rgburrill

There is a world of difference between balloons and aircraft. Just because your sponsors have done high altitude work with balloons before does not mean they understand the legal ramifications of working with drones. It's your project; it's your aircraft; it's your responsibility to make sure that drone meets the FAA requirements and your flight plan is approved.

bpbrinson

"Hemmorage of airspeed" = induced drag. There is no difference between hard turns and slow flight only airspeed. High AOA that is required to fly a delta (tailless) slowly requires lots of power. "Glide" at lower airspeeds for deltas is very steep. Wake turbulence that the pusher has to "chop" through is the reason for lower efficency. Also the other downside of pushers is that they usually have to spin a lower A/R propeller (diameter) due to interference issues with airframe structures and the ground at rotation. Loss of efficency. The beauty of electric is when you want a sample period, turn the motor off. If it is a sailplane type platform you have, glides can be well over 50:1. @Iron, Tell that to the guys that fly their sailplanes over 400mph....oh, nevermind they do blow them up. We are flying our "powered sailplanes" over 150, even 200mph without the issues you describe. Visit the "High Performance" forums on RCGroups. My deltas I designed 20 years ago, needed 2KW (glow) to get in the neighborhood of 180 and the "sailplanes" of about the same size only need 1KW to break 200. Yeah, build them out of wood, they explode. Composite is the only way to go.

Brooks

bpbrinson

RG is correct. The projects that I consulted and flew, had to be flown in restricted airspace. Controlled military base. We were also required to have a recovery parachute. The parachutes purpose was not to save the aircraft, but to stop flight in case it flew out of control and restricted airspace and caused a threat to public/ risked capture. Lets just say the biggest I test flew, took a 16 ft chute to stop flight. If we needed to use it, we would have had a crash site covered with a parachute. Current laws MAY have exceptions. Texas A&M have experience in all sizes of UAS. Ask them. Trust me, Pattern, you guys are not the 1st to do things like this, save yourself some time/money/failure and ask for help from your fellow universities. This is not a football rivalry.

Brooks
Skirting on the edge of saying too much.

IflyPATTERN

Well it must still be a feasible option since the Predator UAV used nby the milatary can reach 50K feet? I understand the "dirty air" that governs the defficiency of pusher- I dont think my setup would be horribly inefficient?

Brandon

bpbrinson

No, never said horrible, run as big and slow turning a prop as as possible. Keep it as clean and smooth as possible ahead of the propeller. I will volunteer to make power plant suggestions when you get to that point. I will need final airframe dimensions, weight, desired airspeed in flight, max alt. desired, ground and structure to centerline of shaft dimensions so I can suggest the correct propeller/gearbox/motor/esc/batteries. All will be off the shelf, (internet) equipment. But it must be final or near final numbers. This stuff is not cheap. Need to only buy once. Be sure, if going to pusher, that the dimension includes distance shaft to ground at maximum rotation angle. Always have a seperate battery and switch for flight and other onboard systems (no BEC). If things fry, you still want to see and glide. I send a nice spreadsheet. Free of charge.

Brooks

IflyPATTERN

People, can we please stay away from the legal implications of all this. Its not what this thread is meant for. I am a grown man, and therefore will make sure everything that I put my name and hard work into is legal because it is my responisibility- i understand. You can PM me with these type of concerns if need be, that is fine.

Thanks

iron eagel

Oh yea, I've learned that over the years.
The beauty of it today is that it's actually becoming affordable for hobbyist such as myself.
You have to forgive me I do tend to look at the more positive aspects of deltas with rose colored glasses, must be an age thing.
I am finding I like a blend of composites and wood as far as building theses day. Plus composite technology isn't all that cheap, well neither is balsa.

jessiej

I am sure that you are and will be very responsible. Also please be aware that "Grown Man" does not establish credentials acceptable to all authorities. (I am aware there may be exceptions in Cajun country).

jess

bpbrinson

Yep, Jessie. Junior student engineer just killed his thread with arrogance. At least I waited till after graduation before I went through my arrogant stage. I have had vast experience with all types of engineers. Bad engineers don't listen to anyone because they already know it all. Good engineers get better because they listen to everyone, and not just engineers.

I am done here.

rgburrill

Hey, the legal implications are what makes this a problem so no, leaving the legal implications out is NOT an option. We live in a SOCIETY. We play in a SOCIETY. And that SOCIETY has RULES. We don't just do things because we want to and sweep the problems under the rug. There are way too many people like you who do not want to understand that we damn near lost our right to flly because people were flying UAVs just the the military does. Do you really think you have the same influence with the FAA as the United States Air Force?

The first law of MODEL aviation is that you MUST KEEP THE AIRCRAFT IN SIGHT AT ALL TIMES!

IflyPATTERN

Wow. Just wow. If arrogance is trying to keep the thread on topic- please crucify me. This is whats wrong with society... What ever happened to the American Dream? You guys may have more experience me, but I refuse to allow this to get under my skin, as it appears to have gotten under some of others because you so quickly want to shoot down something that hasnt even started. This is just pitiful, honestly. I am requesting that this thread be taken down. All I wanted was some DESIGN advice- this is an aerodynamics forum- not a legal one.

