Novel Pitch Trim Control

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GESchwarz

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This thread is a take off from the the thread titled "Novel Flaperons", because it has to do with the use of the trailing surfaces of the wing to perform a little different function.

My tandem seat, pilot in front design has an aft shifting CG problem to contend with. Can I address this by having partial or full span flaps that deploy aft 3 or 4 inches to shift the center of lift aft?

When I want to apply flaps, the surfaces are deflected downward in a conventional manner, whether they are in the forward or aft position.

What issues would I have with this design?

Is there a better way to address the rear CG instead of elevator/stabilizer trim?
 

GESchwarz

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Move the battery forward, or lengthen the engine mount, depending on the severity of the problem.
The plane is still under construction. The fact that it is a tandem seat, passenger in back, means that this is an issue that has to be addressed as a matter of a normal operating procedure. I am wondering if a wing with a variable chord would be a viable solution.
 

TFF

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Lots of complex answers for probably very small gains. I have one friend who has movement disability and he has to fly his RV8A with some ballast in the back solo. Another with a 8 sometimes does. but not a all the time. And another sold his 8A after he finished his 7 and one of the reasons was to get rid of the tendency. I would have a water ballast tank. Full is normal solo; drain put passenger in; keep flyable CG if empty. Trading utility for a little performance loss. Adding complexity will probably add more overall weight than the water ballast.
 

Radicaldude1234

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Without correction, what's the static margin at the aft-most CG? That and what's the desired amount?

The simplest solution might be be to be slightly nose heavy (over stable) when lightly loaded and slightly tail heavy (but still safe) when fully loaded.
 

wsimpso1

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My solution uses a bit of Rad's comment, plus my tail arm is long and tail volume coefficient is on the high side. Mine is mostly to allow some rather aft baggage. Analytically, at forward CG, I can still flare to stall plus a little; at aft CG, I still have stability margins at full power. Testing will tell me if I need to add a water vessel in the tailcone for full forward loading... Hmm, might be good to build it now, and have it ready.

Billski
 

wsimpso1

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Compute your retracted vs extended 1/4C difference, then your new neutral point. I suspect that you will get very little movement in the neutral point for all of that flap complication. Adding a bit of tail volume by running a little longer arm is really pretty efficient. A water vessel aft might be a really good idea for those solo days.

Billski
 

Hot Wings

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ballast is the better solution.

Thank you!
Rather than water ballast how about the method used by Rotorway - a small weight that is moved to one of 2 positions depending on if solo or dual? It is always with you and draining filling water may not be an option all the time.
 

VAPORTRAIL

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Seeing as you are still designing, how about 2 fuel tanks 1 fore 1 aft and a transfer pump to put the fuel (weight) where you want it.

You gotta have gas on board anyway, use it to your advantage, no worthless ballast required.
 

Jay Kempf

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Rather than water ballast how about the method used by Rotorway - a small weight that is moved to one of 2 positions depending on if solo or dual? It is always with you and draining filling water may not be an option all the time.
Water can be shuttled back and forth as well between two tanks. Always with you.

But it can leak and people can forget to shuttle and also I suppose it could freeze but it could just be coolant good to like -50F.
 

GESchwarz

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Seeing as you are still designing, how about 2 fuel tanks 1 fore 1 aft and a transfer pump to put the fuel (weight) where you want it.

You gotta have gas on board anyway, use it to your advantage, no worthless ballast required.
Too many people have died as a result of even the simplest of fuel system nuances. I have a very simple and robust fuel system design that I am happy with. No nuances, everything about it is designed to ensure that fuel gets to the engine, period.
 

SVSUSteve

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Too many people have died as a result of even the simplest of fuel system nuances. I have a very simple and robust fuel system design that I am happy with. No nuances, everything about it is designed to ensure that fuel gets to the engine, period.
Exactly. Fuel systems are probably the number one system on an aircraft you don't want to add complexity to especially in a homebuilt where you don't need anything additional to increase the risk of your engine failing. So many pilots screw up something as simple as a "LEFT-RIGHT-OFF" that it would be hilarious if it hadn't killed and injured a lot of good people over the years. That's exactly why the Praetorian is designed with a "BOTH" setting for the fuel system (to minimize the risk of lateral imbalance) and it won't be moved except to shut down the engine or in the event of a leak or other abnormal situation (to minimize the chance of human error).
 

mcrae0104

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When designing a system for a plane--especially one outside of the norm--I would think that one of the most important questions you could ask is, "what could possibly go wrong?" and then being very imaginative and pessimistic in how you think things will actually go. It's tough to be honest with yourself about risk assessment when designing/building your own plane can be such an emotional thing.
 

GESchwarz

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There is simply no place for emotions anywhere in the design, manufacture, maintenance, and operation of an aircraft. Emotions and other forms of fuzzy thinking are the root cause of perhaps all fatal accidents.
 

SVSUSteve

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When designing a system for a plane--especially one outside of the norm--I would think that one of the most important questions you could ask is, "what could possibly go wrong?" and then being very imaginative and pessimistic in how you think things will actually go. It's tough to be honest with yourself about risk assessment when designing/building your own plane can be such an emotional thing.
A very experienced aeronautical engineer once told me that the best way for a conscientious person to pick out human factors failures mode that most would classify as due to "stupidity" or "hubris" is to drink some whiskey and think about how you'd do whatever is in question. As he put it, "Stupid or egotistical pilots think a lot like drunks. They don't think about their actions or their consequences."

That's one reason why I am going to build a wood and cardboard mockup of the cockpit and use flight simulator software to get a sense of the "flow" of different scenarios. It will also help to get things positioned where they are most convenient and logical.
 

Aesquire

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Pumping fuel around for ballast had led to at least one crash of the B-1b. Big change in center of lift on a swing wing.

The other problem of fuel as ballast is you get all this unusable fuel. And you get stuck with flammable ballast, in the fuselage with you.

The Sparrowhawk sail plane uses a steel or aluminum tube in the horizontal stabilizer mount you choose when assembling to compensate for different pilot weight. The cleanest solution I've seen for ballast.

Ballast is seen as a failure by engineer types. It means you didn't get your sums right or you missed a more elegant solution. Ymmv.

The original Rotorway helicopter had you move the battery to change balance solo or dual. The weight you move on the current model is less pure from an engineer view...... but worlds better from the pilot's.
 

Rockiedog2

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The Zenith 701 with a Rotax 912(originally designed for a 2 stroke) usually runs nose heavy so most guys put the battery behind the seat; no worries. Ours had a 16# batt/#4 welding cable back there and I went to a 4# lithium on the firewall so obviously nose heavy now. A while back I had built a new sharper leading edge on the HS and installed a 1" tube/access door in it in anticipation of the batt change. Can put any round bar in there and it being so much further aft than the old 16# batt location the ballast required is obviously less. I'm still fooling around with loads and cgs but looks like most forward load is gonna require ballast about a 6# net loss. I despise ballast but could accept it in this particular case.
My dad built a Bakeng Duce way back and the pilot sat in front. When a pax was on board we had to be careful it was unstable in pitch. It mostly got flown solo. If I was still in the guessing phase(I can't in good conscience claim "design" phase for myself) and it looked like the battery wasn't gonna balance it out I would have to change something...drastic if that's what it took. Previous talk about planes/design/emotion...I can get downright emotional about ballast.
 
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