How many Rotor Revs are still safe ?

[troed@aon.at]

An interesting discussion came up last weekend concerning "are 500 Rotor-Revs still safe?"

When flying an LSA-Gyro (let´s take a standard: 450kg MTOW, Rotax 914, `840cm-Rotor with 7`chord) at maximum MTOW at Vne at approx. 7000 ft Rotor-Revs. are about 390-410 in most gyros.

When doing then a steep turn revs go easily up to 450 to 480.

Unfortunately I could not find any hint on the internet nor by the rotor blade manufs about tested safe rev-range (I searched for explicit rev-envelopes issued by rotor manufs Aircopter/France, AVERSO/France, Magni/Italy, ELA/Spain, Sportcopter/USA, Dragonwings/USA: didn´t find nothing).

There are some rumours that revs should not exceed 450 but also that people are know to have tested up to 600 revs, producing hypersonic sounds for a short time period.

So my question is: does anybody know HARD FACTS about this issue at least from one rotor manufacturer ?

Angelo

[RotoPlane]

C.B. says: ~400 fps blade tip speed is optimum. The required rrpm of a 30' rotor would be less than that of a 24' one. The rotor manufacturer would know the material and thickness used in their blades and so would know what the stress levels are at in the different rotor diameters, at different rrpms.

[Master Roda]

I think you've answered your own question to a point.

The fact is that your rotor RPM will vary consistantly depending on conditions, weight, power, and the manuevers you are performing.

Do you want a MAX number?

I'll give you one, 400 rpm's +/- 25 rpm's

I wouldnt advise flying MY blades over that simply because it's a liability.

600 is just stupid. You certantly wouldnt cruise at that rpm.

If you want a math equation talk to Chuck B.

Cruise, climb, max bank, and vertical decent should be between 300-400. This simply cannot be an exact number because of the machine and the conditions. (my short answer)

[C. Beaty]

The upper limit of rotor speed is set by compressibility if factors such as attachment strength, chordwise balance and pitching moments have been properly addressed.

Rotors will begin to suffer from the effects of compressibility at advancing blade tip speeds in excess of 700 fps.

Some military helicopters operate with advancing blade tips near mach 1 with special airfoils but there’s no need for that on a gyroplane.

A set of Dragon Wings, 23’ diameter as I recall, was run up to 600 RPM on a test stand just to make sure there were no surprises.

A set of well known fiberglass skinned blades, known to be tail heavy, became violently divergent at much lower speeds on the same test stand.

[troed@aon.at]

Well then, thanks for the sudden replies. Theoretically it´s clear to me BUT: still don´t find any hard facts on specific blades tested to their limits.

Of course there will be very little chance to exceed 450 rpms but the question still is: in certain maneuvers You could exceed 500......

Don´t wanna take a chance on having my blades break away in case ....... :rapture:

[birdy]

Angelo, for wot its worth, iv had my 30 footers up to 430 rpm, and i had to try hard to get them that high.
I dont have a rotor tac on my little machine, but i was flyn a similar one yesterday, with same power, blades [ 26'], prop, and AUW and they were go'n to 420 without even tryn. So id hate to think wot the 26'ers get up to wen im getn upset. [ 500+??]

BTW, i see you know sumthn bout necks and backs.
Want a job?? ;)

[troed@aon.at]

At least for the Aircopter Blades/France I found out now the envelope: they are tested for safe to 500 rpm. That should be enough

[MikeBoyette]

CB already told you this however, I will tell you again. Dragon Wings have been tested on a test stand in excess of 600 rpm's.

[Xavier AVERSO]

Rotor AVERSO, test in flight

http://www.youtube.com/watch?v=J1Oj16kirak

[troed@aon.at]

Thanks Mr. Averso ! Now we have:

Aircopter: tested to 500 rpm
Averso: tested to 590
Dragonwings: tested to 600

Hope the list will still be growing .............

Angelo
---------------------------------
only if You ask You will get answers ............

[birdy]

Dont forget Angelo, everythn is relitive.
A set of 30' ers at 600rpm wouldnt be very comforting.
But a set of 22'ers at 600rpm is near normal at S/L.

[C. Beaty]

The strengths of attachment fittings are relatively simple to calculate.

The centrifugal force exerted by the blade against the attachment fittings is:

CF = W x r x (rpm)²/2900

W = weight of blade, lbs.

r = distance from center of rotation to blade CG, ft.

rpm = revolutions/minute

If a blade weighs 20 lbs. and its CG is 6 ft from the center of rotation, at 400 rpm the CF exerted against the root fittings is:

20 x 6 x 400 x 400/2900 = 6621 lbs.

Federal publication ANC-5 specifies the shear strength of an AN-6 (3/8”) bolt as 8287 lbs. and normally, the bolt would have to shear in 4 places in the typical arrangement for a total of 33,148 lbs.

33,148/6621 = 5. Since CF increases as the square of rotor rpm, the rotor would have to turn the square root of 5 times as fast to shear the bolts or 894 rpm.

It’s as easy as falling off a log. Maybe easier.

There are other aspects to rotors that are more likely to kill you than flinging them asunder.

[All_In]

Wow
Talk about quality.
My brothers was just asking a helio IA for this, but he didn't know!
Thanks
Did I mention I love this site!!!!!!
John

[troed@aon.at]

Hi Beaty !

Thanks for this precise explanation and maths. Unfortunately I am only a mere handworker (surgeon) and not THAT firm with maths but I got the core idea.

For my thoughts I always refer to 8,4m rotors which are common to all dual-seater gyros here in Europe. Unfortunately we don´t have an X-gyro scene here. There is only one excellent mechanic (we call him Dr. Gyro) that is now building his gyro from the scratch.

Thanks again

Angelo

[MAX711]

Federal publication ANC-5 specifies the shear strength of an AN-6 (3/8”) bolt as 8287 lbs. and normally, the bolt would have to shear in 4 places in the typical arrangement for a total of 33,148 lbs.



Chuck, when you say shear in 4 places, do you mean the typical setup has 4 bolts or that the same bolt would have to shear in 4 places along its length? If that latter, what are the 4 places, I'm having trouble visualizing that (nothing new there :) ). Also, wouldn't more bolts help share the shear load between them?

[C. Beaty]

Picture a hub bar with attachment straps top and bottom. With 2 attachment bolts, the top strap would have to shear the bolts in 2 places as would the bottom strap. That equals 4.

The more bolts, the stronger.

[MAX711]

Picture a hub bar with attachment straps top and bottom. With 2 attachment bolts, the top strap would have to shear the bolts in 2 places as would the bottom strap. That equals 4.

The more bolts, the stronger.

Thanks, that makes sense. Presumably the bottom of the bolt (bottom attachment strap) would have the additional load of the blade trying to angle up due to coning, creating a bending moment?

[Passin' Thru]

Thanks, that makes sense. Presumably the bottom of the bolt (bottom attachment strap) would have the additional load of the blade trying to angle up due to coning, creating a bending moment?

Most hub bars have the coning angle machined in them specificaly to avoid, or at least minimize those stresses.

[MrGrey]

I have 23' Dragon Wings on a 400 lb machine and at cruise the RPM is near 370 RPM give or take and I have loaded them up regularly up to 500 RPM without consequence... apparently that is OK even though Mike Boyette told me his ol' man may be a little nervous with those numbers.... I love those blades!

[MikeBoyette]

Matt,
You sure do make them Dragon Wings talk. I really enjoy watching you fly.