|Apr 16, 2014, 10:51 PM|
The Dive Test
Back in the day when trimming out your new prize sailplane included collecting data for fine tuning your CG location, from your successful madien flight we all used the Dive Test.
The Dive Test was simply pointing your plane into a 45 degree dive (preferrably from altitude) and noting whether or not the frontend lifts or continues the 45 degree dive.
The problem seems to be that different people recall the meaning of a frontend, differently. I had learned that a lifting frontend meant you were Nose Heavy.
What do you recall?
|Apr 16, 2014, 11:01 PM|
From a 45* downline flown hands-off where the glider is allowed to gain significant speed, if the glider:
- Pulls Up... it's noseheavy and/or has too much positive decalage
- Stays on the line... it's neutral and probably 0/0 decalage (best decalage for slope use)
- Tucks (steepens dive)... it's tailheavy and/or has negative decalage
A plane can be just aft of neutral and not tuck, which is why it's important to follow up a 45* downline test with a level inverted flight test.
From level inverted flight at cruise speed, if the glider:
- Requires forward ("down") stick to maintain level flight, it's noseheavy
- Flies hands-off inverted, it's probably neutral
- Climbs when inverted, it's probably tailheavy
Plank flying wings like the Moth or Weasel will also show a noseheavy condition when you roll them. Because of the extra "up" trim they've got (due to noseheaviness) they will tend to barrel roll instead of axial roll. Once the CG is moved back the roll will get nice and axial.
With a new plane, it's always best to remember the old saying:
"Noseheavy flies poorly, tailheavy flies once"
For aerobatics you want a neutral plane. It should go where you point it, and not anywhere else. I assume this is the case with most other types of high performance flying.
|Apr 16, 2014, 11:06 PM|
I'd say the dive test is irrelevant in determining CG if you have a symmetrical airfoil and the plane is trimmed neutrally
|Apr 16, 2014, 11:14 PM|
I'm sure pitching moments of the airfoil used come into play in determine exactly where the best CG position is, and a lot of other things.
I still use the dive test when dialing in aerobatics planes but mainly focus on the difference between upright and inverted flight. When there is no difference, then the plane is balanced properly
|Apr 16, 2014, 11:45 PM|
I think you've got something else going on with that Interceptor,.....maybe an incedence misalignment between the wing and stab, or some artifact of the airfoil used on the recut wing.
That little bird originaly was a Bat-outta-Hell
|Apr 17, 2014, 12:06 AM|
Some people still get this backwards and will defend their opinion violently, albeit still wrongly.
The thing to remember is that the aircraft is supported by the lift generated by the main
wing, at roughly the center of pressure (aka "center of lift"). Any non-symmetrical airfoil
will generate a forward pitching moment which pulls the nose down plus nose
weight, which also pulls the nose down. To counter that, you have a horizontal stab,
and elevator surface that generates negative lift which pulls the tail down. When a
plane is "nose heavy" it means you have more weight pulling the nose down, and the
only way to counter it is with more up trim, and thus more negative lift from the
The basis of the dive test is simple. The weight pulling the nose down is static,
but as you increase air speed, the negative lift generated by the horizontal stab and
static up elevator trim increases, which causes the force pulling the tail down to increase, and
the nose to pitch up. If you remove some of the nose weight, then less static up elevator
trim is required to balance the nose (you must re-trim for the new CG), and it flies neutral.
Remove even more, and now the CG ends up at or behind the center of lift,either the
forward pitching moment of the airfoil dominates, or it actually requires down trim to
hold the tail up. Again, as you increase airspeed, the pitching moment and/or positive lift
of the tail increases, causing the nose to pitch down.
One more thing that confuses folks is the difference between Center of Pressure and
Mean Aerodynamic Center (MAC) which is usually in front of the center of pressure.
People often balance their planes at the MAC but for slope planes that usually results
in a plane that feels nose heavy. If you want maximum efficiency, you'll likely find
yourself moving the CG well back from the MAC, closer to the Center of Pressure, but
may find that the plane becomes more neutral at the top end of its speed range.
|Apr 17, 2014, 12:40 AM|
And Chip, you are right...it's darn hard to trim a plane when one has no idea if the trailing edge is up/down/neutral without any sort of reference point other than "that seems close"...especially with it's particular planform.
My Interceptor isn't fully symmetrical either. Dunno if it's supposed to be. Wing was cut by prior owner 20+ years ago...so he said. He does lots of cutting/bagging, etc.
|Apr 17, 2014, 12:46 AM|
Another thing i've found is that cg is very personal preference-based.
I got my Spider60 from Lars...he likes nose heavy. I moved cg from 55mm to 80 and now it's almost perfect! BUT!!! If I were specifically doing higher altitude thermaling, I'd certainly, definitely want more nose heavy, as it can be hard to tell if the plane is pitching one way or the other until the speed becomes significantly faster/slower...and that porpoising that I HATE on the slopes becomes a valuable tool at altitude.
Just my .02
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