Weight vs. Wing Area in windy conditions? - RC Groups
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Oct 02, 2017, 07:30 PM
Registered User
Question

Weight vs. Wing Area in windy conditions?


First post, my apologies if this isn't the right place.

The Aerospace club at my university is taking part in the AIAA DBF competition in April, so I'm sure you're all used to the inundation of students asking silly questions - I've looked all over for a definitive solution here with no success.

I've had little luck finding any real answers as to how the weight and surface area of a plane, especially RC aircraft, relate to their susceptibility to wind. I get that, to a certain extent, the heavier a craft is for a given surface area, the less affected by wind it will be.

That being said, I'd like to be able to (at the very least) determine wether or not the scale of the craft matters. The empty weight and wingspan are crucial to scoring considerations this year, so currently the only reason we would have to scale up our plane would be to ensure that it is able to fly in Wichita, where wind has historically been an issue for many teams.

From our internal discussions there seems to be a good amount of disagreement - I'm inclined to believe that the scale of the craft doesn't matter, and that a 15mph wind will impact a 2lb plane with a certain span and chord just as much as the same design scaled up to 15lbs. Others are concerned that "too small" of a plane won't be able to handle the wind and will be blown off the runway, while larger planes will be more steadfast.

I appreciate any insight you've got!
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Oct 02, 2017, 08:51 PM
B for Bruce
BMatthews's Avatar
All models are affected by wind. But some are fans of a SLIGHTLY heavier wing loading for a given size. But I'd say an equal number do not agree with the other half. I'm in the "light is right and lighter is righter" school. I'll list off a few reasons below.

First off a heavier wing loading raises the stall speed. And often the motor/engine and prop sets the top speed so that means the range of speed from top to stall is reduced. And that means more of a work load for the pilot, not less.

Even fairly large models are small enough that they are susceptible to the wind shear effect during windy flying that results in a lower wind speed along the first few inches from the surface and the transition to the ambient wind velocity some 10 feet up. For more open areas there's little change of note in the average wind speed once over 10 feet. A model coming in to land in such conditions needs to ensure it maintains some speed so as it drops through this shear effect it still is flying as the apparent wind velocity falls. This is very much a transitional thing but then so is the approach and touch down event. And because it's linked to a effect produced by the earth that reaches up it's not the same as the so called "river of air" or the "dreaded downwind turn". In this case it's very real. A lower stall speed can in fact aid this.

All else being equal it would seem like a little more mass in the wing tips and tail would aid with stabilizing the model, right? Well, it will. Heavier extremities means it takes more energy to get them moving to any degree. And that seems like a good thing. But it also means it takes more control surface deflection or the same deflection held for longer to first stop the unwanted motion and then to bring it back to the desired position. On the other hand lighter extremities might move more but are more easily corrected with less input held for a shorter time. And that means less drag too.

I've seen both lighter wing loading full control model sailplanes as well as heavy scale models flying in wind. Oddly enough with good piloting both are easily flown. But I would suggest that the sailplane puts less workload on the pilot and produces fewer "temporary emergencies".

Also if your aircraft will be part of some weight carrying competition then it'll have all the weight it needs to steady it. I would not purposely try to make the extremities at all heavier by design.

Now to some of your specifics. If both the 2 and 15lb models fly at the same speed then you can expect some aspects to be affected the same but other things to be less affected. And in some ways the smaller model is better and in others at a disadvantage. A larger model cuts through more of the sky and that can lead to one wing being in air moving one way and the other wing moving through air going a different way when conditions are turbulent with localized effects from ground based objects or if there is heavy thermal activity. If the location you fly is prone to "dust devils" this spells trouble in terms of lots of very localized vertical air movement to to thermal conditions causing serious up and down drafts. Oddly enough a smaller model can put a smaller work load on the pilot who has to deal with the issue.

But a small model cannot carry very much payload..... And due to the lower relative wing loading it might well be blown off the runway if care is not used. All else being equal a smaller model needs a lighter wing loading to fly the same as a larger model at even double or more the wing loading. It's that old Reynolds Number thing ya know....

You've likely learned that a higher aspect ratio is a good thing. And it is for lower drag and lifting more weight for a given wing loading and power loading. But in turbulent conditions a higher aspect ratio can hurt by increasing the work load on the pilot. And also if the design is not carefully thought out the needs for stronger spars and other parts can result in relatively heavy extremities. So a little compromise in recognition of the frequently found conditions is not a bad idea. After all the old racing adage does come into play. "To finish first, first you have to finish". This doesn't mean you should just build an Ugly Stik or Piper Cub. But it does suggest that you exercise a reasonable degree of pragmatism when aiming for the "best" numbers.

"Large" can still be built "lightly". Just don't go so light that you start risking durability. And if you can locate the payload mostly in a centered manner so the inertial moments are lower you'll be making things easier for the pilot. At least if I were the pilot I'd appreciate this effort because that's the sort of style of model I prefer to fly where it reacts rapidly and predictably to small inputs. The times I've flown over weight models with higher swing moments I often felt like I was in one of those "whip the tail" human chains where the model was slower to react to both disturbances and my inputs but also had this "thing" where I had to start moving the controls before I was finished so I didn't over shoot. All in all I found it rather disturbing. In time we can become used to just about anything. But will your pilot have that luxury?

Planes with heavier extremities also tend to need tail surfaces that are on the larger side of the bell curve for the "size sweet spot". This is because they tend to need more ability to damp out unwanted motion... like when controls are released.

At least all this is how I feel about such things. Best of luck with whatever you and the team decides on.
Oct 02, 2017, 08:58 PM
Registered User
Quote:
Originally Posted by daftdunk
I'm inclined to believe that the scale of the craft doesn't matter, and that a 15mph wind will impact a 2lb plane with a certain span and chord just as much as the same design scaled up to 15lbs. Others are concerned that "too small" of a plane won't be able to handle the wind and will be blown off the runway, while larger planes will be more steadfast.
What do you mean by "scaled up"? Will the 15lb plane have the same wing loading and configuration (shape and aspect ratio) as the 2lb plane? Or are you talking about keeping span and/or chord constant as weight increases?

I expect you are familiar with lift and drag coefficients, so you know that large and small airplanes with the same wing loading will have the same stall speed -- PROVIDED that CL_max is the same. If the airplanes are the same configuration, the bigger one may have a little higher CL_max due to Reynolds number effects.

What will NOT be the same is moments of inertia. The smaller airplane reacts much faster to control inputs because it has less inertia. In the same way, it can be much more susceptible to turbulence, and harder to fly in choppy conditions. "Big airplanes fly, little ones flit" as modelers say.

Do the competition rules allow use of a stabilizer? This could make the smaller airplane handle much better in the wind. Could be well worth the weight penalty if you lose a lot of points for excessive wingspan.
Oct 02, 2017, 09:13 PM
Registered User
Are you using the 2018 DBF rules available here?

http://www.aiaadbf.org/Rules/

If so, I think a 2lb plane is a non-starter.
Oct 03, 2017, 12:07 AM
Registered User
As I see it the most important design criteria is: RAC = EWmax * WS. Being a dividing factor in the final score you will need to constantly check your target performances and overall payload capacity versus the RAC. You will have to do sort of a simulation to see when it becomes a penalty.

So, at first glance, as jruley pointed out, a 2lb empty (but operational) aircraft is not going to win. You "just" need to design the lightest airplane with the shortest wing span and the biggest payload capacity!!!
Last edited by fnev; Oct 03, 2017 at 02:02 AM. Reason: corrected heaviest to lightest
Oct 03, 2017, 01:55 AM
Closed Account
So you're right that will be an interesting factor (or is that Fractor/denominator)
Last edited by Bryce.R; Oct 03, 2017 at 02:20 AM.
Oct 03, 2017, 02:00 AM
Registered User
Quote:
Originally Posted by Bryce.R
I would have thought it was the lightest aircraft (not the heaviest)

What am I missing?
You are right... My bad and corrected!
Oct 03, 2017, 02:21 AM
B for Bruce
BMatthews's Avatar
I didn't read the rules totally but it's pretty obvious that the total payload potential and the compartments is going to play a part in the minimum airframe size. That RAC thing counts for a lot too. But not at the total expense of the passenger and payload factors.

