Constant Speed Prop Question

[aerobat 1971]

Hi There,

I've done a search for this, and can't find any reference to it, so I'll post the question here!

Can anyone explain why some constant speed prop installations have a small 'red' zone indicated somewhere in the middle of the green zone? For example - RPMs are in the Green from x to 2000 rpm, Red between 2000 and 2300, then Green again from 2300 up to the actual red line. What's going on here??

Many thanks,

David

[GotTheTshirt]

Its an engine/propellor/airframe combination that produces an harmonic vibration.
It is normally only in a transient range and so very short duration.
Continuous operation in that range would be detrimental.

[aerobat 1971]

Thanks for the info. This makes good sense to me. I was trying to think of a purely mechanical reason why a given rev range could be bad, whilst above and below were OK. The harmonic vibration would certainly be a problem!!

Cheers,

David

[pigboat]

Have a visit to the Aircraft Engine Historical Society (www.enginehistory.org/). Click on "Piston Engines" on small menu on the left of the home page and read how the engineers at Pratt & Whitney overcame the problems they initially had with the R2800 crankshaft. Fascinating reading.

[aerobat 1971]

Sounds interesting. . . I think I'll do that one from home, don't think I can get away with it at work 'cause I may be there some time!!

Cheers,

David

[barit1]

PS - The adverse harmonic vibration issue is not unique to constant speed props. I've seem similar avoidance bands for light singles with fixed pitch props.

Truth be known, some turbine engines have "critical speed" regimes where prolonged operation can be damaging. However, these are generally sub-idle, so the engine passes through the band fairly quickly during the start.

[RatherBeFlying]

The Grumman Tiger has a yellow band on the RPM gauge and the prop is fixed.

As I recall, another prop was made availble on an STC that eliminated the yellow band.

[Max Angle]

If it's a Lycoming engine it's probably because it's got a hollow crankshaft, some of them where built that way and had an "avoid" band in the rev. range due to vibrations.

As said above it's not just with constant speed props, ours were fixed metal props, it was the crank that was the problem and the vibration in the avoid band was quite prounounced.

[yachtno1]

Had something similar on the V2500 up till recently ..:ugh:

[planemad_bk]

what sort of light singles/twins would have this red band in the ranges of 2000-2300 RPM?

could someone also explain to me (like i'm a 5 year old :ok: ) what harmonic vibration is?

[barit1]

planemad_bk:

Did you ever see the movies of "galloping gertie", the suspension bridge at Tacoma WA. that oscillated so badly that it collapsed? (1940)

A Crankshaft is similar in that it is subject to vibratory excitation in what is called a "spring-mass system". When in resonance, it can break - the same way a soprano can shatter a wineglass with her voice.

[pigboat]

Barit1 you are correct about some turbine engines having a restricted RPM range. The RR Dart 529 installation in both the G1 and the Fairchild F-27 had a restricted range of between 6200 and 7200 RPM. I can't remember the reason and I'm too lazy to go look, but I believe it was something to do with the accessory gearbox drive.

[PAXboy]

planemad_bk: any movement, like twirling your keys around your finger, produces vibrations. The complications produced by having an engine + gearbox + prop all vibrating and generating more vibrations in the aircraft to which they are attached ... the designers have to try to get the vibrations to balance each other for, if they interact they can multiply the vibration and this leads to uncomfortable moments. :uhoh:

Sometimes, the vibration from the engine will resonate with another item and that leads to the kind of doubled-up vibration that is destructive. During design and testing, they can find and eliminate most of these. If, however, there is a vibration only present in a narrow bandwidth of revs, then it may be best to just avoid it, providing of course that it is not in an optimal place on the torque curve!

Many motor vehicles have a similar problem and they are tuned to be stable at all legal speeds but it is not unusual for a car to have a sweet-spot when the vibrations are minimal and the speed is maximal, particularly as the car gets older and components are worn in unpredictable ways. Then, when you find that sweet-spot, all you have to do is explain this to the traffic police. :}

[GotTheTshirt]

Some big pison engines like the big Pratts have Harmonic dampers on the crankshaft to try and reduce the vibration in the affected range. And yes it is ANY engine/propellor/airframe combination fixed pitch or CS and tubine or piston. The big radials seem more prone because of the sheer mass changing direction every few seconds !

Also it is a combination in that you can see the restriction on one installation but the same engine or same propellor in another aircraft do not have the restriction.;)

[john_tullamarine]

I wouldn't worry too much about getting into the DEs side of things unless you fancy a touch of math brain strain.

A characteristic of rotating machinery (of which all our engines, piston, jet, turboprop are examples) is a phenomenon referred to as whirling or whirl mode which is a resonance thing where the shaft does interesting gyrations and can suffer severe structural distress. While the combinations of different machinery assemblies complicate the issue, the basic phenomenon occurs with something as simple as a spinning rod.

Suggest you do a Google (or whatever search engine turns you on) for "whirl modes" and sift amongst the hits. I ran a Google search and turned up near 200 hits including helo and fixed wing references ...

Happy reading ...

[barit1]

Oh, and it may not be the crank that fails first; early P&W 9-cylinder engines occasionally induced prop blade failures. A Lockheed 10-A shed a prop blade near Waco, Texas in 12/38, and it shook the engine out of the mount. Remarkably the plane landed intact, albeit "well shaken".

The fix was in the pair of dynamic balance dampers installed in the crankshaft counterweights; one tuned to cylinder firing frequency (4.5x crankshaft speed) and the other to double that (9x).

[BEagle]

If I recall correctly, the TSR2 engine being tested under a Vulcan demonstrated a resonant mode failure in a somewhat spectacular, fiery and terminal manner in the mid-60s, destroying both it and the Vulcan in the process.

Fortunately it was on the ground at the time.

[barit1]

I have seen the results of a similar "inadvertent disassembly". There was a mild resonance that was being explored in the test cell - it should not have caused a big problem - but the turbine shaft broke at its weakest point...

...which happened to be the stress riser created by an electro-etched marking of the part number on the shaft. :}

Cut the bloomin' engine in two. :{

[Oktas8]

Going way back to the original question...

I used to fly a Socata Tobago TB10 that had a restricted rpm range for instrument approaches. Apparently this was to prevent electromagnetic interference with the glideslope antenna when flying ILS's.

The antenna was under the fuselage behind the propeller disc, needless to say.

Now is this because the two-bladed propeller at 2250rpm has a blade passing in front of the antenna 75 times a second, and one part of the glide slope signal is modulated at 2x75 = 150Hz? Wouldn't have a clue - perhaps someone else can elucidate.

Merry Christmas to all,
O8

[barit1]

2250 rpm = 37.5 rev/second, so the blade passing frequency is indeed 75 hz.

And one side of the GS is modulated at 90 hz, the other at 150 hz.

I'm not sure what to make of this though: Why should the Tobago be unique in this restriction? It should apply to any SE aircraft with the GS antenna behind a 2-bladed prop arc.

(also for a 3-blade prop at 1500 rpm, for that matter...)

:confused: