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Airfoiled Stab, CG and Stall Speed - 1/7/2008 5:27:54 AM   
efish


 

Posts: 459
Joined: 8/12/2005
From: Singapore, SINGAPORE
Status: offline 		Hi, I'm having some problems setting up a 65.5" wingspan Extra 330. No specs from the manufacturer, but I approximate the wing area at about 900 sq inches by pure guess work. The plane has airfoiled rear stabs, elevators and rudder. Think I read somewhere that an airfoiled horizontal stab had the effect of increasing the effective wing area and allowed the use of a more rearward CG.

I first flew the plane with a 26cc gas engine that seemed to balance the plane nicely with 2 batts in the engine box. AUW was about 9 lbs. Power, take off and general flying was no problem, however, I found that the wing loading seemed to be all but destroyed - all was well above about 1/3 throttle, but below that, the plane would tip stall unpredictably. Landings had to be hot otherwise the plane would just seem to quit flying. Thrust line tests both in level flight and on vertical lines showed the factory setting to be reasonably close. At this point, the CG was setup just behind 1/3 of the root rib chord. Even then, it took quite a bit of up elevator trim just to hold level flight. Rolling inverted while climbing at a 45 degree angle, the plane would hold the climb before gradually pitching towards the ground.

My theory was that the gas engine was too heavy and that I needed to lighten up the AUW. I have on hand a FZ-110 that puts out more power than the gas engine but weighs about a lb less. As I type this post now, I'm reconsidering - could it be that I had the plane too TAIL heavy even with the gas engine? - that would account for the seemingly optimum results during the inverted 45 degree climb test but wouldn't explain the up elevator trim needed for level flight.

With the YS, the plane seems rather tail heavy - CG is about middle of the root rib, so I'm loathe to risk the plane by flying like that. I generally like my CG towards the tail, but this plane is something of an unknown and doesn't appear too forgiving so far! And correcting the CG by adding weight would negate the benefits of using a lighter engine.

Would have thrown in the towel, except that the plane flies decently at speed and does show some promise. Besides, I'm hoping to salvage some of the considerable time and expense I've already put into the plane.

Does anyone have any suggestions/theories?

Thank you.
       Post #: 1

RE: Airfoiled Stab, CG and Stall Speed - 1/7/2008 8:05:54 AM   
BMatthews



Posts: 8893
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline
quote:

ORIGINAL: efish

Hi, I'm having some problems setting up a 65.5" wingspan Extra 330. No specs from the manufacturer, but I approximate the wing area at about 900 sq inches by pure guess work. The plane has airfoiled rear stabs, elevators and rudder. Think I read somewhere that an airfoiled horizontal stab had the effect of increasing the effective wing area and allowed the use of a more rearward CG......


First off your model will have symetrical airfoils So it's not the same as a lifting airfoil. Symetrical equals flat plate as far as that goes.

Second it's not the airfoil choice at the tail that determines the CG and lift or not. The allowable CG position is related to the wing area, mean aerodynamic chord, tail length and tail area. That relationship determines the overall aircraft neutral point and the CG needs to be at or ahead of that point for nondiverging flight. On some designs the tail length and stabilizer area are both long and large enough that the neutral point is well back on the wing. In some extreme cases it can be near the wing's trailing edge. On such setups IF YOU CHOOSE TO PUT THE CG THAT FAR BACK then the stabilizer will have a positive lift coefficient of a fairly high value. At that point is is worth using a cambered airfoil to help the stabilizer carry that lift coefficient in a more efficient manner.

In your case this isn't really an issue. Although any time we have the CG positioned back behind the wing's 25% MAC point the tail is operating with a positive lift coefficient rather than a negative one. But until you have a model that has a neutral point position that is back around the 50% chord point there's no point in trying to use a cambered airfoil back there.

That help?

Check for the Neutral Point calculator web site in the stickied Aerodynamic Tools thread and go input your numbers and see where the NP is located on your model. You don't want to fly with the CG at or behind that point until you test it out more. If your present CG is at or behind that point you'll need to add nose weight or perform surgery on the rear portion to get the balance point ahead of the NP by at least a small amount like 1/2 to 1 inch. Even then it'll be REALLY neutral feeling in flight.


