Forward CG

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danerazz
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Forward CG

Post by danerazz »

So quick update:

I only flew my highlander twice last year due to a bunch of unrelated personal obligations.

When I did, my CG was calculated to be just short of 10” with the recommended range 9-15”.

The flight characteristics were NOT GOOD. I was getting tail stall and never got the wing to stall in testing. This led to very fast, flat, landing attitudes and a very unnerving landing characteristic. I COULD NOT get the plane to achieve what I would call a “normal landing attitude.”

Between the first and second flight I added VGs to the wing and underside of the tail.

Over the (long, painful) winter here in Maine I added 10 lbs of ballast to the tail, and yesterday before I flew I added 3 gallon jugs of water to the back of the baggage compartment totaling about 24lbs.

This moved my calculated CG to just slightly aft of 12”, roughly center of the envelope.

The result is the wing now stalls before the tail (as it should) and landings are much more docile and predictable.

I have not yet calibrated my ASI, though it roughly matches a GPS (haven’t been out of the traffic pattern yet). Pre-ballast the tail was giving way at about 40mph indicated and I could not achieve level flight below that speed. If I increased my pitch I had to be going faster and it would climb. Slower and the tail gave up. I could not descend or maintain level flight with any kind of nose up attitude (“slow flight” was not possible).

After moving the CG, the wing clearly stalls, though I don’t know what speed since I am now at the bottom of the ASI range when it happens. There is a slight wing buffet and tendency for the right wing to drop a little during the stall. Mostly benign stall behavior. I can achieve a normal landing attitude and land like I think I should be able to.

More testing is needed, but I’m pretty sure my forward CG limit is going to be something AFT of the factory recommendation.

I have NEVER had a tail stall in 11,000 hours of flying, and it is disconcerting.

This thing definitely flies better with a more rearward CG!
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SheepdogRD
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Re: Forward CG

Post by SheepdogRD »

There are a lot of Highlanders built by relatively low-time pilots, myself included. This could have been really ugly for someone unprepared and inexperienced. For this first-of-its-kind engine installation, I'm glad you had enough experience to handle the unusual behavior. Well done, Dane.

What's next in your testing?
Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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danerazz
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Re: Forward CG

Post by danerazz »

Just more landings, reliability and performance testing. Number one is check the ASI accuracy. I will test with the water ballast removed. It should be fine, but I had added 10lbs. of lead to the tail in addition to the water jugs in the baggage compartment.

My first ever landing last year the tail stalled and drove me into the ground causing me to bounce. I hit the power to go around, but my idle mixture was not set right (too lean, my fault) and the engine quit. Fortunately I pushed the stick forward as it went airborne then jerked it all the way back as it settled after the engine quit resulting in a not-bad touchdown.

Second landing was a little better: I had the idle mixture set right. Still tail stalled.

CG moved aft, I did 4 landings at 12” aft of datum and it was what I think it is supposed to be like.
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BDA
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Re: Forward CG

Post by BDA »

*SuperStol*
I put 60# sand bag against bagage bulkhead for all testing on the first flight.
Granted I am testing some radical changes these days, but it likes the weight.

Dont add Lead - put your tool kit in the tail, it should always be with you. And it should not change in weight substantially.

I made a large extended bagage all the way to the vertical stab for large bulky engine/ wing covers that alows for aft tool kit.
But you can do a very simple small “oil door” at the bottom longeron for your tool kit.

Billy makes a nice small one.
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danerazz
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Re: Forward CG

Post by danerazz »

Yeah, I needed 15lbs of ballast to balance my engine. That put me “in the range” when empty with no threat of loading aft CG when loaded. The 15lbs of lead is all the way in the tail. It would have had to be nearly 30lbs if it was in the baggage area.

I didn’t do any massive airframe changes. I DO wish I had put a baggage access door in the side. It is a LONG reach to get to the back of the baggage area!
Keye
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Re: Forward CG

Post by Keye »

Something else that may help is sealing the gap between the elevator and the horizontal stab; if you haven’t already done this.
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SheepdogRD
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Re: Forward CG

Post by SheepdogRD »

Any subsequent builders with the radial engine should probably build an XL. It would have been nice to have that option when you started the build...
Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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danerazz
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Re: Forward CG

Post by danerazz »

Yes, I definitely would have built an XL if it was available at the time!
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BucF16
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Re: Forward CG

Post by BucF16 »

Dane Keeps posted! I retired last week from AAL. After I Finnish the honey-dos I've put off for YEARS, I finish my Apex/Highlander. hopefully in early August. AND THANKS (to whom ever) FOR FIXING THIS SIGHT!
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SheepdogRD
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Re: Forward CG

Post by SheepdogRD »

You're welcome, Bruce. Glad to be of service, especially to my fellow sheepdogs.
Richard Holtz
Highlander N570L -- Ms. Tonka -- in gestation

If just enough is really good, then too much ought to be perfect.
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BucF16
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Re: Forward CG

Post by BucF16 »

8)
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