|Dec 14, 2012, 06:40 PM|
And for the majority of this thread, for everyone else Target. One thing I htink that folks need to learn is how does a high performance ship feel like and that means finding someone to let you fly a ship that is set up correctly. If you dive test one of my ships, they will tuck and go straight in, and I have moved my CG forward compared to what I use to fly.
But I let the "feel" of the ship tell me if I am right or wrong and go from there. In moving the CG back forward a bit, that came from reading David Hobby's writings about his set up and when having a chance to fly particularly in the early morning at the Masters and the Nats (most contest do not start early enough to get to fly under those conditions). When it is really light, I fly the rudder and I want the ship to stay put and not wander around. They say that every major input you have to make to correct the ship costs you 20 seconds of time from the potential flight time you could have (I have heard that Phillip Kolb did some hard work coming up with the figure).
So, you have to make the ship fly for you and your touch. There is no absolute perfect set up, CG, throws, declage, etc.
|Dec 14, 2012, 08:37 PM|
United States, WA, Gig Harbor
Joined Aug 2007
Thanks for that input Marc. Thats another good data point.
How about a quick summary so far.
Some people love dive tests and think they are the cats meow.
Several others - in addition to Gordy - not so much.
I think we all agree that IF you're going to do a dive test, you need to trim first, dive, adjust cg, RE-trim, dive repeat as needed.
All the other discussion of neutral points and pitching moments, etc are not relevant to the central question which is:
How do you physically balance your sailplane?
Im not talking about the aerodynamic theory or design or stability margins or any of that. At this point i dont care about any of that.
Im interested in what you do out at the field to get your sailplane balanced optimally for thermal flying and making your times.
I want to fly as well as Gordy and Marc and Joe Wurts - or at least have a shot at getting as close as I am capable of getting.
Theory isnt going to get me there. I want to balance my sailplane the way they do.
Gordy's main beef with the dive test seems to be that its done at higher speeds than where you will normally be flying. That seems like a valid beef to me - and thats aside from possible aeroelasticiy issues, bending push rods, broken elevators, highly cambered foils, poor set up, dumb thumbs etc etc. It seems there are a number of things that can be problematic with the dive test.
So he is suggesting that you balance at the other end of the speed range as well as inverted testing.
That makes sense to me, so Im going to give it a try and see where I end up vrs where my model is balanced now.
Im flying an RES poly sailplane right now and that makes it a little dificult to do a smooth inverted flight test. So Im going to give Gordy's method a shot and see where that gets me.
I will throw a tiny bit of theory in here because I think its interesting to note that Dr Drella suggest you START testing CG at the point where there is zero load on the elevator - and then move forward.
If you think about Gordy's balancing system - it gets you to that same zero load elevator CG and then moves you a tad forward.
How can you argue with that?
|Dec 15, 2012, 01:40 AM|
hehehe, good compromise!
However, I'm feeling like more and more of an outsider...I don't fly thermal duration...I fly it all...
In one single flight on the slopes, I'll be in full camber and scratching at every bit of (slope) lift I can get off any jutting rock and working every eddy of wind to keep it up, and then progress to full-wind rip-roaring half-pipes and high speed passes...then, dang it, back to scratching for lift again!! And then there's the boomer thermals that are perhaps 30' wide even when the wind off the bay is 20mph+ and there's very little slope lift (sucks when the wind is high and the lift is low!!) A plane can slermal 150 yds altitude in what seems like 45 seconds.
Hence the need for a "neutral" plane that flies well in all those conditions. A neutral plane. Not necessarily task-specific. A do-it-all setup.
|Dec 15, 2012, 02:20 AM|
hehehe, somewhat off-topic, but I previously said the math means nothing to me...
I just remembered I used to live for math waaaay back in freshman/sophmore year of high school (late '80's), where my absolute favorite subject of all time was physics (My, have I forgotten!!) (Just say no, kids!)
It was an absolute joy watching the natural world around me and seeing the formulas in my head and seeing how it all just melds together... How an 8yr old can understand it without realizing it (how else can he throw a snowball at a moving car and hit the intended open window?!)
I've completely forgotten that I used to know so much so many years ago, and how it used to be so second nature!
I tried to do some simple division recently and couldn't even remember how to start! That's how badly one loses something they don't use.
This all came flooding back from another thread where I remembered a movie I used to watch waaay back when...Pi...forgive the digression.
|Dec 15, 2012, 05:44 AM|
So you are not interested to hear what Mr Daryl Perkins believes or knows about this subject? Or you may know it already? I think more than I would appreciate his view.
It's not the first time I see him sit in the lounge eating popcorn, and this time I took the opportunity to try to get his views.
For my part, I understand very little in terms of mathematics in the aerodynamics area, but is still very interested in the subject, if it can be presented in a popular science so that you understand what is meant. I do not care a damn about mathematics more than the four basic arithmetic operations; these have been enough so far in my 56 year life.
|Dec 15, 2012, 11:40 AM|
He's chimed in before to threads like this, and often times been argued with.
You don't need DP or JW or Mark Drela to tell you what has been already discussed many times (IMO)...
On a tailed aircraft with a large horizontal tail volume (like most of todays TD/F3J planes), You can have a WIDE range of flyable CG's.
As you move the CG back, the required AoA of the wing for level flight is reduced. That means the decalage between the wing and tail will also be reduced. This also means the plane will show less "ballooning" with an increase in speed....
Conversely, as you move the CG forward (and re-trim for level flight!!!), you will have to make the wing fly at a higher AoA caused by that added nose weight/re-trimming, which is also adding decalage. And the plane will now "balloon" or pitch up with added speed more....AND it will be less sensitive in the pitch axis.
