Graupner prop airborn failures

[Cherokee Flyer]

Has anyone else been having prop failures in the air with Graupner 14/7 props. Went to Hobby Lobby; they suggested I ask my questions here, so that is my question. I have had two fail in flight in the last couple of weeks on two different airplanes; both time causing considerable damage to the plane. I was using them on a Saito 100 that tacked 9150 RPM static on the ground on both airplanes. The first time I thought it might be a fluke, but they both failed in the same manner. They appear to have bubbles in the prop that had to be caused during the manufacturing process.

Might want to watch them!!

L.

[bhchan]

I would take photos of the broken props, the area of the break and send them to Graupner.

I was surprised to see HL did not want to do anything about it? It sounds to me that it is defective parts.

Brian, an EAJ

[Cherokee Flyer]

Graupner says see H/L. and they don't have a clue!!!!

L.

[Cherokee Flyer]

Pictures- this happened in the air.

[bhchan]

You are correct, that looks like manufacturing issue with bubbles (voids) in the plastic. I have use these props for electrics and no issue like this. I have prop strikes (to the ground) and they held together.

I am surprise that Graupner does not pay attention to issue like this, someone can get hurt seriously when the blades departed. H/L just a distributor/dealer.

Brian, an EAJ

[village_idiot]

If you have more of them, do you have a video camera? Might be an idea to set up the motor on a test stand with a tack on the prop and record what happens. At least this way you can prove that you aren't spinning it faster than designed and that it failed, and it won't cause a crash.

[JimDrew]

We are a distributor for Graupner's high-end products (turbines and radio systems). We also provide Graupner with their 2.4GHz technology. I will forward this thread to the proper contact at Graupner. This is not normal, and I am surprised to hear that Graupner would tell you what you have reported.

[Flyboone]

Sorry I missed this one guys. We will gladly replace the prop. Please fill out the form here http://www.hobby-lobby.com/returns.htm and we'll get you fixed up.

Thanks,
Jason Cole
Hobby Lobby

[Cherokee Flyer]

Thanks, I didn't think so either, but no one seemed to care. It wasn't like I was asking them to replace airplanes that were damaged or anything, just wanted them to know they had a problem. Have used them for years and never had this issue.

L.

[Cherokee Flyer]

Jason;
I am not sure that I now trust them nor can afford to test the replacements on more airplanes. and by the way it happened twice. Look at the pictures. Oh yes, thank Scott for the return email or call-NOT!!

L.

[aero104]

I'm glad to see Hobby-Lobby.com came through for you.

You picture appears to show defects.

As a side note, I think these graupner props come with an instruction sheet that has you estimate the forces on the prop. The instructions include the formula for determining max rpm for each prop by calculating max feet/second of the tips. I have been running these for years and never had a problem. They are a very durable prop. You might want to run the formula to make sure you are within limits.

Just my 2cents worth.

Chris

[aero104]

I found the information I mentioned earlier. Here it is:

"Please note

When used in conjunction with large and very powerful engines the ultimate load factor of SUPER NYLON propellers may be reached or even exceeded. In cases like these wooden propellers should preferably be used. The rotational speed, that is the velocity of the airscrew tip in the plane of rotation, provides some information on the limiting factor. The rotational speed must not exceed 180 m/sec. Maximum airscrew rpm are then governed by the diameter of the propeller. Max. revolutions can be calculated using the following formula: Constant (includes safe value) 3438 divide by diameter of prop (in meters). Taking a 28 cm diam. propeller the formula reads: 3438/0.28 = 12279 rpm. The actual ultimate load depends on several other factors in addition to rotational speed though, such as engine vibrations and propeller warping as a result of improper installation, to name but a few, which can affect the ultimate load factor negatively. Propellers should be mounted perfectly centered on the prop shaft and with a minimum of play. If necessary install a propeller adapter of No. 198. In order to preserve the good physical properties of SUPER NYLON propellers, in particular their strength, their water contents of approx. 2 - 3% must stay trapped in the material. For that reason they should not be stored in dry rooms or near warm heating units. A lack of water contents can be compensated either by putting the propeller in boiling water for a short while or by submerging it in water at ambient temperature for a couple of days. Be sure to abide by the safety guidelines concerning
rotating propellers! Always remain clear of the propeller arc! Do not ever use a damaged propeller!"

So for your application, here is the formula for a 14x7 prop and a static rpm of 9150.

Design Constant divided by prop length in meters=Max rpm of prop

3438/0.36 = 9550 max rpm for prop

So, since your prop most likely will increase well over a 400rpm (9550-9150=400) during flight, you are exceeding the max design loads of the propeller.

Graupner's blue sheet that is included in the box suggests you use a wood prop. I would guess that a four stroke naturally causes excessive firing pulses which is causing your props to fail at the hub. Also, if you are flying aerobatics or 3D, that would also cause hub failures. I wouldn't use the replacement prop if I were you.

I hope that helps.

Chris

[Cherokee Flyer]

Chris;

I too have been running them for some time; about 15 years. I don't know how you figure H/L came through for me, they don't even know what the problem was.

If you will note in the original post, I had tacked the engine on the ground at 9,150 RPM; you might get a big 4 stroke to increase by 1,000 RPM on the down leg in the air I.E. 10,150 RPM. Using the formula in the instructions that has come with the props since I have been buying them, they should be good to 12,250 RPM's. That give about a 15-18% safety margin.

Something was wrong with these props. I had the good luck to not ruin the bearings or destroy the engines, either one.

It is interesting to me that in more than 50 years of flying R/C aircraft this is the first time I have ever had an in-flight failure of a propeller. And then have two in a couple of weeks of the same type?

L.

[Cherokee Flyer]

Chris, your numbers are correct, I will be forwarding them to all of the fellows who fly with me and have been using that prop on 91 four strokes for the last ten years and tell them to STOP now!! After all they have been turning about 9,200 RPM static on their broke-in engines.

L.

[aero104]

Chris, your numbers are correct, I will be forwarding them to all of the fellows who fly with me and have been using that prop on 91 four strokes for the last ten years and tell them to STOP now!! After all they have been turning about 9,200 RPM static on their broke-in engines.

L.

I' m glad I could be of some help. :)

Graupner should make their data sheet a little less confusing. I modified the text above to use the division symbol "/" instead of ":". And thank goodness for online length converters to go from inches to meters. Then they use a sample formula that looks like the one for your prop. VERY confusing.

I personally like wood props on four-strokes because the wood dampens the engine pulses and is easier on the airframe. My current favorite are XOAR and 3W props.

Chris