Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Dexacare, Mar 28, 2016.
Very well said Bob.
First of all, I never thought much of this design from the beginning. But I’m a guy who likes my wings on top! I do however admire the effort, speed and transparency of the build.
Agree on the comment that CFIs seem to know little about aircraft. Had a CFI friend who killed himself in a Questair after adding a turbonormalizer and not considering the CG implications with minimum fuel. He was also a medical professional and they seem to have problems with hard limits.
However I don’t accept the arguments that Peter had to pay his dues before designing a successful aircraft. Well, all Elon Musk did to design a rocket was read a couple of books. So he should not have founded SpaceX? There are first principle people who can do good designs. The difference may be that once they get a grounded mental model, they talk to LOTS of experts and have a good filter to take good advice.
So the saga continues, Peter won’t give up and we’ll see what happens. As for the believers who lost $$, think of it as cheap education.
Yeah, the CEO and owners call the shots, not the designers or engineers usually.
The Klapmeier's had to learn the "shaft" lessons with the first design before switching to aircraft engines. No doubt some concerns of experts were ignored. But they eventually had a product.
Haha! A cheap education! Today that's unfortunately very accurate, even at $20k and probably better value!
Some life lessons you can't learn in school. 19 years ago I bought a wood fishing boat on the word of my neighbor that it was in good shape. It looked good too. Turned out to be rotten wood faired nicely with cement and epoxy. Paid off the 15k it cost and gave it away. Those lessons you remember. I survived, feeding three kids. I'm sure these guys will too. And they won't forget very quickly. Probably take a much closer look at their next investment.
Here's what Peter has to say about the flexible coupling:
A replacement for the broken Elpex has been found and ordered. It's a different product produced by Lord. Should be twice as strong and will slip instead of break if the torque max is exceeded. Shouldn't take too much effort to fit into our existing setup. Will show it in the next vid. We're back in the game!
Mat Helm the Elpex has 4 separate torque numbers, rated, max, fatigue and overload. They are 400nm, 800, 200 and 1000 respectively. My estimate is that the engine puts out max 700nm. The Elpex we chose was the one with the highest ratings. Obviously it was not enough. As with everything we needed to test it and now we have and it failed.
David Hanson no that's not the case. If it slips because of max exceeded torque it will stop slipping when the torque is reduced. Likewise the max torque of this one is 2260nm. Our engine probably maxes out at 750nm. I think that's enough margin.
[Question asking about coupling being limited to 3500rpm]
Martin Renschler actually the one I saw today is set up on a Dyno that has run to over 5000 rpm without a problem. However, it's easy enough to limit our engine to 3500rpm if necessary.
Looks to me like he's chosen a Lord LCD-0600 (edited---thanks BoKu) which has a "nominal RPM" of 2000. Lord says "Operation up to 3500 rpm is permitted with reduced misalignment."
Based on the 2260 Nm torque rating that he cites, I think it's actually the LCD-0600. It actually looks like a pretty neat unit. According to the Lord catalog it weighs in at 62 lbs.
No problem: just add a little depleted uranium in the nose ….
To be fair, we don't know what the Elpex connector weighs. So it could go either way.
Edit add--Whatever connector they get, they should probably get two of them. My bet is that this rig is gonna eat them like popcorn. Near as I can tell they've done no TV analysis on this driveline (and little rigorous analysis of any type on it), so at this point they're just throwing stuff at the wall to see what sticks. I wish they'd get Ross in there with some accelerometers to see what they're actually up against.
If the system is in resonance, peak engine torque is meaningless since we've seen stuff go 30X over this value. Secondly, as discussed previously, you should never mount the drive to an external structure not directly bolted to the engine as has been done in this case. There will be serious misalignment between the crankshaft and PSRU when the engine is developing high torque and the rubber engine mounts distort due to that reaction. Using the coupler at or above its max RPM rating is also a bad idea, especially with the high misalignment likely to be present here. There are several other problems with the drive design discussed previously too.
