50+AirYears

In fact, for a long time, and maybe to now, the absolute model speed record of something over 240 mph, was held by an unpowered glider, a slope soarer in Australia.

DipStick

Loubub & Agcat.

Brothers what a great invention, no better person to share the sport with. Glad it worked out well for you both.

Loubud

Hi Steve.
Agcats isn't my bro but I wouldn't mind if he was. We seem to be a lot alike. I actually wish he was closer to where I am. I crash, he builds. Sounds good on paper.

DipStick

Loubud, Agecat

Sorry misread the post. I have been building planes for my brother for many years, I build he crashes, its only been the last few years that I've been learning how to crash planes. Getting pretty good at it and only cry half as long now.

I did run a few more calculation not sure if you would be interested in the graphs. In short, with the 275 watt set up I'm showing that the plane would have a climbrate of around 20 fps by going to a 8x4 or 8x5 you could gain a little in the climbrate department about 2 or 3 fps but your prop would be spinning higher RPMs, not sure if it would be worth the change. This plane should be able to climb about the same as a average 40 trainer. Did you do a static thrust test? I'm showing that you would get around 43 oz.

The other thing that is interesting is that the power requirements to maintain level flight would only be around 25 to 50 watts with the 8x6 somewhere around 6,000 to 7,000 RPM. I have flown a 2 meter glider with an .049 engine and a 6x3 prop at around 10,000 to 12,000 rpm and it took for ever to gain any attitude, climbrate was about 2 fps, ran out of fuel in about a couple of minutes, by comparison your plane rocks.

One final thought, with the pusher design and the body shape I think that staying with 8 inch prop was a really smart design decision. Besides the obvious rebuild problem, I don't believe that you would have gained any great advantage from a large folding prop. You would have needed about the same power to get the climbrate your getting and would have about the same drag penalty because the fuse on your plane, the pusher design, and the small diameter have substantially reduced the drag penalty that would be incurred from not having a folding prop. If your thermal hunting, even if the prop is fixed and not folding, the parasitic drag is really not that big of an issue because you operating at such low speeds. I could be wrong about all of this but my "calculation" and intuition would lead me to believe that you made a excellent choose based on your design and budget considerations.

Loubud

Hi Dipstick,
Actually, I think we are at the upper end of econo motors. Agcats is using a heli motor that I have some experience with. No, can't fly the heli yet but still spending...er...a...trying. From the threads that I have read re: heli motors and torque and all I chose the Align 2800KV motor. It will handle 25 amps and seems to be rugged enough for me to use it. The esc is also Align. 35 amp. One of the problems Agcats had was debri in the motor from manufacturing. It cause overheating and vibration. We also believe that it caused a failure in the esc. I sent Agcats another esc that he'll get next week. So we'll wait for the new test flight.
We both appreciate the trouble everyone went through to help us on this prop thing. I think the only thing left to do is try the different pitch props and pick the one that doesn't fry the esc or motor. It'll be fun for me and a bit nerve wracking for Agcats.
Ok. Fun all around.

AgCat1982

Way to go, Dipstick!!!!!!! Finally someone has translated my posts into English and actually understands them! I'm serious. I always thought I could put my thoughts into written words decently. Apparently I have been fooling myself!!!

I really appreciate your input. Did you happen to watch the short video of the maiden flight? It does fly and I could tell that, if I had been able to keep the throttle open, the glider was going to climb very well! The fear of burning out the ESC, causing loss of flight control, caused me to keep pulling back. The next flight will have a separate receiver battery.

As for as watts, I was using a rule of thumb of 50 watts per pound as providing a decent climb rate, assuming the prop would provide enough pull. I think you can see, in the video, that we don't need a lot of speed to stay air born. We were looking for an electric power system that would provide somewhat more than that and still be affordable and actually work under the relatively unique problems presented by this glider. Now I know that it will, if the new ESC is able to hold up o.k..

Thanks again

taylorcraft1947

agcatsbest,

What is the pitch of your propeller, and how many blades does it have? If it is an 8 pitch 2-bladed propeller, you may find that an 8x6-or-7 3-blade, or even an 8x5-or-6 4-blade might provide more thrust for the same load. This should allow you to trade some of the unusable high-speed capacity for some extra thrust in the climb.

Also, it was not that your question was unclear. Instead, people responding looked past your question and tried to give you answers that would produce the best results. From past experience and study, they were trying to give you what you "needed", not necessarily what you wanted. I have noticed that this is what many of the people that I consider good teachers try to do.

taylorcraft1947

AgCat1982

Thanks, it's just that they kept missing the point that 8 inches WAS the limit on prop length! No way around it without redesigning the glider! What I kept getting was that I needed a bigger prop with a gearbox! Some of the help was an honest attempt where others weren't reading, or understanding, my posts and were going where I couldn't go.

I appreciated those who tried to understand my real problem.

The prop I am using is an 8X6 Master Airscrew.

On the extra blades, I had read in places that it probably wouldn't give any added benefit and might be worse. Are you saying a three blade will provide more grab? I heard some guys say they put the 3 or 4 blade props on for display and put the 2 blade on to fly, on some planes.

Thanks again.

50+AirYears

Just a little sidelight on studies done on the difference between reading something on paper and reading on a screen. Might help explain some of the problems with the misunderstandings about the situation with the prop diameter limitations. Several studies have been done in this area, and every one have similar results. The people who read something on a screen show about 75% of the immediate comprehension of the subject that someone reading it on paper would, the short term retention also shows that the person who read the article on screen will retain significantly less after 5, 10 and 15 minutes than the person who read it on paper. After a day, the person who read something once on paper will have a recall of about 50 % of what was in the piece. A person who read it on screen will have about 20% recall.

It's a physiological thing, not psycological, too.

No wonder some of our fresh-outs who grew up using mostly computers have trouble understanding engineering specs and writing up understandable reports. When I started this current job 24 years ago, I was working with engineers who could do multivariable and differential calculus in conversations on a problem and come up with the answers without even using paper. Some of the young guys I work with now can't even recognize a calculation error a computer comes up with until you show through a hands on prototype that the error was there, and then because of their faith in the computer, question the real situation, not the virtual answer. Just like a lot of executives.

AgCat1982

Heard that!!!

Also, I have noticed that there are some who get into a thread reading only a couple posts before their entry. Whether or not it's intentional I don't know. I know I started to do something like that a few times when I first got on RCU, if I started at the last post. If I realized what I did, I would go back to delete the body of the post, feeling very silly![&:]

AgCat1982

In case you were posting while I was editing my last post, my last post has added comments.

Good Day!

Leslie Ward

Just read thru this whole thread, quite interesting , and reminded me of when I repowered my fishing boat. Twin screw diesel. Needed to absorb the extra horse power.
Same problem, props close to the bottom of the hull, and the only solution was higher pitch & more blades. 24x24 4blades vs. 23x17 3blades. It worked. I could stay with all the heavys ( top speed) but got way better fuel consumption.

Also have a friend who built a powered glider very similar to the design in this topic. Same dilema, engine wants to over heat. I assume because of the lack of airflow and/or excessive propload.

thanks to all , trades are backing off, time to fly

Aloha, Les