Replies to This Discussion

Permalink Reply by Doug Weibel on November 20, 2009 at 7:16am

Lots of disagreement with your assumptions. I do have some experience in this area by the way. I am a full scale glider pilot and have made high altitude flights in mountain wave lift.

Dropping at high altitude will not produce high g forces. G force will be 1g prior to drop, momentarily drop to 0g, and return to 1g as drag limits speed. The need for careful aerodynamic design is related to issues like flutter and airfoil behavior at relatively high subsonic Mach numbers.

Ailerons are not necessary to stabilize an airframe. Simple design elements like wing dihedral are sufficient. Your mission does not require turns fast turns. An inherently stable platform with rudder control is simpler and less prone to failure.

Glide ratio and speed are independant. You can have an airframe with a low glide ratio, a high sink speed, and a low airspeed. Pull full flaps in a SuperCub and you come down like a brick while at a nice slow airspeed.

Gliding time matters because it relates to the required size of your batteries. If you have an airframe with a 75 meter/minute descent rate you will have a descent time of over three hours.

Loops are a very bad idea as a strategy for increasing descent rate at high altitude. In full scale gliders we use control surfaces called spoilers (sometimes called airbrakes or dive brakes). In an emergency if spoilers are insufficient we would most likely use a flat spin. I have a good aquaintance, Dr. Joach Kuetner, who used a flat spin for a fast descent from around 12000 meters during research in the Sierra Wave project. You can produce a spin in many airframes without ailerons.

Rereading your post, perhaps you did not mean loops, but circles?

Finally Andrus brings up a good issue. Study the typical wind profile in your area to make sure your intended mission profile is feasible with a given airframe.

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Permalink Reply by mo fakahany on November 20, 2009 at 1:45pm

Hi Doug,
Things are clearer now!
Now we are looking for a design that has:
1.gliding ratio of 7:1 -10:1 (guess that is more than enough to reach the target and then descent in circles afterwards)
2.Withstands flutter on the wings, like this one you mean?
http://www.reggiepaulk.com/2008/05/low-frequency-high-amplitude-win...
3.Withstands airfoil behavior at relatively high subsonic Mach numbers.
4.An inherently stable platform
5.Only fitted with rudder(how about elevators? Will a V shaped tail do the trick?)
6.No Ailerons (for any reason, if glider went upside down will it be able to roll back to its normal position?)
7.Speed and time, as you mentioned we don’t want to be carrying loads of batteries up there so we are talking about one hour-one hour and a half (in descending)?

Now design’s calculations are a bit technical for us, to be involved in, as we are trying to focus more on the hardware, control and programming
So does anyone have any suggestion about a glider that will fit the bill, even if we had to reinforce it with carbon fibers or fiberglass?
Plans for gliders that you have done and tested them will be appreciated also

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Permalink Reply by Doug Weibel on November 20, 2009 at 9:31pm

Now we are more in agreement.

Nice find on the flutter video. Always scared me to see that. Flutter is often catastrophic.

On item 5 I did not mean to imply that you would not want elevator control. I certainly would. I just wanted to point out that ailerons were unnecessary for your mission. A V tail will work.

On item 6 - yes, stable airframes (for example with ample dihedral) will roll back upright from inverted.

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Permalink Reply by bGatti on November 22, 2009 at 2:32pm

If your airspeed is slowing than wind speed; then your glide-slope doesn't matter.

Presumably you care about being able to operate as many days out of the year as possible; that means incorporating the largest possible performance envelope.

Yours is an interesting application of math to flight envelopes.

I am pointing out that a high glide slope is probably not consistent with a maximal flight envelope.

Less say the winds are 25km/h. You spend 2 hours getting to altitude, You're 50km downwind:

Options:
1. a glider with 12:1 and forward speed of 25km/h.
2. a delta wing with 6:1 and airspeed of 50km/h


1. Flight test: your downward speed is 1/12 of your forward speed or ~2.5km per hour. In the 6 hours it takes to reach ground, you lose 150km of additional winddrift and make up 150km of hours aloft, so you miss your launch point by 50km.

2. Flight test, your downward speed is 1/6 forward speed. It takes only 3 hours to get down, you've lost 75km of extra winddrift, and gained 150km of forward progress. So you reach your launch return with 25km unused potential.

There is a 75km difference in the two flight envelopes.

In the end you're far better off trading glide ratio for forward speed.

You won't miss the target by "diving down", you'll miss the target when your maximum forward speed drops near or below the wind speed.

So your airframe should be optimized for high speed.

You'll want to do the flight envelope analysis for various airframes, but it's trivial to predict which airframes will be in contention, look for the fastest designs, as you don't require a slow landing speed, you can improve return distance by eliminating wing drag. Long wings have drag, short wings have less, you only need enough wing to sustain a 1:6 glideslope at very high speed. Try a long delta wing with 15% dihedral; use two servos for elevons; lay the components into the wings.

Look at the planform of a cruise missile. This is the kind of airframe which is optimized for high speed without the pesky slow-speed landing requirements.