The odd time limits on the passenger load out will strongly affect the size of the air frame. It would be nice to have a large airframe and carry a whole bucket full of "passengers". But the 5 minute window to load out passengers AND do the required flight prep will set a limit on the number of passengers. Some suitable way of handling would be needed to raise that number able to be processed within the time window. If some wholesale method could be devised to allow for more passengers then it would justify a larger plane.

That RAC factor favors a lower aspect ratio. But go too low and flyability will suffer and the takeoff and landing rolls will become longer. If time and team members permitted it would be nice to test "scale" models of different configurations that all have the same wing area and weight but which vary in span via changes to the aspect ratio. Then test to see which takes off, handles and lands more easily and within some reasonable distance.

I started to read the LRU section but my brain started to go foggy.......
Oct 03, 2017, 08:43 AM
An itch?. Scratch build.
eflightray's Avatar
Quite a few videos on Youtube regarding the AIAA DBF competitions.

Ray.
Oct 03, 2017, 10:00 AM
Registered User
IMO the first thing each design team should do is read the rules. Then put them on a poster and re-read them at each meeting. There are a number of interesting "traps" here:

SCORE = Written Report Score*Total Mission Score/RAC

So first of all, put your best writers to work on the written report. No excuse for this score being less than 100%. There's no right or wrong; you just have meet documentation requirements and clearly describe what you did.

The total mission score includes three elements:

Total Mission Score = M1 + M2 + M3

If you look closely, the maximum possible score is 3.0, because M2 and M3 are divided by the best team's score.

M1 basically just means the airplane flew successfully. No partial credit.

M2 and M3 favor carrying the maximum number of passengers and/or payload around the pattern in the shortest time, so they favor big, fast airplanes. However, if you push it too far and don't make it, you get zero. An "80% solution" (80 percent of the best team's score) might win the contest overall.

Also notice the order in which missions are flown. If you don't pass the "Ground Mission" (LRU replacement), you don't even get to fly M2 and M3! So making components accessible and replaceable is very important.

And then there's RAC, which favors the lightest empty weight and the shortest tip-to-tip span.

Did anyone else catch that LiPo batteries aren't allowed? (page 14)
Oct 03, 2017, 12:27 PM
B for Bruce
BMatthews's Avatar
Quote:
Did anyone else catch that LiPo batteries aren't allowed? (page 14)
Yeah, I saw that while looking to see if there was a maximum power pack size or spec.

There is SO much more to this than just designing and building a flyable air frame. I tend to agree with your suggestion about the chart... except that it may require more than one chart since there are so many requirements in so many different design areas. It might be more correct to call it the "Reminder Wall" than a "Reminder Chart"....
Oct 03, 2017, 03:24 PM
Registered User
Quote:
Originally Posted by BMatthews
But a small model cannot carry very much payload..... And due to the lower relative wing loading it might well be blown off the runway if care is not used. All else being equal a smaller model needs a lighter wing loading to fly the same as a larger model at even double or more the wing loading.
This is really where my real worry lies. The RAC scoring means that low-aspect ratio biplanes are going to be abundant to minimize span, which will result in extremely low wing loading, coupled with a very light plane. I really like the concept you mentioned of building similar configurations at different scales, which will definitely be on our prototyping plan.

Quote:
Originally Posted by jruley
IMO the first thing each design team should do is read the rules. Then put them on a poster and re-read them at each meeting.
I very much agree, I actually ran a sort of "Rules Trivia" at the meeting Sunday so that everyone could get a better handle on the importance of certain factors.

