|Nov 07, 2012, 11:00 PM|
CG, decalage, and trim
Anybody have a handy link that fully explains how to systematically go about sorting out the interactions of those forces in order to "neutralize" a plane?
Just having a hard time figuring out multiple planes having similar yet way different issues (if that can make sense).
|Nov 08, 2012, 03:08 PM|
What specific issues are you having?
You could read "CG Position" and "Decalage" here:
|Nov 08, 2012, 04:53 PM|
yeah, I used to know a lot about planes, now I just think I know. I was hoping nobody would ask to explani because I don't think I can.
I thought porpoising was tailheavy...but then a dive test has it pulling up quickly which wuold indicate up-trim i.e. noseheavy??
And another plane wallows around badly like it's noseheavy, but its dive test has it diving more and more nidicating down-trim i.e. tailheavy...which it doesn't feel like in flight.
I have an incidence meter somewhere, but four moves in a few years means I need to dig some more.
I was kinda hoping to find a source of all the info in one spot...such as the video, but which I had hoped to find free.
I tried minor balancing change but only seem to make the plane fly overall worse. I just need to start from scratch.
|Nov 08, 2012, 05:47 PM|
Thanks, that helps a lot.
1) Launch model
2) Trim the elevator for hands off flying at what appears to be your best glide or L/D speed
3) Now push the nose over into a 30-40 degree dive, after a few seconds release your elevator to neutral
4) Observe what happens.....
1) If it pulls out very gradually you are all set
2) If it pulls out quickly, land, remove nose weight and repeat starting at step #1, but trim some down elevator prior to launching
3) if it tucks or continues straight ahead it is tail heavy, remove tail weight or add nose weight. Trim a little up elevator and go back to step #1
The key to all of this is getting the model trimmed for straight and level flight to begin with. If you trim for a different speed you will get different results.
|Nov 08, 2012, 05:56 PM|
Porpoising is normally caused by a nose heavy model. As the model banks into a turn the nose will drop due to lift vector changes thus it picks up speed slightly since it was trimmed with a lot of up elevator to compensate for the nose heavy condition, as it does the model will self correct and climb. Then the pilot is always correcting for climbs and descents thus the porpoising or over controlling. A properly balanced model is easier to fly than either a nose or tail heavy model as it requires less pilot correction.
Changing incidence will only cause the fuselage to fly at a different angle to the flight path.
Hope this helps.
|Nov 08, 2012, 11:16 PM|
Sorry, I indeed misspoke regarding porpoising as nose-heavy.
See? I've been out of the hobby so long I can't even get the terms right!
I think my failing memory is simply not remembering what the flying characteristics of a nose/tail-heavy plane. That's gotta be it.
The only thing I know for sure is that if there is trim given to a surface, that it will have a greater effect the faster it's flying.
Another issue is that they seem to fly differently every time...perhaps it's the wind speed being different each time. I had it dialed in so nice on the slopes, then took it out on a winch for thermaling and I just couldn't seem to get it to fly right.
Additionally, I recall the Radian Pro thread going into decalage mods to get it to fly better...and since there's a lot of interaction between the three forces, I figured it'd be best to start from scratch with the decalage, then re-dive test and adjust balance accordingly...but now I think I need to just work on one single plane, do one single thing at a time in small increments, and just experiment.
Then there's my personality trait of not wanting to do something experimentally without being sure of what I'm doing. My head was getting all confused trying to wrap around seemingly conflicting info, such as having up elevator trim, yet diving in dive-test...and now I'm just blah blah blahing without giving any useful info.
|Nov 09, 2012, 02:26 PM|
Had the same basic discussion several times in the hand launch thread and I am sure in this thread.
this is the one I started:
I went through the RC Soaring Digest also (I listed the articles from each I was interested in. Some may help you with this question). Lots of guys with many letters after their name discussing the validity of the dive test, etc.
designing a sailplane pt 3
Understanding thermal Soaring Sailplanes pt4 - Ballance, Trim and Vortex Drag
Flying in wind and weather
CG, Elevator, Trim and decalage - theory
Flying in wind and weather
CG, Elevator, Trim and decalage - practice
The joy of competitive soaring
Flying in wind and weather
CG, Elevator, Trim and decalage - The Utility
Vacuum Feed through fitting
Flying in wind and weather
CG, Elevator, Trim and decalage - Trimming techniques
Programming the X347
On flying upside down
Addenda to Programming the X347
Jer's Mold Making Part 1
And it goes on after that a few times as well.
