PDA

View Full Version : Do jet engines adjust pitch?


davethelimey
20th Apr 2004, 14:56
Just out of curiosity, really. Can jet engine blades adjust their pitch to take bigger/smaller bites of the air as propeller blades do?

Cheers.

Notso Fantastic
20th Apr 2004, 15:07
No. If pressure imbalances build up across the engine, strategically placed valves can open to reduce the pressure, but all they do is go faster or slower according to how much fuel you chuck in.

simfly
20th Apr 2004, 15:23
Some engines have variable inlet guide vanes (VIGV's) which change angle to keep airflow smooth through compressor/turbine (can't remember which!!), it is possible for the engine to stall/surge.

Felix Lighter
20th Apr 2004, 17:10
many turbine helicopters have VIGVs because the inlet airflow is often disrupted by the rotor wash.

the VIGVs direct the air onto the 1st stage of the compressor at the optimum angle based upon several factors such as N1 speed & acceleration, Fuel Flow, Power demand/required etc.

VIGVs don not rotate and are often hollow and fed bleed air to prevent inlet incing.

i dont know of any fixed wing aircraft with either VIGVs or variable N1 fans.

ferrydude
20th Apr 2004, 18:28
Yee Olde GE J-79, used in the F4, F-104, B-58 and other applications certainly has variable inlet guide vanes

Flight Safety
20th Apr 2004, 19:01
While I'm not aware of any engine that has variable pitch compressor blades, variable pitch stator blades have become pretty common. This started with the J79.

Here's a link to a RAND Corporation reference to military jet engine development, and the following extract comes from this reference.

jet engine development (http://www.rand.org/publications/MR/MR1596/MR1596.appb.pdf)

The move toward high-pressure-ratio engines first advocated by P&W soon confronted designers with new difficulties. As pressure ratios increased for optimal efficiency at cruise conditions, problems arose with the design of the compressor operating efficiently at low speeds and especially during acceleration. Under these conditions, airflow patterns over the compressor airfoils were very different than they were under their design conditions, and small disturbances that could cause compressor stall became common. GE made the revolutionary technological breakthroughs that solved this problem by developing variable-geometry compressor systems, which used variable-geometry stators. A row of stators redirects the airflow between each row of rotating compressor blades in the compressor assembly. Variable stators change their angle of attack for different airflow conditions, thus addressing the compressor stall problems. This technological breakthrough led to the development of the famous J79 turbojet engine, made Mach 2 flight possible, and was critical for the development of modern very-high-bypass commercial engines that power today’s large airliners.

Hawker-rider
20th Apr 2004, 19:51
the older learjets especially the one with the straight jet engine have VIGV's Also some of the straight pipe HS125 had them. Don't know about the turbo fan engines on these planes....

The Nr Fairy
20th Apr 2004, 19:56
The Allison/RR 250 fitted to JetRangers, Hughes 500 and the likes doesn't, as far as I remember, have VIGVs. What it does have is a bleed valve on the compressor section which, at start, is fully open.

As the start progresses and pressures build up along the (logical) length of the engine, the bleed valve is closed. If this didn't happen the pressures at low compressor RPMs would lead to compressor stalls. However this mechanism is totally automatic, and the only thing the pilot needs to do is ensure the valve is open before start by physically checking it.

ferrydude
21st Apr 2004, 01:37
yes, and the bleed valves on the larger 250 series tend to be quite useless to engine operation and causing unecessary maintenance. Many lower elevation operators (GOM) remove them and limit the aircraft altitude to 3000 ft max

lomapaseo
21st Apr 2004, 02:56
P&W had a ducted prop (geared fan) on the drawing board in the late 80's with variable pitch, including reversible fan blades.

Dan Winterland
21st Apr 2004, 08:15
Variable IGVs and stators are common on larger turbofans. Engines which use this sytem are the P&W JT9, the GE CF6 and the RR Pegasus as fitted in the Harrier.

davethelimey
21st Apr 2004, 10:55
Another example of why I come here all the time. It's addictive I tells ya.

Thanks folks. :p

WJman
21st Apr 2004, 20:22
As someone stated the larger Turbo fan engines have VSV's (variable stator vanes) and VBV's (variable bleed valves) to port air into or unload the compressor. In older engines such as the CF6-50 theses were controlled by the MEC, and moved according to fuel pressure, with lines coming of the MEC(main engine control) to actuators. Today on newer engines the MEC has been replaced by a HMU (hyro mechanical unit), but info to move the vsv's and vbv's is now electronic inputs. The thing I found really interesting was rigging the VSV's and VBV's on the old CF6-50, the first time I participated in it I was amazed at the precision with which it could be rigged. I have only changed one HMU and no rigging was required, I think it's all done automatically via the EEC and the HMU. Has anyone had to rig something like a CFM-56?

