# Rotor blade tensile strength safety margin?

#### twistair

##### Living in the Skies
I found an old article in which Jim McCutchen described their SkyWheels tests and become curious - what safety margin should be provided for rotor blade and blade-to-hub tensile strength? Next question is what is (if any) difference for such strength calculation between, say, DragonWings blade for gyroplane and for Mosquito helicopter? Some guys here measured Mosquito blade section square and immediately whined that there is not enough safety margin in tensile strength if calculated for 3.5 G load and safety factor of 1.5

#### Vance

##### Gyroplane CFI
As far as I know there have been no blade failures on the Mosquito helicopter.

Ernie built a test stand that spun the blades up much higher than they would ever go on a gyroplane or a helicopter.

I felt this demonstrated adequate tensile strength for any rotor RPM likely to be encountered.

Have you seen any signs that the blades are being overstressed on Mosquito helicopters?

#### Jean Claude

##### Junior Member
The centrifugal force in steady autorotation under 3.5 g would be 3.5 times the nominal value (i.e 1.9 time the Rrpm). And in this case, the drag of the rotor would be 3.5 times larger. The driving thrust can not reach that. Otherwise, 3.5 g can not be reached with the nominal rpm, because the blades would be already stalled . Therefore, my opinion is that 3.5 g is a very sufficient standard.

#### twistair

##### Living in the Skies
I agree, JC, that 3.5G cannot be achived in a gyro. But the main concern is about same DW blades for a Mosquito helicopters where regular tip speed is higher and blade behavior, I believe, is a bit more complex.
Some Russian designers (don't know how deep is their knowledge in fact) state that for helicopter blade spar the rule of thumb is to use tensile strength for, say, 6061-T6 not their regular 310 mPa but 6 times less - they insist that this margin is for safe simplification which includes different blade stresses deviations. Don't know how much sense in this, I'm just trying to find any rule of thumb for simplified blade stress evaluation.

#### Alan_Cheatham

##### Member
A 1.5 safety factor is too low for rotor blades as the loads are too variable to accurately apply, larger safety factors are warranted.

I found this information in a Helicycle newsletter online.

"A picture of helicopter rotor blades probably won't get very many people overly excites. However that's not the case with the folks at Eagle R&D. To us, delivery of these blades means that a whole bunch of HELICYCLE's will soon be able to take to the air. Tooling up to produce certifies quality rotor blades for less than we sold then for at Rotorway in the late 80's had kept our noses to the grindstone.

The key quality feature is the 2024-T351 aluminum spar. In days gone by, 2024-T351 spars were simply purchased as extrusions and the price was reasonable. These days, if by some miracle you could find an extruder who would sell you a critical part like a rotor blade spar, their price would give you a heart attack. It's for this reason that most of the kit helicopter manufacturers now utilize some type of composite blade.

Don't get confused, there are a few kit helicopter blade spars and a lot of gyrocopter blade spars made from aluminum in the market place. The difference is these spars are extruded from 6061-T6 aluminum alloy. The centrifugal force loads on a gyro are lower so 6061-T6 might be used in this instance, although it is not the best material. To restate, in a powered shaft rotor, we must use 2024-T351 to meet the 8/1 safety factor required at the blade root. 6061-T6 does not have a high enough tensile strength to meet this requirement, given cross-sectional area, airfoil shape, blade C.G. and overall weight considerations.

Another key reason for the high cost 2024-T351 is that unlike the 6061-T6, it is a solution heat treated material. After extruding and preliminary straightening stretch, a 2 piece die is carefully wired around each part and it is heated up to 900 degrees F before being quenched in water, It is then aged prior to delivery. The customer must continue the straightening process with special tools and press fixtures tailored to his airfoil. 6061-T6 is extruded, stretched and aged, a much simpler process.

In order or provide a high quality, yet extremely low cost aluminum rotor, Eagle R&D had to take an alternative route. We had to find a way to machine our spars right from the certified quality billet and we had to do it quickly and inexpensively. In the past, companies like Boeing Vertol, Bell Helicopters and Eurocopter have machined their rotor blade spars on huge multi-million dollar planner mills. No one in the kit helicopter business that we know of has succeeded in this process because of the time and costs involved. It's taken us months of work, but the proof of the pudding is in the picture.

The issue of quality materials is only one of a multitude on non-compromises in the HELICYCLE. If forty years of helicopter flying has taught us one thing, it's that safety has no compromise. In addition, producing high quality in low volume at low cost is not just an oxymoron, it's a whole lot of mind bending, back breaking work, We Believe the HELICYCLE is worth it. !!"

#### Jean Claude

##### Junior Member
The safety factor for aluminum alloys is mainly determined by the number of cycles of effort it will have to undergo. The maximum value that it can bear on a few cycles is irrelevant.
Force maximum or flexion test on one cycle proves nothing.
It can break with several times less, after the cycles is repet 50 millions of times https://apps.dtic.mil/dtic/tr/fulltext/u2/007610.pdf

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#### Doug Riley

##### Platinum Member
Jean Claude is exactly correct. Aluminum alloys (2024-T3 OR 6061-T6) have fatigue characteristics that are troublesome. We use these materials anyway because they are light and somewhat corrosion-resistant. But the safety factor of 8 mentioned in the Helicycle newsletter, and Jean Claude's implied safety factor of 10, are both in the correct range.

In other words, an aluminum alloy part that has a one-time strength of 60,000 psi, if subjected to constant flexing in use, must be designed around a fatigue strength of under 10,000 PSI. Either that, or the part must be rated for a limited life.

Early riveted-metal helicopter blades had useful lives of under 200 hours. Bonding helped us make blades with more reasonable lives, in that the glue distributes the loads better. On certified helos, however, bonded metal blades still have limited lives, and must be thrown away once they reach that number of hours.

Steel has friendlier fatigue characteristics. It has its own issues, though. Bensen used steel spars on his wood blades.

There's a bit of exaggeration in the Eagle newsletter, though. You can use either 2024 or 6061 in blades if you design around the appropriate fatigue strength. In fact, you can employ crepe paper if you use enough of it and distribute the load evenly.

#### Jean Claude

##### Junior Member
Regarding the blades themselves, I must add that the section of materials used does not only serve to resist tensile efforts. It is just there as ballast to decrease the conage of the seesaw rotors.

#### Jazzenjohn

##### Gold Supporter
T3 is solution heat treated and then cold worked. T6 is solution heat treated and then artificially aged because cold working doesn't improve strength. T651 is stress relieved T6 but is generally equivalent. I believe the Helicycle newsletter statement is incorrect. https://www.engineersedge.com/aluminum_tempers.htm

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