HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Sep 05, 2012, 07:56 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
Discussion
Glide Test

So I built a solid balsa wing and mounted it to a fuselage and gave it a glide test. The test seemed to have mixed results. I'd toss it and it would start out straight and then at the end of the glide it would flip over and cartwheel. I gave it a ton of opposite rudder and it would stay straighter longer, but even then the glide ended in a skidding turn.

It's just a flat plate really with a rounded front and sloped back half, cant call it high precision. Could there be that much difference in lift from one side to the other?

Could moving the CG forward stop it from wanting to roll over at the end of a glide?

Could it just need more dihedral?

It needs something to fly nice.

I tried it with a motor, BTW, and it started out flying and then it stalled and spun in. I noticed the Rx was acting up when I tested it after the crash, would lose link and the servos didnt like that. Not sure if that caused the crash or was the result.
HELModels is offline Find More Posts by HELModels
Reply With Quote
Sign up now
to remove ads between posts
Old Sep 05, 2012, 11:47 PM
KE your cub.
Curare's Avatar
in the gutter, again....
Joined Jun 2005
4,200 Posts
If it can't fly straight and level, then you've got something wrong. If it constantly falls over one way, I'd suggest you have some form of assymetry in your aircraft. Recheck all your dimensions, angles and check your wing and tail for warps too.

Also your tail looks a little short...
Curare is offline Find More Posts by Curare
Reply With Quote
Old Sep 06, 2012, 12:39 AM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
OK, thanks. Yeah, I'm trying to see where the problem is and cant really find it. I used a contour gauge to compare and it matches The only thing that has changed is the wing, so makes sense that's where the problem is, just cant find it. With the original wing, it flew fine and handled fine.

I ruined the original wing when I installed an autopilot and in FBW mode it went into a dive and folded the wing and crashed and prop strike sliced a hunk out of the trailing edge.. I figured time to build one.

Back to what I observed during the glide test. It seemed like it needed speed. Maybe I could lower the incidence and just fly it faster?
HELModels is offline Find More Posts by HELModels
Reply With Quote
Old Sep 06, 2012, 12:43 AM
12th Pursuit Squadron
TheAeronut's Avatar
Garland, Tx.
Joined Mar 2002
1,111 Posts
I have three suggestions:

Yes, more dihedral. This won't fix the tip stall problem but will fix the lack of roll control with rudder input (flat skidding turn).

Add several degrees of washout to help with the tip stall problem.

Check the plane for lateral balance. My guess is that part of the problem is that the wing or whole plane is heavier on the side that it falls off to. That, and possibly one side of the wing is a bit longer than the other. Check for dimensional symmetry first then lateral balance.

J.P.
TheAeronut is offline Find More Posts by TheAeronut
Reply With Quote
Old Sep 06, 2012, 01:00 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
Thanks, J.P. I cut the tips at the last taper and took a wild guess on what the angle should be and sanded accordingly. I checked the lateral balance and sure enough it wanted to lean to the left wing. As for which wing is longer, the left is longer by about 5 mm. That actually favors falling off on the right.

I might give it another try.
HELModels is offline Find More Posts by HELModels
Reply With Quote
Old Sep 06, 2012, 02:50 PM
12th Pursuit Squadron
TheAeronut's Avatar
Garland, Tx.
Joined Mar 2002
1,111 Posts
The new tip polyhedral looks good. Still, if you have not already, get the wing centered as closely as you can measure. Once that is done, add weight (tape, pins, whatever) to the light tip to get the lateral balance as close as you can.

Until you remove all asymmetries you will be fighting the controls and trim changes contantly.

J.P.
TheAeronut is offline Find More Posts by TheAeronut
Reply With Quote
Old Sep 06, 2012, 03:30 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
Thanks again, J.P. I gave it another glide and it flew much better, straight. I took all the trim out before I tossed it too. It's starting to act like it wants to fly.

Just need to replace the servos before I test power again.
HELModels is offline Find More Posts by HELModels
Reply With Quote
Old Sep 07, 2012, 01:13 AM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
I might be able to give it a try tomorrow.

I got the HS-55 servos in and what a difference they are. I had HK junk in there. The Hitec's are silky smooth and quiet.
HELModels is offline Find More Posts by HELModels
Reply With Quote
Old Sep 07, 2012, 06:16 PM
Registered User
Joined Sep 2012
2 Posts
I don't see the wingspan.

Tiny abberations (warps or mismeasurements) in SMALL planes show up as radical unwanted performance.

For instance, if you are off by 1/8th an inch in a 120 inch wingspan model and it flies great - - - - and you build a smaller version of a 24 inch span with that 1/8th inch error then the effect of the error will be 6 times greater than in the large 120"
model.

Then where were you testing? Was it indoors or outside?
HybridFlier is offline Find More Posts by HybridFlier
Reply With Quote
Old Sep 07, 2012, 07:16 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
Yeah, those tiny errors add up sometimes they cancel each other if lucky. My basic design has proven itself before, so I know it isnt proportions or the tail alignment. This wing is hand sanded so lots of opportunity for those tiny errors.

Anyway, I've been glide testing off a grassy hill. I just did another glide and it was pretty good. The wind was in my face at the start but I saw it get hit by a side gust. What was good was it got upset by the gust and then the dihedral kicked in and it righted itself. That's a major improvement.

Maybe tomorrow I'll feel brave enough.