Brandon

jessiej

I was just worried that ya'll might hit the 50 yard barrier.

jess

beepee

Not just turbulence, IE, but different air velocity off the top vs. bottom of the wing. If you ever stood by when a Beech Starship flew by, it had a very unique sound. That was the reason. Makes the propeller measurably less efficient.

Good luck,

Bedford

Von Ohain

Guys, this thread started so nicely, please don't corrupt it by TROLLING.
"This can't be done" attitude is trolling in my book, and if i were moderator I would delete such posts.

"Dirty air" for a pusher is in reality just a minor problem, and nothing to get caught up in.
It is not caused by turbulence either, as propellers is in reality very insensitive about turbulence.
(Ex. think about counter rotating propellers, or coaxial helicopters. The rear/lower propeller/rotor is constantly running up close in the turpulence of the propeller ahead, and it is practiually unaffected by it).
The thing with pushers is something called P-factor, which is a shift of vector direction and length in thrust when freestream into a propeller hits the propeller at an angle other than parallell with spin axis.
For a pusher, this angle might be different depending on where on the propeller disc you measure, because the freestream direction vector is different on, for example, on the upper side, and the lower side of a wing of which the propeller is mounted behind.
This difference in P-factor on upper and lower half of the propeller disc is causing a minor efficiency loss, and some extra audible noise.

For all practical pruposes this efficiency loss is NEGLIBLE unless you got SEVERE difference in P-factor from upper side to lower side.

(ex. think about a helicopter. When its moving forward at lets say 15 degree pitch attitude, your freestream relative to spin axis angle is 90 - 15 = 75 degrees.
But the rotor still works, and the helicopter flies. For an aircraft with pusher, you will never ever encounter p-factors as a result of such extreme freestream angles).

Long story short: Stop Whining, pusher isn't a problem! Stop derailing a great project with neglible "problems"!
Stop being surrender monkeys!

None of you guys got acces to neither the equippment nor the knowledge that the designers of any full size pusher aircrafts do, so the fact that full scales can measure a difference in efficiency doesn't mean that YOU can. and if YOU can't tell the difference, YOU can't care either.

jester_s1

I think this has been a really interesting thread. The project is ambitious no doubt, and the OP may possibly be over his head, but he's working with a professor and (I'm assuming) a competent team who will take the necessary precautions to avoid legal and safety issues. If the project fails, it will still be a learning experience for him. So let's be helpful, ok?

A thought that hasn't come up here but I've seen in other discussions on similar design goals is to use a rocket or a balloon to get up to altitude. Have you considered this? It would allow you to drastically reduce your power system to save some weight and have longer loiter times at altitude. Sure, it doesn't replicate what one would see in a full scale plane but if this is intended to just be a useful research vehicle that shouldn't matter. I could definitely see a successful design where a balloon takes the plane up to 5k feet or however far you want to go, then is released as the plane's prop starts the cruise. Of if you really want to fly up to altitude or be better in the wind, a rocket system that can be jettisoned when it's used up gives you some free altitude.

motomike

My Feedback: (4)

If already suggested, forgive me.

What about taking it aloft via weather balloon with a remote release? You could collect your samples on the way down and require much less engineering and weight in the airplane to do it.

I guess the advantage of using an airplane as opposed to taking the collection lab aloft with a balloon and bringing it back via parachute, is that you can drive it right to where you are?

Good luck

Mike

IflyPATTERN

Ok guys, thanks to some people I might just hold on to this thread. But if things get out of hand again I will simply stop posting. As you can see from my track record on RCU, I rarely post on these forums anyway bc Ive seen the shananogans that can go on on the internet... Fact of the matter is I am very interested in haearing multiple opinions so I can see everything from multiple perspectives! I have always believed two minds are better than one!

So JUST so everyone can sleep well at night: safety and being in compliance with the FAA IS VERY important to me... I just believe myself my colleagues will take on this responsibilty ourselves. In fact, my proffessor/team will be meeting shortly to discuss such safety issues so that we can be prepared and know what we can\cant do.

This leads me to another point/question. Just out of curiousity- how high can the soaring/glider guys fly without needing any sorts of permission? Because 99% of the testing will be conducted under 5K feet since it is believed 99% of airborn microbes live in this range. I read several articles online but havent got any clear-cut answers.

Brandon

da Rock

Guys, please proofread your posts. If you discover that the post is about another member and it includes derogatory adjectives, do everyone a favor and don't click OK. It'll be considered off topic for one thing.

This is a pretty good thread. Let's keep it on topic.

iron eagel

I just answered the OP question of why a pusher is considered to be less efficient than a tractor setup. Does that mean a pusher wont work in this application, of course not.

Still it is a very ambitious project with lots of technical issues that need to be addressed, as well as the paperwork needed to operate a UAV in the NAS.
I think it is interesting that they are going to the extent of a purpose built airframe to collect their samples. I suspect that the speed of a full scale airframe may damage the samples they are looking to collect and hence that is the reason for this approach.

