor more or less appropriate for a particular application.
A 'bearingless rotor' is rarely totally bearingless, but 'reduced bearing count rotor' doesn't sound as sexy. Same comment applies... there are specific bearing types that are well suited to particular sized components based on loads and motions. It's a matter of seeing which ones package reasonably into your specific hub needs. As far as the flexbeam portion of a bearingless design, my comment on strain levels being relatively constant with rotor diameter still applies. In practice, of course, there are plenty of other design considerations that make a bearingless rotor more or less appropriate for a particular application.
It's a matter of seeing which ones package reasonably into your specific hub needs. As far as the flexbeam portion of a bearingless design, my comment on strain levels being relatively constant with rotor diameter still applies.
A quick example....Sikorsky had a design problem with the Rotor Blade Retention design on the S-76A.....despite the Ira best efforts.....they had two failures.
We might also add the Gear Box problems on the EC-225....the Engineers thought they had that sorted out as well but discovered they did not.
Read the latest Report about that in the 225 Thread.
Then say we bring up square windows on the Comet.
The Titanic was unsinkable.
The F-117 Stealth was invisible....till one got shot down and a second one was almo st lost to a pair of SAM's.
The list goes on at great length.
Reality always trumps theory!
We might also add the Gear Box problems on the EC-225....the Engineers thought they had that sorted out as well but discovered they did not.
Read the latest Report about that in the 225 Thread.
Then say we bring up square windows on the Comet.
The Titanic was unsinkable.
The F-117 Stealth was invisible....till one got shot down and a second one was almo st lost to a pair of SAM's.
The list goes on at great length.
Reality always trumps theory!
Last edited by SASless; 28th Apr 2017 at 18:59.
Spline claims that:
Exactly how do you propose you achieve stiffness for the same strain within a linearly scaled aero envelope without increasing structure and in turn weight?
An increase in size by a factor of 2 would require a quadratic increase in stiffness (EI), as your material modulus is invariant in our example case. Greater stiffness with the same materials requires more material, and results in larger weight and therefore larger loads. This is a snowball effect. Mach scaling a rotor radius 200% is a cubic function for mass, square function for loads.
A blanket statement like:
in light of the immense structural changes required by scaling up is dubious. I've seen it time and time again on trade studies of rigid rotors - from coax to tiltrotor.
In any case, the added mass has a cascading effect on mast moments and rotor hub structure also increasing the sizing in the non-blade rotating system as well.
If rigid rotors in general scaled as easily as you suggest here, then existing conventional medium and heavy helicopters would already implement that rotor system as it offers significantly better control power and a far less complex and costly rotor head assembly. Sikorsky also would not have candidly admitted that X2 technology does not scale beyond the medium class in 2011
Edit: The link is now dead, I wonder why? Alt reference
Exactly how do you propose you achieve stiffness for the same strain within a linearly scaled aero envelope without increasing structure and in turn weight?
An increase in size by a factor of 2 would require a quadratic increase in stiffness (EI), as your material modulus is invariant in our example case. Greater stiffness with the same materials requires more material, and results in larger weight and therefore larger loads. This is a snowball effect. Mach scaling a rotor radius 200% is a cubic function for mass, square function for loads.
A blanket statement like:
in light of the immense structural changes required by scaling up is dubious. I've seen it time and time again on trade studies of rigid rotors - from coax to tiltrotor.
In any case, the added mass has a cascading effect on mast moments and rotor hub structure also increasing the sizing in the non-blade rotating system as well.
If rigid rotors in general scaled as easily as you suggest here, then existing conventional medium and heavy helicopters would already implement that rotor system as it offers significantly better control power and a far less complex and costly rotor head assembly. Sikorsky also would not have candidly admitted that X2 technology does not scale beyond the medium class in 2011
Edit: The link is now dead, I wonder why? Alt reference
My previous comments on strains is also true for articlated and rigid rotors and it's why I've been able to use the same materials for all of my rotor designs despite an order of magnitude range in their diameters.
The reason we don't see more rigid rotors is as you alluded... they do weigh more. The flapping hinge, despite the mechanical complexity, is a wonderful elegant invention. A rigid rotor produces high vibration, huge moments, and not always in helpful directions. They're twitchy, high gain beasts. Do they have their applications? I believe yes, but certainly not for every helicopter design.
I was under the impression that the AH-1Z/UH-1Y is a true bearingless design, and it is successful. However, I suspect that if one wished to scale that up the design team would encounter, and have to account for, the loads and other issues you pointed to.
OK, thanks, appreciate the insight.
OK, thanks, appreciate the insight.
Sans, when you double the rotor radius and hold solidity, blade number, and airfoil family constant, you get a fourth order increase in cross section stiffness as part of that scaling. Mass per unit length is a second order scaling which leads to natural frequencies being inversely linear with the radius doubling, which is good, because so does the rotor rpm to hold tip speed constant. That also means that fan plots tend to look similar for each type of rotor, somewhat regardless of diameter. Of course, specific design details like abrasion strip thickness and other details don't scale linearly with rotor radius, so there is some variation in resulting fan plot, but the principle holds. This is true for articulated and rigid rotors.
Even with bleeding edge UHM materials the weights still increase to a point where hub attachment and inboard structure gets to be the root (no pun intended) of the challenge for a high speed flying vehicle, not to mention the potential ILS issues that are readily apparent with very thick laminates making use of such a high percentage of stiff materials attempting to keep weight down.
Incorrect Referenced 'By' Line
Originally Posted by Lonewolf_50 
I was under the impression that the AH-1Z/UH-1Y is a true bearingless design, and it is successful. However, I suspect that if one wished to scale that up the design team would encounter, and have to account for, the loads and other issues you pointed to.
