Ok guys, I have read the thread on the WC and have found that there is a tangent thread going on about quad copter props and my name kind of came up. So as to not hijack that thread, I am "taking it outside" so to speak.
I thought I would post what I have been doing and what has happened with my props that I am using so everyone can get it from the horses mouth.
First off, the prop that came up was one that I had used and it did crack right at the base of the blade at the hub. One flight it was fine and flying at 5.3 second lap times. I landed, set up and took off again and it was 6.3 second lap times. Ok, I did not change anything and since this is electric I knew something was amiss and just flew out the flight. It was a long 5 minutes and 30 seconds. When I got down I noticed one of the blades was out of line. I could also wiggle it a bit so I found what the problem was.
What caused this? Well, I fly off of a friends front yard. They are kind enough to mow a spot right next to their driveway for me to take off. Landings are a little different. I land in taller grass. I really need to move my gear foreword a bit more but have not and the plane can tend to nose plant at the very end of the landing. Now if the prop is horizontal like it normally is on an IC engine, no problem but since my Enterprise is electric the props stops where it wants too. In some cases it tends to be vertical. Not a good spot. I think I just nosed over a bit and hit the prop cracking the hub and did not know it. This is not a fault in the design of the prop. The props were never designed to be used as a brake. I have cracked IC props once in a while this way too. Not very often however but still have. I brought this prop to the NATS just to show as a conversation piece. Little did I know just how well that would turn out to be true.
In the case of Kenny Stevens losing his prop. It sounds like to me that his was fully running and just tried to be used as a plow. Again the props were never designed for such as thing. I have tried to do this with IC engine props too (again not too often, doing so really hurts your pocket book not to mention your pride). When this happened with the IC engine props, I got the same result. Maybe not at the hub but somewhere along the blade length it broke. Again not the fault of the prop because it was asked to do something it was never designed to do. No matter what prop, when it comes to prop vs. ground, ground wins.
Bob Hunt did fly my plane (thanks for the advice after words on how to trim out my plane better!) and yes he was asked to make a short take off roll. The reason for this is because I do not have a lot of ground clearance. With my tip plates on the stab it makes it hard to see if you have neutral. I learned to look at the reflection in the rudder as to what is neutral but Bob flying the plane for the first time would not know for sure. So it was better to ask for caution on the first flight especially. I like large diameter props, I really did not want a shorter one. Did that once on an official at the NATS with an IC engine. Real exciting in a bad way. Carbon prop vs. asphalt and asphalt wins every time.
I do believe these props are quite safe if taken care of. I do have a couple of things I do want to mention as to look for.
At the NATS I had a 14 inch prop on my plane. It has a smaller hub on it. This is the same size hub that the 13 inch props have on them. Or at least the ones that I have and have seen. I also think these are the same size as the 12 inch, 11 inch and probably the 10 inch props. At 10 inches these are overbuilt. 13 inches work fine. I have never had a problem and since Orestes uses them (he is the one that told me about them) I would assume he has never had a problem because something like we are discussing (shedding blades) would be disastrous and I am sure he would not use something that dangerous. Howard obviously has used them and liked them without any problems. Kenny same way unless you hit the ground with the motor running. So with all that, I would say the 13 inch props are quite safe.
Now I did have another interesting thing with one of my 14 inch props with the same size hub as the 13. I did have another get weak at the hub with both blades. It was the one I had on at the NATS. What happened was the plane slowly slowed down. I had a fair number of flights and things were holding fine. Then one day the plane went a tenth of a second slower. So I increased the RPM by a 100. Flew fine at where it was. Then after a few more flights it slowed some more. Again added a bit more RPM. Then I switched motors and noticed that I can move the blades a bit at the hub. I compared this to a new one and the one I was flying wiggled a bit more. So the blade was getting weak. The funny thing is the amount of current that I used out of the battery also went down even though I increased RPMs. Not sure why but it did. Now why did I not notice the props sooner. Simple, the weather was getting hotter and more humid. The plane (Enterprise and last years Stuka) always slowed down in hot weather and sped up in cool weather due to the density of the air. So I thought it was normal since things were heating up here in the Midwest.
So what is the moral of the story? I am thinking that the hub strength for the 13 and lower is just fine for out purposes judging by the number of flights I did, Howard has done and of coarse Orestes who probably has more flights on this size prop than anybody. 14's "COULD" be pushing the envelope just a bit. Really more testing is needed first. I would not doubt that my prop may have been used as a brake now and again. This would also explain the hub joint getting weaker on my example. I am going to talk to someone this weekend at the FCM contest who knows more about composites than I do (and it don't take much to know more than I do in that regards) about the possibility of reinforcing the blade to hub area.
What have I done to help correct this. I have gotten my hands on some 15 inch props that I cut down to 14 inch. These 15 inch props have the next size up hub. The hub is not only bigger in diameter but thicker as well. The blades at the base are also thicker giving more strength at that area. I used this prop at the WKSI contest this last weekend. Several people saw this and noticed just how beefy they seem to be. Sparky even thought you could run these on an IC engine although I don't know if I would go that far. I do think I mowed a bit a grass with it but did not use it as a plow and don't want to test that part of things either.
So, what is the final moral of the story? I feel these props are strong enough for our purposes judging by the number of flights by the people mentioned earlier. Just watch what size hub you are getting. I know there are some out there that have a very small hub. I would not recommend these. But to me the thought of just outlawing them outright as some have implied is a bad idea and way premature.
One thing I am interested in is how they are made. I would be very interested in seeing a video of the process that is used in the factories that make these if one exits.
Boy did this get long.
Matt Neumann (probably should get Sparky to fix the spelling of my last name)
Topic: Quad Copter props (Read 2788 times)