Quote:
Originally Posted by jruley
If you look closely, the maximum possible score is 3.0, because M2 and M3 are divided by the best team's score.
I could be mistaken, but the maximum total mission score is actually 9: M1 =1, M2=2*(1 maximum) =2, and m3 is 4*(1 maximum)+2=6, for a total of 1+2+6=9.


Quote:
Originally Posted by jruley
What do you mean by "scaled up"? Will the 15lb plane have the same wing loading and configuration (shape and aspect ratio) as the 2lb plane? Or are you talking about keeping span and/or chord constant as weight increases?

I've been working on a MATLAB script that optimizes score with a handful of assumptions. Of of the major ones that the mass of the plane can scale 1:1, as in that a plane with 10x the mass will be 10x larger (span, etc). Same aspect ratio, while increasing chord and span. Obviously that is a huge assumption, and is one of the more difficult things to really map out numerically. I can also have the script scale span with increasing mass, by whatever factor I like.
That same method can be applied to any of the calculated factors - payload fraction, etc. RAC, from the analysis I've done so far, is the most important metric by a pretty large margin.

Currently, assuming that a plane does not get significantly faster as its weight increases, (which would decrease lap time), and that a the mass and wingspan scale 1:1, the only thing that determines the best size is the plane : payload ratio. (I know payload fraction is the actual useful metric, I've just gotten into the bad habit of using the ratio.) If it increases significantly with increased mass, as in goes from about 1:1.3 to 1:2.5, then larger is always better, up to a 55lb gross weight. That is equivalent to a 3lb plane carrying a 3.9lb payload, which does not seem at all out of the question from past competitions. That also means a 15.7lb plane carrying a 39.3lb payload, which to me sounds pretty damn unfeasible. If the plane : payload ratio does not increase that much, then score rapidly decreases with increased weight.


Quote:
Originally Posted by jruley
Do the competition rules allow use of a stabilizer? This could make the smaller airplane handle much better in the wind. Could be well worth the weight penalty if you lose a lot of points for excessive wingspan.
It does as far as I know, and that's part of the plan to make up for a very small, light design with a low aspect ratio.

Quote:
Originally Posted by BMatthews
That RAC factor favors a lower aspect ratio. But go too low and flyability will suffer and the takeoff and landing rolls will become longer.
Agreed, and that's one of the things we really have to determine in testing, like you said. The 2007 competition was the only past year to use the same RAC scoring scheme, and the ended up with a lot of high-aspect ratio biplanes, which have their obvious stability issues. That being said, a good pilot can fly a well-built, hard-to-fly plane, from what I've been reading and hearing. The 2007 winning plane was very unstable, but also had a final score more than double the second place team due to a crazy low RAC.

Quote:
Originally Posted by BMatthews
I started to read the LRU section but my brain started to go foggy.......
Oh we've got some ideas in the works for that
Oct 03, 2017, 03:41 PM
An itch?. Scratch build.
eflightray's Avatar
Quote:
Originally Posted by daftdunk
This is really where my real worry lies. The RAC scoring means that low-aspect ratio biplanes are going to be abundant to minimize span, which will result in extremely low wing loading, coupled with a very light plane. I really like the concept you mentioned of building similar configurations at different scales, which will definitely be on our prototyping plan.

...................................
Not necessarily biplanes, those Youtube videos show some canards.

Ray.
Oct 03, 2017, 03:45 PM
An itch?. Scratch build.
eflightray's Avatar
An example --


WashU AIAA DBF 2017 (7 min 13 sec)



Ray.
Oct 03, 2017, 04:04 PM
Registered User
Quote:
Originally Posted by daftdunk
I could be mistaken, but the maximum total mission score is actually 9: M1 =1, M2=2*(1 maximum) =2, and m3 is 4*(1 maximum)+2=6, for a total of 1+2+6=9.
Yes you are right , so the missions are of increasing importance to the overall score. However, if you don't pass the ground mission, you don't even get a crack at M2 and M3. So LRU replacement may trump aerodynamics, and drive structural approaches as well.


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