Almost forgot, the wing span is 42"
HELModels is offline Find More Posts by HELModels
Last edited by HELModels; Sep 07, 2012 at 07:22 PM.
Reply With Quote
Old Sep 11, 2012, 03:26 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
I'm ready to try it again. It balances right, servos are good, motor is attached. Why am I nervous? Could be that I havent been able to put together two successful flights in a row since they cut me open.
HELModels is offline Find More Posts by HELModels
Reply With Quote
Old Sep 11, 2012, 05:17 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
OK, that was a relief!! I took it out and flew it with power and brought it back in one piece. I cheated though, I installed the autopilot and put it in stabilize mode for takeoff. That worked like a champ as it allows nudging and it needed some to climb. I waited until it was a couple hundred feet up and switched it to manual with no surprises. I landed in manual after flying with the buzzards. As it was making the landing approach, glide was straight as an arrow. The added dihedral was noticeable in turns. I'd let go of the stick and it would return to level.

One thing I noticed is it just wants to climb with any power. Should I tilt the motor down to counter that?

Thanks
HELModels is offline Find More Posts by HELModels
Reply With Quote
Old Sep 11, 2012, 09:32 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
Joined Oct 2002
11,619 Posts
Climbing under power can come from two major factors. First is the location of the thrust line in relation to the rest of the model. Second is the pitch trim and CG location teaming up to produce a model which is TOO stable in pitch. Such a model can be tricky to fly as it wants to raise the nose with ever little minor gain in flying speed be it from power applied or simply from a little bit of a dive in a turn or some other maneuver.

Your model clearly has the thrust line up at or above the main drag creators so you're likely find there.

So that leaves two things in your case. One is the glide trim and CG location. You can test that out yourself by getting up high and trim for a steady and slow but non stalling sort of glide. Then nose over to a 40 degree dive and let go of the elevator stick so it comes back to neutral. Watch the way the model pulls out of the dive. You want it to have a reasonably slow and lengthy recovery without it looking like the recovery is a little "iffy". For a model size such as yours I like to see it return to level before it lifts up past to another stall over an altitude loss of about 50 to 60 feet.

If it tends to recover with a sharp rise of the nose into a really sharp stall then the model is also going to respond to power by raising the nose equally strongly.

It seems counter intuitive but if the model snaps the nose up really quick it means that your CG location is too far forward. The proper steps then are to move the CG back a little and re-trim the elevator to regain a nice level glide.

Along the way with all this you might find that the elevator gets quite sensitive to inputs. If that happens reduce the amount of throw by moving the pushrod mount points outwards at the control horn or inwards on the servo arm.

Pitch trim is sort of like Goldilocks and the bear's porridge. You want some but not too much OR too little. The key is to learn where the happy "some" amount for each design occurs with the CG at the best point.

From there in your case the prop blast over top of the stabilizer and elevators may be causing some funky interactions as well. If you work with the glide and do the dive tests and adding power still produces a strong nose up response then you may be causing it by the prop blast being over the tails in the V area only. In that case SLIGHTLY tilt the back of the motor down a little. Only a degree or two at a time though. If you can just barely see the adjustment it's enough to try for that time.

Also it's entirely normal for any model or real plane to want to climb with adding power. The key is to set it up so that it's not an over whelming response.

Real pilots work with this by using power to control their rate of climb and elevator trim to control the airspeed. It seems all wrong but that's what they do.

The thing is that the airplane wants to fly at a given speed set by the elevator trim. It doesn't matter if it's in the glide with no power at all or if it's at full power. The response to power is to fly faster which makes the model want to raise the nose just like it did when it sped up during the dive testing. It'll raise the nose until it's back to the trimmed flying speed. If the power you add is enough it'll be in a climb at that airspeed.

So to actually speed up a full size pilot actually alters the trim of the aircraft so that it wants to fly level at a faster airspeed with that amount of power.

Now we modelers tend to not like that so much. So we tend to use downthrust and other tricks to minimize or eliminate that tendency. I'd suggest that you temper the amount of climb once you optimize your stability and CG location but at that point you allow the model to retain SOME of the climb with power. Enough that it doesn't shock you but still leave some in there. If you simply want speed either click in a little down trim or simply hold a hair worth of forward stick for the time you want to simply go fast.
BMatthews is online now Find More Posts by BMatthews
Reply With Quote
Old Sep 11, 2012, 10:07 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
Thank you, appreciate the refresher. The previous version was right as it would climb with power, but it wasnt excessive. I have a little latitude with the CG and where I put stuff. The good thing about the stabilize mode of the autopilot is that it knows nothing about the airspeed in that mode and keeps it level. That's fine as long as there's enough airspeed from power.

I'll try it tomorrow maybe and give it a dive test first before I tweak the thrust angle.
HELModels is offline Find More Posts by HELModels
Reply With Quote
Old Sep 12, 2012, 05:17 PM
Registered User
HELModels's Avatar
Joined Aug 2003
2,655 Posts
I'm getting tired of this, crashed it. Took it to the same spot, almost identical conditions and crashed on launch. That did something to the wing which wasnt visible. I gave it 3/4 throttle instead of full and it went up. I'm making the first circle back and the wing snapped off. Sure, I should have just packed it in after the first crash and taken more time to inspect.

I really used to be able to fly these things. My IQ must have dropped 100 points after surgery and I only had 100 points to start.
HELModels is offline Find More Posts by HELModels
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Testing C of G; Test Glide? Minimole Electric Plane Talk 12 Mar 14, 2006 12:34 PM