UStik

Let's be helpful - very good idea. Seems to be not that easy though...

To throw in my 2 Eurocents, I think the general concept is a given and requested is only information helpful to put it into practice. Shoe showed on the back side of an envelope that it can be done, so we think about how it can be done.

Climbing to 15k ft and cruising there is possible with an airframe as already suggested. For me the discussion showed that a narrow (high aspect ratio) wing is most important, but not planform or streamlining. Airfoil may play a role. The airframe's overall design determines how good it climbs and at what speed.

Now the electric drive has to match it to get the high efficiency required (65%) at that speed. It has been said here several times that an as big as possible propeller should be used. That's far more important than pull/push or other design features.

There are some well-designed (basically efficient) carbon folding props, just the suitable diameter and pitch has to be found out. High efficiency at low speed requires a slow-turning prop and drive. There are some low-kv outrunners, but maybe a more conservative outrunner or even inrunner with a gear would be better. (Gear has been mentioned before as well.) There are ready-made outrunner/gear combinations (e.g. AXI).

Attached pictures show characteristics of an F3A drive by AXI. They go to show that the prop determines the drive's overall efficiency (prop, gear, motor, ESC, battery, cables, connectors). And that efficiency has a marked maximum at some speed (the light blue line in the second diagram). In this case, though, the required 65% are achieved between 27 and 46 m/s. That should be too fast for this project but that just means one has to choose/layout the drive differently. (That can't be done on the back side of an envelope, but it's not that hard, either.)

Another small but maybe important aspect comes to mind. Climb and cruise are quite different modes for the drive. To get better efficiency in both modes full-size airplanes have variable-pitch propellers. There is even an automatic variable-pitch hub for model folding propellers, as presented in the glider forum a while ago ([link=http://www.rcuniverse.com/forum/m_10815942/tm.htm]here[/link]). Might help a bit. (Dang. Seems to be no longer available.)

Last picture a typical pusher configuration, for aerial pictures in this case.

Von Ohain

Good post UStick.
I might add that full sizes also use variable pitch for much the same reason that a car uses a gearbox:
To allow the engine to spin within its useful operating range, across the whole speed envelope of the car.

Electric motors has much broader and efficient operating range than any combustion engine, and hence is the benefits of variable pitch much reduced.

Ex. Lotus Elise, the "mother car" has a 5 speed gearbox and combustion engine, while the Tesla Roadster "daughter car" has a 2 speed gearbox and electric engine, and still outperform it in a straight line.

So the question is wether or not the extra complication, weight and "source of things that can go wrong" with a variable pitch propeller is worth it, when mated with an electric motor which has much less benefit from such a propeller.
I believe this project is complicated enough as is, and that keeping things simple (or as simple as possible) is important.

UStik

I agree, but I didn't want to go into that detail.

I forgot another aspect: Flying such a thing will be much different to flying F3A, and I don't even mean the instrument flying using the control system in the FPV. It has been said as well, a stable airplane is required since even if it can be flown "IFR" it is much workload. And why use an expensive and (a bit) heavy autopilot when a stable plane flies on its own, anyway. Like in the old days, it should be trimmed for a good power-off glide and then the drive set up (downthrust) for best climb.

The plane could even have a polyhedral wing without ailerons. There are such 5 meter rudder-elevator gliders! And there's an idea to place the two halves of the inverted V-tail seperately quite far outbords on booms attached to the wings. This way they have a rudder and aileron effect at the same time. Here are pictures. Not exactly a pusher but you get the idea.

BMatthews

I'd have to bow to those that know more about how propellers are affected at lower altitude density. But consider that this is pretty much a climb then glide style flight pattern. To reach up to 5000 to 15000 feet with this craft about half or more of the flight time will be spent in the climbing. So for THIS case it is the climb phase rather than the cruise phase that will be the mode that shapes the design of the craft.

There's been some mentioning of the idea that there may already be suitable craft available from other universities or research groups. It would certainly be worthwhile looking into what they have already.

There is also a lot to recomend the idea of twin tractor motors over the idea of a single motor pusher. The tractor motors could be out on the wings where they can still have a good size propeller diameter for good climbing efficiency while keeping the nose of the fuselage clear for sampling intake openings and camera lenses. Such a configuration also gets away from the rather cumbersome twin boom layout that is needed for a larger diameter pusher prop. I know it doesn't mattr much when at the field or in the air. But at some point you need to pack the whole thing up and take it back to the storage area. It also reduces the need for additional strength in the tail surfaces since they are no longer a structural element that joins the two tail booms.

UStik

Yes, I agree, that's what I wrote in my first post. Even if I still don't know whether a marked cruise is intended at all.

opjose

RC soaring aircraft are subject to the same restrictions imposed upon other RC aircraft. Technically you are restricted to 400' above the pilot.

You are contemplating operating a UAS which has far more restrictions...

Please see this link:

http://www.faa.gov/about/initiatives/uas/uas_faq/#Qn4