Now for a slightly less obvious (to some) observation of my own-
TD planes have HUGE stabs, compared to what I am used to. This is a great thing, it makes them very CG tolerant (The CG can be flown easily over a wide range), and I believe the reason they are designed that way is because the planes are asked by the pilot to be flown very slowly at times (thermaling and landing) and then the large stab really helps controlling the plane. The fact that it also allows a wide range of CG's is just co-incidental to this.
If you look at a slope racer like the Ceres, or even some earlier V-tailed designs, the horizontal volume calculated using the various formulas available is much smaller.
This is where the guys like me, that are "anal" about measuring CG's down to the mm, come from. Guess what happens to the "CG range" when the tail volume is so much smaller? The range gets smaller too, and more important. Too far aft and you are likely to have the wing depart flight with a little too much elevator.
But still the same principles of AoA apply, just over a lesser range.
Now look at plank flying wings-
Guess what? The same ideas apply to that, only even more. That is why the CG on a plank wing is so much further forward than your typical TD or even slope plane, the tail volume is less yet. Think of a tailless aircraft as having a super short tail boom. The CG range is VERY small, and yet, as you move the CG forward, you will still have to re-trim (the elevons this time) a little "up" to accomplish level flight. And moving the CG back will require the same lessening of the up trim, but you can't effectively make the cg back so far that you have down trim for level flight with a plank wing.
The same can be said for every instance of the above examples with the cg further back: First is that the tail will require less up trim, or more down trim (depending on how you look at it).
Next is the plane will become more sensitive to pitch (a good thing, most times!). Because of this, you should "turn down" your throw on the elevator, since it is now doing less work lifting the weight of the nose (by providing downforce) it doesn't need as much throw to accomplish the same result.
The planes will all fly with less ballooning when the speed is increased. Another good thing.
The planes will fly inverted with less down elevator/elevon throw needed. A side benefit not likely used flying TD, or a good thing if doing aerobatics on the slope.
The planes will all become more sensitive to lift, and likely will accelerate more with this aft CG when doing flying through lift.
Here is my get-out-of-jail card:
I am not an aero engineer. I'm a boat captain. All of what I wrote above I completely believe in 100% based on my OWN observations. Some of the phrasing may be technically incorrect as far as aerodynamics are concerned, but all of what I have intended to write (I will be editing to make things clearer if possible, and in case of a typo) is true, to the best of my knowledge.
You will notice that I didn't give wing chord percentages in my info... Wing plan forms, while they mostly follow trends, can vary, and statements like, "Set your CG at XX% of the root chord and it'll be fine" really make my hair stand up on end.... While that percentage statement may be true for most molded TD planes, it simply cannot be used as a one rule to follow for every molded plane. Someone will inevitably get a plane with some appreciable forward sweep designed into the wing, and the plane will end up in parts and pieces on the first flight...And that is not what anyone here wants to happen.
I meant no disrespect when I said that DP would be wasting his time posting here. I was just saying that the information is out in both technical and layman's terms in lots of places, you just need to look for it, and it needn't come from DP, JW, or Tom Keisling or Mark Drela to be useful to you.
|Dec 15, 2012, 01:03 PM|
Obviously, such information need not come from authorities in the field, but it does not hurt either.
Similar issues have always been and will be repeated many times over here at RcGroups, for it will always be new model aviators seeking answers. Then there is also a great need for repetition and renewal in many subjects, even for us "old-timers".
|Dec 15, 2012, 02:09 PM|
You are welcome.
I hope that someone, especially the original poster, finds the stuff I wrote useful....
Again, these are my opinions, not statements of fact (but they are for me alone), I am not an aeronautical engineer, just a guy that has tried to improve his duration flying as of late, and who has spent plenty of time on the slope.
PS. here is one little tip that I find is handy for CG adjustments in the field-
Instead of "taking weight out if the nose" at the field, measure, find, and mark the approximate spot on your tail boom that is the same distance from the CG that your nose weight is. When you are at the field, ADD a 1/4oz or what have you, to that spot on the tail boom (taped securely), and test fly the plane. Its much quicker, and you can easily see what you have going on with regard to adjustments.
When you are happy with the adjustment of the CG you have made, check and mark the CG with a balance stand at home.
Take off the tail boom weight, weigh it, and take the same amount out of the fuse in the nose. Your CG should stay exactly the same as it was in the field, but with a lighter AUW.... You can confirm with your balance stand if you want to...
I'm writing the above coming from flying with some skinny nosed F3F/B style fuses, where the weight is not usually easily accessible. I usually STILL do it this way, even if the nose weight is easy to get to, flying TD planes. Its just easy for me to know what is happening this way.
|Dec 15, 2012, 05:29 PM|
United States, WA, Gig Harbor
Joined Aug 2007
Well said Target!!
I'll add one more tip on adjusting CG to yours. I mark the final flying CG location on the wing with everything assembled, then I remove the wing and see where the fuse balances without the wing in place and mark that on the fuse.
Then when Im fiddling with adding or removing weight or moving servos or battery packs to get my new CG, I can quickly check the new balance without having to put the wing back on.
|Dec 15, 2012, 05:52 PM|
ooo, great tip MrE! (fuse balance)
Thanks Target for the info. I like (for me) non-too-technical writing.
So that's why the little free flighter has no much decalage/incidence...to somewhat keep it flying at one speed? Without RC we wouldn't want it getting out of hand. So the RC version would be better suited to a "closer to zero" angles.
MAn, I need to find my incidence meter.
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