< Message edited by BMatthews -- 1/7/2008 8:09:16 AM >


_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to efish)
       Post #: 2

RE: Airfoiled Stab, CG and Stall Speed - 1/7/2008 8:15:44 AM   
pimmnz


 

Posts: 499
Joined: 11/9/2005
From: Auckland, NEW ZEALAND
Status: online 		Efish, you need to work out the MAC and balance the model where indicated (30% MAC?). The stall problem may be weight related but no matter what you put up front you will always need to ballast one way or the other to get the cg in a safe range. There are plenty of posts in this section which will point you to a Cg calculator where you simply plug in the direct model measurements and it will calculate it for you. The root rib measurements you are using do not equate to the actual MAC CG position, only where it ends up. A quick indication will be to mark on the wing with a soft pen the chord half way from root to tip and then measure 1/3rd the back from the leading edge on that line. This will give an approximate safeish cg. Check that there is no wash in on the tips either as this can lead to nasty stall characteristics. Highly tapered wings will also have the same effect. It may be that the actual model design is the culprit. In the end you might just be trying to fly this particular design too slowly.
Evan, WB #12.

(in reply to efish)
       Post #: 3

RE: Airfoiled Stab, CG and Stall Speed - 1/7/2008 9:00:28 AM   
efish


 

Posts: 459
Joined: 8/12/2005
From: Singapore, SINGAPORE
Status: offline 		Thanks so much guys.

For most of my models, I generally just eyeball about 1/3 the MAC and work out my preferred CG through repeated flights. I've got a soft spot for 3D flight, so quite a few of my models have the CG well aft of the recommended range. Just a little concerned here cuz of the cost and safety issues.

Bruce - thanks for your explanation on the symetrical rear stab - one of those things that I've always pondered on; makes sense. The NP concept is new to me, too. Thanks. Will look it up, plus run the numbers and see where the NP comes out to. At least that's a plan!

Evan, thanks for reminding me to check for wash in. Forgot to have a look. Was hoping to avoid having to do any extensive reengineeering to balance the plane.

Frankly, although I really really hope otherwise, I think the model design may just be the culprit.

Thanks again.

(in reply to pimmnz)
       Post #: 4

RE: Airfoiled Stab, CG and Stall Speed - 1/7/2008 8:41:19 PM   
feihu



Posts: 178
Joined: 11/17/2002
From: Sun City, AZ, USA
Status: offline 		efish:

If your weight estimate of 9lbs and your wing area is 900sq in, then your wing loading is 23oz/sq ft; which is fine for that size airplane.
Your CG location at 1/3 back at the root rib is NOT 30%MAC, so you could be flying near the borderline of pitch instability.
pimmnz has given you a way to locate a "safeish cg" - so please don't overlook that! And with a corrected cg, you may not require washout.
Also, the increased weight to rebalance would still leave you with an acceptable wingloading.

feihu

(in reply to efish)
       Post #: 5

RE: Airfoiled Stab, CG and Stall Speed - 1/8/2008 1:12:08 AM   
da Rock



Posts: 6370
Joined: 10/11/2005
From: western, NC, USA
Status: offline 		Want to find out the safe CG range the easy way?

It takes a yardstick and 10 minutes.

http://www.geistware.com/rcmodeling/cg_super_calc.htm

(in reply to feihu)
       Post #: 6

RE: Airfoiled Stab, CG and Stall Speed - 1/8/2008 9:42:55 AM   
efish


 

Posts: 459
Joined: 8/12/2005
From: Singapore, SINGAPORE
Status: offline 		Thanks feihu - that's just the problem. 9lbs was the figure off a set of reasonably new and fairly accurate bathroom scales. I ran the wing loading figures myself before maidening the plane and it seemed acceptable. But I can tell you that the plane handled like a dog at slow speeds. I've had a few planes this size and smaller with heavier wingloading figures that handled so much better at slow speeds. I did worry about the CG being too far back as well, but like I said, in the air at speed it handled pretty much like it should. Knife edge flight was ok but I was tempted to try shifting back the CG even a little more just to see if that would add more rudder authority. I've flown tail heavy planes but this one just didn't feel tail heavy at all.

Da Rock - thanks, that does sound like a quick way to work out a cg range. Will use that as an indicative range to start with.

But I guess after all that's said and done, a nose heavy plane is easier to handle, so I'm thinking I might as well go back to the gas engine and play with the CG. Don't really need the extra power (which I will lose by adding weight) or cost of running glow.

(in reply to da Rock)
       Post #: 7

RE: Airfoiled Stab, CG and Stall Speed - 1/8/2008 12:45:06 PM   
da Rock



Posts: 6370
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

ORIGINAL: efish

But I guess after all that's said and done, a nose heavy plane is easier to handle, so .............


A nice thing to believe in, but not always what happens.