What I got was this:
1. CG sets stability. Back is less stable but more readily indicates lift. Unfortunately, you will need to be on the sticks more to correct for the turbulence. Forward dampens turbulence in pitch but also can mask thermals. It takes practice, not settings to differentiate lift from turbulence.
2. Decalage sets trim speed. How fast do you want to fly? you can adjust the decalage for faster or slower flight. Flatter is faster.
In the end, it is a preference thing. Set all three the way you like the plane to fly. All three can change for some based on weather (wind and turbulence) conditions.
Unless you are way off, no setting will make a plane horrible or suoper fantastic. It is still the pilot behind the sticks that wins contests.
Enjoy the reading. I have.
|Nov 09, 2012, 09:09 PM|
Oh...this'll prolly happen a bit as I remember things as they come up...
I mentioned decalage because one plane pulls up in a dive and so I assumed it was needing tailweight...yet it has about 1.5-2mm up elevator trim to begin with...which, if given no other info whatsoever, one could assume it was noseheavy.
So conflicting, right? up trim=noseheavy, yet pullout from dive=tailheavy.
P.S. I've only just started reading all the wonderful references above, so hopefully not asking something that could have been answered already.
|Nov 09, 2012, 09:34 PM|
United States, MA, Waltham
Joined Dec 2001
Stalls get sharper and nastier as the c.g. goes back. A tail heavy glider may diverge without a correction. For instance, it may go faster and faster and faster, pitching down all the while. Or it may pitch up a little and get really slow, without correcting itself, and stall hard.
|Nov 09, 2012, 10:02 PM|
One day I want to do an instructional video on this subject as it would complement my prior productions and folks ocer complicate it. No offense intended. I have an outline and a title but I need do graphic work and I do not have the skills or patience for it. Perhaps in a year or two from now.
|Nov 09, 2012, 10:32 PM|
Joined Apr 2011
It's not really so confusing. Assume the plane is flying straight and level. It is staying level because it is balanced. By balanced, I mean that either it's weight is perfectly balanced and the horizontal stabilizer/elevator is not exerting any up or down force OR the weight is not perfectly balanced but the stab is exerting the perfect amount of up or down force to keep the plane level. Assume that the second case is true. When you put the plane in a dive, the nose weight does not change, regardless of the airspeed. However, the force exerted by the stab increases as the airspeed increases, so the balance achieved during level flight no longer exists. The out of balance plane will now either nose over even further, or recover from the dive, depending on whether the stab was exerting an upward force or a downward force during level flight. If the plane is nose heavy, then during level flight the stab had to exert a downward force to keep the plane balanced. When you dive the plane, this downward force increases but the nose weight stays the same, and the tail is forced down, which then forces the nose up, and the plane pulls up. If the plane is nose light (tail heavy) then during level flight the stab is lifting, and during a dive the tail lift increases, forcing the plane into an even steeper dive.
|Nov 09, 2012, 11:17 PM|
hmm...you're all saying what I thought was true, but previously adding nose/tailweight didn't yield satisfactory results.
hmm...perhaps I am a bit hung up regarding having the control surfaces neutralized for a neutralized flight, and need to ignore the "up-trim" for now.
Therefore, as I mentioned before, I need to start from scratch.
I need to do this more specifically than just "add/subtract weight".
I need to mark the current CG and make incremental adjustments and re-measure and re-mark CG...then fly.
Previously, I made adjustments on the slope (which means no way to actually check CG due to non-static/windy conditions) and then fly. I get too...what's the word?...pppffff and just "do something" rather than take an analytical and incremental approach.
It's good to know that, at the very least, the way I saw the plane was on the right track...but what I did with that info was not.
More importantly!!! I can't just go with three or more planes on one trip and try to remember exactly what each one is doing differently!
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