Anyone who thinks using a 60 lb. drive coupler on an aircraft is a good idea needs a slap in the head. The entire 300 hp PSRU on my RV weighs 54 lbs., including the flywheel and weights.
If this powerplant and drive is really going to used for production, get a professional vibe engineer involved and instrument the thing to do a proper analysis. IMO you're asking for trouble proceeding down the present path with a blindfold on and fingers crossed. This is serious stuff.
That coupler weighs more than my entire 80" aluminum propeller!
Okay, I just looked the Elpex up in the catalog. Based on Peter's description in the video comments, it's a ELPEX-S size 220 with SN rubber element and SAE #8 drive flange. The Elpex catalog shows it weighing in at 6.4 kg (14.1 lbs). So unless they can shave some more weight off of the Lord than they did by trimming the flange of the Elpex, they're in for about a 48 lb weight gain.
The empty weight for the plane will soon exceed the original gross weight projection once they add the required ballast to the nose. Time for a total rethink.
Hmmm, has he reinforced the structure, reduced the g ratings, or changed the safety factor?
My dollar is on the last option, because it was so well designed that it can take the extra weight.
Bob, is that weight gain with the nose ballast that will be needed?
Nope, that is just the pure delta between the catalog mass for the Lord coupling (62 lbs) minus the catalog mass for the Elpex (14 lbs). However, the engine is closer to the CG than the nose compartment, so the trim mass is probably less than 10% of the delta.
Haven't followed this thread very closely, but the video of the coupler failure mode was...surprising. Are there any other couplers on flying a/c that are totally dependent on the 'rubber' to couple the engine to output? Everything else I've ever seen have the 'hard' parts of the drive & driven ends overlapping, so that if the soft stuff fails, there's at least some chance that the prop will keep turning as long as the system stays away from resonance while in flight.
I've seen a couple of drives using rubber couplers with no fail safe but it's not a good idea. I'd say from his comments, Peter doesn't appreciate the possible destructive effects of TV at all nor does he think there is anything fundamentally wrong with his PSRU design.
The more I look at the fin/ rudder area on Raptor, it seems like they have far less area than something like a Velocity and more fuselage area forward. I wonder if directional stability will be adequate?
I had the same concern when I saw the Commuter Craft prototype with extended endplates on the canard.
Are you saying more fuselage area forward because it is longer or because it is wider? (or both)
I think the RC model had some problems and he had mentioned "Dutch roll".
I don't remember what Dutch roll is actually, the term isn't mentioned in Perkins and Hage stability book.
What I know is uncontrolled free flight models have larger dihedral and smaller vertical tails. Pilot controlled RC models and full size airplanes have less or no dihedral and bigger vertical tails.
I obviously don't spend a great deal of time and effort defending Peter's design skills, but I'll throw this in for consideration.
When the first Long-EZ was built, it was basically an enlarged Varieze, per Burt's design, with slightly larger wings and strakes. The winglets weren't enlarged. I have it on good authority that it was very happy to fly in one direction and point in another. Apparently, the current winglet size was arrived at by adding foam and glass in small increments by Mike and Dick, until it was interested in pointing where it was going. Hence, the current size of the winglets. The rudders, which were the absurd small square ones to start with, were soon changed to the longer, skinny ones that you see on almost all EZ's now. Having flown both kinds, I can attest to the efficacy of the higher A/R rudders . And with respect to area forward of the CG, having recently flown a Long-EZ with a VERY large extended nose, as well as minimal lower winglets, the directional stability at low speeds was not as good as the stock Long-EZ. Not dangerous, by any means, but definitely less directionally stable.
Getting this stuff right isn't trivial, and is many times a matter of tuning in flight test on a new design. Even SS2, with all the CFD and analysis done prior to flight (and there was a LOT), needed tweaking of the tail sizes and surfaces.
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