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Permalink Reply by Tom Kent on November 22, 2009 at 2:51pm

Another good platform to compare to would be the small diameter bomb...it is a long-range glide weapon.

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Permalink Reply by Timothy Ritzema on November 26, 2011 at 9:35pm

Lol on the slow-speed landing requirements :P

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Permalink Reply by Andrus Kangro on November 20, 2009 at 12:24am

Hi, I think that this similar project you referenced provides quite a lot of valuable information. I would assume that main concern is cold (below -40 degrees/celsius) and very high winds up there, so you may want to look into airframe design that flies very fast and may even be more similar to jet than to glider. If you go with glider design, then it may be good to seriously clip its wings at least. If you land with parachute, then high landing speed is not an issue.

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Permalink Reply by Gary Mortimer on November 20, 2009 at 10:18am

Cold is an issue, I did a job at 30k and all sorts of batteries failed, the only ones that did'nt were inside a beer cooler with heat sticks broken in it to keep the radios alive.

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Permalink Reply by mo fakahany on November 20, 2009 at 1:58pm

check out this link:
http://translate.google.com/translate?client=tmpg&hl=en&u=h...
see how they insulated their stuff and how they tested everything with dry ice

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Permalink Reply by Morli on November 22, 2009 at 3:47pm

Hi @ Gati, Tom
That was interesting to read and I see some merits to it with my very limited knowledge of such project. There are however few things that poped up in my mind, no disrespect but both your example( roc ket/ mis sile and Tom's( bom) were never expected to be recovered undamaged/ reusable with payload inside once the mission
(self dest ruct/inflict dam age to the target etc ) is complete :) keeping in mind the terminal velocity, well there is always drag suite and parachute recovery system etc but it will be whole different point of view and issues.

1. What happens when the wind speed is say 75 km/hr for last 2 or more hours of assent assuming it was total of 4+ hours travel? What happens if and when balloon hits the jet stream in upper atmosphere.? Be what ever glide ratio and speeds , will the recovery vehicle make it back to launch site ?
2. when did the project objective change from being safe and successful recovery of payload to high speed impact at launch/target site , coming to that where ever did I hear or read about one of the objective of such high altitude experiment is the get/ land the recovery vehicle on the launch site?!!! , The recovery teams usually track & chase the recovery vehicle with some sort APRS or some form position reporting system or becon with RDF technique is what I have read most of the time.

AP and other form of stabilization in this platform will probably be considered as secondary payload & will be useful for return/recovery journey only. keeping that in mind I do see and agree to the point that battery capacity for such payload recovery vehicle will of concern and a faster return home will be better idea but since it is not expected to land back at launch site does away with any propulsion power requirement( glide and gravity being key) and so to whole of available power capacity is used to run AP stabilization to some decency , logging, photography , position reporting system etc.
Or did I miss the big picture? Thanks any way for making me understand the usfulness of delta wing recovery vehicle over a glider.

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Permalink Reply by Morli on November 22, 2009 at 4:10pm

glider with clipped wings :) looks good

@ Gary , What about small solar panel or small wind mill with dynamo etc to power mini/micro heater for electronic & bat bay ? is it practical ?

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Permalink Reply by bGatti on November 22, 2009 at 4:50pm

Morli,
Interesting responses - but easily answered.

Initially I proposed the space shuttle design as the example of a high altitude recovery-oriented airframe, but let's by honest, the mission of the recovery is to slow down and reduce glide slope, rather than recover to launch against a winddrift, and it lands on its own wings - as a consequence, it may have more drag and higher lift than the objectives of Weather Balloon payload recovery.

The lower drag / less lift options start looking like cruise missiles, as that is what you get when you optimize for high speeds. To be honest, payload recovery requires even less wing surface per weight than a cruise missile as it does not require level flight.

As for landing, the stated goal is a parachute landing, which means the mission profile is really more like a rocket, than an RC airplane. Where I going to land the payload under its wings, I'd go delta with Elevons - no problem - but you've got to be there in person, the flaring required is tricky and AP won't do flaring very well.

On the what happens - I'm with you on this;
If you get a high speed wind during recovery, or a jet stream; your chances of losing the payload are quite high - of course your chances without the recovery system are 100%.

Let's assume you have one-way radio data coming back from the device - even at 15K.
I would think the AP could sort through the list of potential landing sites, and list them in priority based on current position - and send back the preferred and secondary sites (from a preprogrammed list). You might add to this an abort policy which drops the recovery unit if the balloon drifts to far from any landing site. So that gets you a guaranteed landing site within your maximal flight envelope.

The answer to 1. what if the wind picks up.
Is A. maximize your horizontal air speed, while minimizing your time aloft - this means look like a cruise missile, and B. include secondary landing sites - for a last ditch effort, you can assert that every 2kmX2km intersection is a preferred landing site - so as to minimize the number of places you need to search if it drifts beyond the predefined list.

This is not a technically difficult proposition, it is instead a regulatory challenge.
Good Luck.

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