OK, thanks, appreciate the insight.
Originally Posted by Spline Drive:
There is typically an inboard shear bearing between the flexbeam and inboard end of the blade. Often a lag damper is installed there as well. The bearing transmits shears, allows for blade pitch and has some misalignment capability to handle flap and lag motions.
The successful bearingless rotor employed on the brother/sister duo Zulu/Yankee does indeed have a small elastomeric shear bearing at the inboard end of the cuff. It accommodates relative pitch motion between the torsionally rigid cuff and the yoke and reacts shear loads at that point as well. As I understand it, the name 'bearingless' is used to indicate the larger bearings used to accommodate flapping, lead/lag and the large C.F. loaded feathering bearing have all been replaced with fiberglass (composite) flexing virtual hinges. And the center of the yokes (blade station 0.0) are connected rigidly to the mast, eliminating the two bladed mast bumping (teetering rotor) flaw and delivering a significant amount of hub moment (control power) to the mast. Otter
I was under the impression that the AH-1Z/UH-1Y is a true bearingless design, and it is successful. However, I suspect that if one wished to scale that up the design team would encounter, and have to account for, the loads and other issues you pointed to. OK, thanks, appreciate the insight.
Originally Posted by Spline Drive:
There is typically an inboard shear bearing between the flexbeam and inboard end of the blade. Often a lag damper is installed there as well. The bearing transmits shears, allows for blade pitch and has some misalignment capability to handle flap and lag motions.
The successful bearingless rotor employed on the brother/sister duo Zulu/Yankee does indeed have a small elastomeric shear bearing at the inboard end of the cuff. It accommodates relative pitch motion between the torsionally rigid cuff and the yoke and reacts shear loads at that point as well. As I understand it, the name 'bearingless' is used to indicate the larger bearings used to accommodate flapping, lead/lag and the large C.F. loaded feathering bearing have all been replaced with fiberglass (composite) flexing virtual hinges. And the center of the yokes (blade station 0.0) are connected rigidly to the mast, eliminating the two bladed mast bumping (teetering rotor) flaw and delivering a significant amount of hub moment (control power) to the mast. Otter
Last edited by Otterotor; 28th Apr 2017 at 18:23. Reason: Correct the 'by' line.
Join Date: Sep 2007
Location: USA
Posts: 601
Likes: 0
Received 0 Likes
on
0 Posts
"Bearingless" rotors still have some arrangement that permits adjustment of the blade pitch. Usually a torsionally flexible attachment structure or a laminated elastomeric bearing. However, they do not provide adequate stiffness for rigid rotor systems. Look at the Sikorsky rigid rotor hub designs that use rolling element blade feather bearings.
"Bearingless" rotors still have some arrangement that permits adjustment of the blade pitch. Usually a torsionally flexible attachment structure or a laminated elastomeric bearing. However, they do not provide adequate stiffness for rigid rotor systems. Look at the Sikorsky rigid rotor hub designs that use rolling element blade feather bearings.
Sikorsky Archives | X-Wing
If you browse around on Google patents, it's clear Sikorsky continues to examine atypical applications of flexbeam type rotors, including for coaxial aircraft. In fact, if you examine good photos of the S-97, the inboard half of the blade looks proportionally thicker than the X-2 does, and early ground run photos with the hub fairings off show the blade composite wrapping around the outside of the hub structure. They haven't said as much (to my recollection), but Raider might well be a 'rigid' flexbeam rotor, at least flexible in the torsion direction.
Spline wrote:
How did that workout for them?
The Sikorsky X-Wing used a flexbeam with a torisionally soft I cross section but was stiff in the flap and lag directions.
AH56 Documentary
This is relevant to the discussion.
https://www.youtube.com/watch?v=ycXEgIRWGqs
Interesting. It appears superior to the 97 in every way, did not take 2 years to get out of hover, and scrapped in early 70's.
https://www.youtube.com/watch?v=ycXEgIRWGqs
Interesting. It appears superior to the 97 in every way, did not take 2 years to get out of hover, and scrapped in early 70's.
Having been involved with X2 and seen S97 blade construction, I can say they are quite similar, despite one being detail designed by a supplier and the other being "build to print" by the same folks.
This is relevant to the discussion.
https://www.youtube.com/watch?v=ycXEgIRWGqs
Interesting. It appears superior to the 97 in every way, did not take 2 years to get out of hover, and scrapped in early 70's.
https://www.youtube.com/watch?v=ycXEgIRWGqs
Interesting. It appears superior to the 97 in every way, did not take 2 years to get out of hover, and scrapped in early 70's.
Solving the rotor stability problem proved more difficult than expected.
The 525 is doomed then I assume....as is the 609 and the Tilt Rotor based upon prototype crashes.
Now of course the Sultan ignores politics and places a focus on just one problem.
It was politics that was fatal to the Cheyenne....not aerodynamics!
He might read his history a bit better!
Now of course the Sultan ignores politics and places a focus on just one problem.
It was politics that was fatal to the Cheyenne....not aerodynamics!
He might read his history a bit better!
It absolutely is, and the flexbeams are what I believe have either failed or been giving them issues hence some rumored new builds. Rumor also has it that next month is supposed to finally be higher speed testing, so it would make sense they would want new components.
Having been involved with X2 and seen S97 blade construction, I can say they are quite similar, despite one being detail designed by a supplier and the other being "build to print" by the same folks.
Having been involved with X2 and seen S97 blade construction, I can say they are quite similar, despite one being detail designed by a supplier and the other being "build to print" by the same folks.