Fragman88
22nd Apr 2004, 03:01
Hi Dave

The RB211 (747) has Variable Inlet Guide Vanes just behind the big Fan at the front, to guide the airflow smoothly into the rest of all the whizzing bits further downsteam.

The best and most impressive feature of this is the VIGV controller, a pressure driven device that works really well except when pushed a little i.e. Typhoon conditions, big crosswinds etc, combined with large throttle movement. It can then deliver the most spectacular results, throwing a doughnut shaped fireball out of the intake beyond the nose of the A/C with accompanying sound effects that make one wish for a quiet lightning strike!

Fortunately the bark is much worse than the bite, and I've only had one run down after the stall/surge, and even then, no damage----Thanks to all the chaps at RR, you build them well.
:ok:

DDG
22nd Apr 2004, 04:11
Hi WJman,

I have changed HMU`s on CFM56-7 fitted to B737NG and found they are so much easier than on the classic.Nil rigging involved,testing involves an
1)actuators test through the EEC
2)dry motor
3)idle power leak check
4)engine ground run EEC bite test.

The HMU is removed with the fuel pump package(quite heavy) due to access to some parts is restricted unless you have some "special wrenches".


The classic (CFM56-3) was quite involved and easily takes double the time due to the extensive rigging/testing involved,
VSV feedback cable rigging
VBV feedback cable rigging
Thrust lever cable rigging
Start lever cable rigging
PMC Adjustment
Specific Gravity Check
Low idle power adjustment
High idle power adjustment
Mec trim ( engine run)
Accel/deccel check(egr)
Power assurance (engine run)


Give my the NG any DAY!

chuks
22nd Apr 2004, 08:15
Even some of the modern, smaller engines have VIGV, such as the PW306B used on the Fairchild Dornier Bankrupt Bust and Gone but Coming Back Anytime from Now 328Jet.

It seems a bit strange to have no physical connection between throttle and engine but just a position sensor that talks to the FADEC to arrive at a mutually agreeable compromise. It works just fine, though, which is all I really care about. Nice, smooth acceleration and care-free handling. Not like the horror stories about early turboprops melting down when the pilot forgets to go to 'ground fine' after landing or asks for a little too much power during taxi.

WJman
22nd Apr 2004, 15:43
posted 22nd April 2004 03:11
--------------------------------------------------------------------------------
cfm56-7 hmu changes
Hi WJman,

I have changed HMU`s on CFM56-7 fitted to B737NG and found they are so much easier than on the classic.Nil rigging involved,testing involves an
1)actuators test through the EEC
2)dry motor
3)idle power leak check
4)engine ground run EEC bite test.

The HMU is removed with the fuel pump package(quite heavy) due to access to some parts is restricted unless you have some "special wrenches".


The classic (CFM56-3) was quite involved and easily takes double the time due to the extensive rigging/testing involved,
VSV feedback cable rigging
VBV feedback cable rigging
Thrust lever cable rigging
Start lever cable rigging
PMC Adjustment
Specific Gravity Check
Low idle power adjustment
High idle power adjustment
Mec trim ( engine run)
Accel/deccel check(egr)
Power assurance (engine run)


Give my the NG any DAY!

Thanks for the info, I juts started working on the NG's -700. The course was good but I am a hands on type of guy. I learn more through chasing snags and changing parts. The only problem is that the NG-700's never seem to break, and when they have it has been easy to diagnose and fix. Don't get me wrong it's great to have a good a/c, it's goo fro the company, OTP and all that stuff, but sometimes a good snag on an overnighter makes the shift go by quicker and helps me learn the a/c quicker. Thanks again.

Basil
25th Apr 2004, 21:19
Another bit to take into account is that whilst the RR RB211-524 series have two actuators on the VIGVs, the RB211-22B only has one - which sometimes was a little slow to, er, actuate with fairly predictable results :eek:

John Green
25th Apr 2004, 22:31
Incase you guys didnt know, Honda with there new jet engine ranges have been developing variable pitch on the compressor blades as well as the Turbine, to improve efficency even more over VIGVs. being tested at the moment on the Honda Jet. Its a bit like VTEC for jets.:ok:

TURIN
27th Apr 2004, 11:07
The RR concept of having a triple spool layout on it's bigger engines also negated the need for more complex VSVs throughout the LP/IP compressor such as needed on PW JT9 and GE CF6.
Having said that, with the advent of the Very Large Engines such as the Trent, RR felt the need to add VSVs & VBVs to the design to get the required results.

The joke going around RR when P & W said they were going to build a geared fan engine was that it would be 'a two and a half spool engine'. (you had to be there I suppose :O )

As for a variable pitch fan.... I think the complexity of this would negate any efficiency gains. The closest anyone came I suppose was the Unducted fan contra rotating efforts of the 80s.

What ever happend to them? Last I heard was the Russians were fitting them to a big 4 eng transport.

:ok: