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Old Feb 27, 2012, 12:16 AM
RIP MC
fnnwizard's Avatar
United States, CA, Midway City
Joined Dec 2003
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43ft/min sink might be a stretch, but the numbers suggest it won't be that much of a stretch.
a 4m Xplorer at wing loading 8.1oz/ft^2 can achieve close to .9ft/sec = 54ft/min.

The Maxa's wing loading is 20% lighter = about 10%less sink = 49ft/min. But it's also got a thinner foil, higher AR ad I am sure it is less drag overall than the XPlorer. The only thing against it is Re numbers because it would fly at a slower speed.

The more span and the higher the ar, the higher the wing loading can be to achieve same sink as smaller span and smaller ar plane.

The issues I'm experiencing with some of the lightweights is that as these planes get so light, the thermalling starts to become a handful. I keep racking my head to figure out why and so far I've come up with a couple of hunches.
1. A disproportional amount of weight remains in the extremities, so the lighter overall plane, though can fly slower, which makes the control inputs less effective so response is not as good (of course not speaking of Maxa here)
2. This slower flying plane won't need to bank as much while thermalling so the speed differential between inside and outside wing means sometimes not optimal TE deflections ( think opposite ail to hold bank).
Could it be the number 2 reason, maybe why so many like a 3.5 Xplorer at low levels more than a 4.0 X? Anyone?
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Old Feb 27, 2012, 01:56 AM
In F3J size does matter!
roydor's Avatar
Israel
Joined Nov 2006
851 Posts
I doubt that lower than 55 ft/min can be achieved consistently in "regular" morning air with an F3J model.
Extraordinary air is another matter entirely. Sometimes the air is "thicker" due to moisture, usually when the temperature is low or after rain, then you get fantastic air.
Last year in the Bulgarian eurotour competition we flew a round in drizzle and overcast skies after almost an hour of rain (no thermal activity, just some better area’s with “thicker air” here and there). I flew my 2050 gram Gremlin (3.45 meters, 70 dm^2) and I managed to fly 9:40 off an average launch in no wind. I estimate a sink rate of 0.29 m/sec or 57 ft/min which is lower than I can make in what I consider as "regular air". Just for the record I suspect "regular air" sink rate for my 2050 gram Gremlin is around 0.33 -0.32, similar as other "ordinary" models at that wing loading.

AR can increase performance. However, the difference between an AR of 20 and 18 is not the same as the difference between an AR of 18 and 16, I doubt models with an AR of 20 are better because of AR, it’s a small contributor, probably no more than 2%-3% to performance at most but with costs to strength, handling and price.
I think that biggest contributor to performance for the larger models is the fact the model gets relatively "cleaner" aerodynamically. This is due to the fuselage diameter staying the same while the wing area gets larger and the parasitic drag due to canopy, servo hatch, elevator slot in the ruder, hook, etc, all these get smaller in relation to the wing and you get a cleaner planform (same drag penalties divided by more wing area equals lower drag coefficient). Together with a lower wing loading (again, parasitic weight of the fuselage divided by wing area) equals slower speed and reduced sink rate assuming L/D doesn't get to messed up (lower Reynolds vs. aspect ratio).

As for handling, in my limited experience, longer wing spans tend to be less nimble and more prone to pilot mistakes due to adverse yaw affects which are span proportionate. Enlarge the rudder on the longer span models and add more aileron to flap mix and longer models will start to fly more similar to the shorter ones.

Having said all this, I would love to get my hands on a Maxa, it’s a beautiful model and I’m sure it flies exceptionally well.

Roy
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Old Feb 27, 2012, 02:04 AM
registered user
Australia, QLD, Gold Coast
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That was very well written Roy. I understood the whole way not like some explanations in here ! Lol
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Old Feb 27, 2012, 02:55 AM
RIP MC
fnnwizard's Avatar
United States, CA, Midway City
Joined Dec 2003
3,269 Posts
Maybe if I wrote it this way, it can clear up the picture.

If we take a Maxa and ballast it to a wing loading of 9.55oz/sq. ft (that would be an 84 oz Maxa). This would equal the Gremlin in wing loading. How would this compare to Gremlin in terms of sink speed?

Now take a 61oz (1730g) Maxa with wing loading 6.9oz/sq.ft. Compared with 84oz Maxa, light Maxa is 28% lighter. Or, said another way, 84oz heavy Maxa is 38% heavier.

How much would sink decrease in light Maxa then?
I do understand that higher humidity air ( more moist air) is less dense than dry air, but I am not sure if that is what you are trying to say.
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Old Feb 27, 2012, 04:30 AM
In F3J size does matter!
roydor's Avatar
Israel
Joined Nov 2006
851 Posts
Thin airfoil vs. thick airfoils…
Thin airfoils have 0.1 to 0.2 less overall Cl. When translated to optimal Cl for flight is comes to no more than 0.1 delta Cl between the two.
This is because we do not fly at the max Cl due to safety margins required to avoid unintentional stall.
In calm conditions (very low turbulence) model behavior is very predictable and everything happens slowly which makes it possible to fly closer to stall speed, perhaps only 15%-20% faster than the stall speed.
In more turbulent conditions we need grater margin to avoid stalling the model and so we increase to 25% of stall speed all the way up to 45% or 50% in very choppy condition.

My Gremlin airfoils are somewhere between the Supra foils and the Perfect foils which means I can fly slightly heavier then the Supra without losing too much but I still need to be lighter than a Perfect (in terms of wing loading only as all these models have slightly different wing areas)
Still, Physics is physics and less than 10% in Cl means less than 5% in air speed (square root of the ratios off course) which means very small differences in theoretical performance.
Please note I write “theoretical performance” because real life performance, in my personal opinion, has much more to do with the pilots ability to fly the models optimally and closer to their stall speeds and this is why I will always choose models with larger elevators, more forward CG and more forgiving wings than a “performance based” model. This allows me to fly my models 5% slower with less control inputs in light air and squeeze out more theoretical performance then I, with my limited abilities, would be able to squeeze out otherwise.

As for moisture in the air, I don’t know the physics behind that but after rain my models fly a slower so I suspect “thicker” air is either denser or more viscous.

Roy
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Old Feb 27, 2012, 06:54 AM
<>< AKA W4BPS
USA, TN, Tullahoma
Joined Dec 2001
2,428 Posts
Some pilots

Supposed that made any difference. The word pilot??

Quote:
Originally Posted by webbsolution View Post
I would have to agree with the skepticism - there were 3 or 4 maxas at the SWC launching from Mono winches...early morning (non active) rounds were difficult to attain the 10 mins and - some pilots flying maxas did not attain their time. If you can do 12 mins in dead air from just 450 feet ...then this should have been a cake walk from a mono winch launch...
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Old Feb 27, 2012, 10:51 AM
Flying = Falling (Slowly)
dharban's Avatar
Tulsa, OK
Joined May 2004
2,643 Posts
Quote:
Originally Posted by roydor View Post
As for moisture in the air, I don’t know the physics behind that but after rain my models fly a slower so I suspect “thicker” air is either denser or more viscous.

Roy

Hey, its a slow morning here and I've got an extra can of gasoline sitting here, so I thought I'd toss it on something and see what happens. I graduated in Mechanical Engineering 45 years ago and your statement tripped a synapse which caused me to remember (correctly or otherwise) that air/water vapor mixes are LESS dense than dry air mixes. Understanding the source credibility of Wikipedia, I nonetheless offer the following from that source:

"Air and water vapor density interactions at equal temperatures

At the same temperature, a column of dry air will be denser or heavier than a column of air containing any water vapor. Thus, any volume of dry air will sink if placed in a larger volume of moist air. Also, a volume of moist air will rise or be buoyant if placed in a larger region of dry air. As the temperature rises the proportion of water vapor in the air increases, and its buoyancy will increase. The increase in buoyancy can have a significant atmospheric impact, giving rise to powerful, moisture rich, upward air currents when the air temperature and sea temperature reaches 25 °C or above. This phenomenon provides a significant motivating force for cyclonic and anticyclonic weather systems (tornadoes and hurricanes)."

It would appear that it is incorrect to suggest that moist air is more dense than dry air. But it still leaves open the possibility that if you are talking about a situation where a body of moist air is surrounded by a body of relatively dryer air that the moist air can have some upward movement.

My problem with the latter notion is that on those early morning and dusk times when sink rates seem to become bafflingly low the geography of the places that I fly is so uniform that there is no structure to provide relatively wetter and dryer areas. Yet I can go to my flying field within a half-hour of sunrise, launch and fly a fixed 1/2 mile square at minimum sink and a) see no variation in sink rates from leg to leg, b) see no variation from day-to-day, month-to-month or even season to season. I see consistent results which may vary from plane to plane which almost never vary for any particular plane.

When a breeze comes up, all bets are off. Go figure.

Happy Landings,

Don
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Old Feb 27, 2012, 11:15 AM
Detail Freak
target's Avatar
Harbor City, CA
Joined Oct 2003
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OK guys-

Here's a quick question to "stir the pot" a little more:

Why does fog form?

R,
Target
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Old Feb 27, 2012, 11:38 AM
Flying = Falling (Slowly)
dharban's Avatar
Tulsa, OK
Joined May 2004
2,643 Posts
Quote:
Originally Posted by target View Post
OK guys-

Here's a quick question to "stir the pot" a little more:

Why does fog form?

R,
Target
That's a little easier.

The atmosphere contains varying amounts of water vapor in it at all times. One measure of how much water vapor air has in it at any particular place is its Dew Point Temperature. The Dew Point temperature is the temperature at which water vapor in the air begins to condense -- form fog. One good way to look at the difference between some place where the air is "dry" and a place which is "humid" is the typical difference between daytime high temperatures and night time lows. In the Gulf, this can be as low as 10 or 15 degrees. In the high deserts it can range over 40. The practical effect is that on a 90 degree day in the desert, the likely nighttime low temperature (without a frontal passage or such) would be in the range of 50 degrees, while in the Gulf a 90 degree day would likely result in a nighttime low of 75 degrees or so.

And in both places, you would expect the formation of dew and/or fog if the earth's radiance to the night sky dropped the temperature to their respective dew points.

Happy Landings,

Don
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Old Feb 27, 2012, 03:19 PM
launch low, fly high
New Zealand, Hawke's Bay, Havelock North
Joined Dec 2004
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Minimum sink is best described as a function of span loading. The Maxa 3.95 has a 26% lower span loading than the Gremlin, the Maxa 3.45 has a 15% lower span loading. Re, and Cl limits may reduce this relative benefit by a few percent.

One comment about the airfoil design for the Maxa. The Maxa is using airfoils that have a bit higher max lift than the Supra airfoils, and also are intended to be more optimized for handling characteristics than for absolute performance. I'd far rather give up a small amount of theoretical performance to buy achievable performance by a pilot that is focusing on the air rather than having to focus on flying the plane "just right".
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Old Feb 27, 2012, 04:17 PM
Where is the lift?
cptsnoopy's Avatar
USA, AZ, Phoenix
Joined Nov 2005
6,398 Posts
Quote:
Originally Posted by webbsolution View Post
I would have to agree with the skepticism - there were 3 or 4 maxas at the SWC launching from Mono winches...early morning (non active) rounds were difficult to attain the 10 mins and - some pilots flying maxas did not attain their time. If you can do 12 mins in dead air from just 450 feet ...then this should have been a cake walk from a mono winch launch...
I could be wrong, but I'm pretty sure I got my times in the morning air. I am not a great example of getting the highest launches either. Why don't you try it or get with someone who has one and find out for yourself?
That goes for all you skeptics... lol

Charlie
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Old Feb 27, 2012, 05:05 PM
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United States, CA, Folsom
Joined Jul 2007
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Quote:
Originally Posted by cptsnoopy View Post
I could be wrong, but I'm pretty sure I got my times in the morning air. I am not a great example of getting the highest launches either. Why don't you try it or get with someone who has one and find out for yourself?
That goes for all you skeptics... lol

Charlie
+1. Fly it first, talk about it later. Not the other way round.

JT
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Old Feb 27, 2012, 05:40 PM
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Quote:
Originally Posted by jtlsf5 View Post
+1. Fly it first, talk about it later. Not the other way round.

JT
But JT this is RCG.. LOL

sj
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Old Feb 27, 2012, 06:25 PM
Detail Freak
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Most folks don't buy a plane though without a little bit of research here or other places.

R,
Target
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Old Feb 28, 2012, 06:26 AM
kdt
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Training wheels

Quote:
Originally Posted by webbsolution View Post
I would have to agree with the skepticism - there were 3 or 4 maxas at the SWC launching from Mono winches...early morning (non active) rounds were difficult to attain the 10 mins and - some pilots flying maxas did not attain their time. If you can do 12 mins in dead air from just 450 feet ...then this should have been a cake walk from a mono winch launch...
The Maxa is not a panacea for perfect scores. In fact it becomes more difficult to fly a model of this performance at the limit. Knowledge of how far away you can fly and make it home becomes more critical as the weight is reduced. There is no way of knowing the boundary of the overall performance without practice and experience. Flying a new model especially one of the high performance of Maxa can be humbling in competition. My score at the SWC proves my point with two zeros due to landing out. I would not say the air conditions that my problems occurred in were non reactive Vladimir has made training wheels for Maxa ... "ballast" My point is there are many variables and the biggest one is the pilot.
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Last edited by kdt; Feb 28, 2012 at 08:19 AM.
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