If you don't know where the CG range ends fore and aft, that old idea will give you an airplane that seems to handle very sedately when nose heavy. But in fact, since most modelers don't adjust the elevator throws to match the CG movements, what most get is simply a less and less effective elevator. Which they interpret as "easier to fly". Don't let your farther forward CG go too far forward. Without knowing the CG range, you got no hope of doing that.


(in reply to efish)
       Post #: 8

RE: Airfoiled Stab, CG and Stall Speed - 1/8/2008 1:12:01 PM   
efish


 

Posts: 459
Joined: 8/12/2005
From: Singapore, SINGAPORE
Status: offline 		Well guys, I'm humbled. Always been impressed by the Geistware site, but for some reason never explored the calculators there - my loss. So I spent some time taking measurements and crunching numbers.

First, my estimate of the wing area was wildy off. According to the calculator, the plane only has 764 wing area (excluding the 189 tail area). So I guess my first thought that I had destroyed the acceptable wing loading was correct.

Second, the "Ideal CG" is aft of what I had flown the plane with - thus partially explaining the amount of up elevator trim required and perceived lack of rudder authority.

So I guess I can conclude that no gas engine has sufficient power to weight to fly the plane without dramatically affecting the wing loading to such an extent that it becomes unworkable.

Next, the plane presently balances with the CG on the NP. Will heed Bruce's advice and at least for the first few flights, add enough weight on the nose to get the plane to balance on the "Ideal CG". Fortunately, the distance between the two is only about an inch, so hopefully it won't take too much.

Just thought I should post my findings just in case someone else faces similar issues. Guess preference and approximations have their use, but you really can't run from the physics of flight.

Thanks again for your help and patience, everyone.

(in reply to da Rock)
       Post #: 9

RE: Airfoiled Stab, CG and Stall Speed - 1/28/2008 11:26:48 PM   
nmking09



Posts: 382
Joined: 10/14/2004
From: Champaign, IL, USA
Status: offline 		You can get a larger cg range by choosing an airfoil that has a steeper cl vs alpha slope. Most symmetrical airfoils have a sharper clalpha than a flat stab, giving a larger cg range.
-Trust me.

But it does sound like you are nose heavy.

_____________________________

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RE: Airfoiled Stab, CG and Stall Speed - 1/29/2008 12:39:12 AM   
Villa



Posts: 860
Joined: 8/12/2003
From: Wilson, NC,
Status: offline 		Hi efish
In your first post and in a later post you state, or imply, that on this model the horizontal stab produces a vertical UP vector because it has an airfoil. I think that is wrong. The horizontal stab on your plane pushes down. There are other more knowlegable people following this and I hope they will dive in and convince you because I am not familiar enough with aeronautical teminolegy and a theory to make a good argument. I do know that the center of lift is behind the CG which results in a moment or coupling in the direction of nose down. The hor stab counters this moment by generating a negative lift.

(in reply to nmking09)
       Post #: 11

RE: Airfoiled Stab, CG and Stall Speed - 1/29/2008 12:45:38 AM   
BMatthews



Posts: 8893
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Villa, the stabilizer only lifts downwards when the CG is located quite far forward. Like at the 25% and further forward point. When it's back at the 28 to 35% point as found on many models this turns around and the stabilizer is lifting up just like the wing.

What keeps the model still pitch stable is that the wing is still lifting a lot harder at any given angle of attack than the stabilizer so you still have a positively stable model.

Go back up and read my first reply to this thread for a bit more explanation.


_____________________________

Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to Villa)
       Post #: 12

RE: Airfoiled Stab, CG and Stall Speed - 1/29/2008 1:03:14 AM   
da Rock



Posts: 6370
Joined: 10/11/2005
From: western, NC, USA
Status: offline
quote:

ORIGINAL: nmking09

You can get a larger cg range by choosing an airfoil that has a steeper cl vs alpha slope. Most symmetrical airfoils have a sharper clalpha than a flat stab, giving a larger cg range.
-Trust me.

But it does sound like you are nose heavy.



Do you have any references that explain the phenomenon you're describing? Something that shows the formula that applies the airfoil's "alpha slope" and the resulting increases to the CG range?

The CG range is basically controlled by how big the tail is and how far back compared to the shape and size of the wing, and not much else. As aerodynamic formulas go, the CG locating ones are pretty simple. They need some wing measurements, the tail measurements and the distance the tail is back from the wing.

(in reply to nmking09)
       Post #: 13

RE: Airfoiled Stab, CG and Stall Speed - 1/29/2008 2:54:10 AM   
nmking09