Horizontal tail construction

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Lendo

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John Roncz suggests 0.045 volume coefficient for the VT Area of small aircraft and that anything under 3 time the MAC (25% MAC wing to 25% MAC VT) for both the HT and VT, it is considered short coupled.

So Area is 0.045 x Wing Area x Wing Span / Lever Arm (25% MAC Wing to 25% MAC VT). It's easier to see the longer the Lever Arm the smaller the Area. Also suggested to me is that the increase of Lever Arm offers less Parasite Drag than the increase in VT and HT area.

If the sweep at the 25% VT Chord exceeds 20°, then the area increases exponentially with the sweep angle.

I find rounding up the VT Area compensates for any minor to average increase in Sweep Angle.
Hope that helps, a bit better than guessing.
George
 

wsimpso1

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Sounds like way too little tail volume. Increasing tail volume means more moment available at the mount of the tail boom. This could be a substantial change. You are talking about redesigning the airplane... Are you ready to do that?

Billski
 

Eugene

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You are talking about redesigning the airplane... Are you ready to do that?Billski
No, not smart enough for that. But hope to make some small improvements that will give me better flying aircraft. There was pretty big smile on my face during my last test flight. So, I am almost were I want to be without redesigning anything.

Here is my small changes:

- improve airflow to horizontal tail

- reinforce tail pipe to prevent flexing in flight

- increase tail area by 20-25%

- convert tail to 8-10% symmetrical airfoil
 
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Eugene

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Here is theoretical question that sitting in my head for sometime:

Picture below from Aeronca Chief manual. With side-by-side seating this airplane have CG spread only between 23-26% or so. Stabilizer is set for neg. - 3.5°.

Screen Shot 2020-02-26 at 19.52.50.png

This picture below is fo Aeronca Champ. Because of tandem seating this aircraft has CG movements 36.6% - 21% MAC. And stabilizer installed at neg. - 5°.

Screen Shot 2020-02-26 at 19.55.38.png

Question is this: - when aircraft designer should stop increasing stabilizer angle and start thinking about increasing horizontal tail volume? After he finds during test flights that tail need to generate bigger balancing force. Is 5° cut out line that should not cross on 100 MPH typical LSA? Both this airplanes have same Vh=0.33 only, but no flaps 85 MPH with 65 HP engine.
 

fly2kads

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Question is this: - when aircraft designer should stop increasing stabilizer angle and start thinking about increasing horizontal tail volume?
When you don't have sufficient control authority in your most critical condition. For many aircraft, this is flaring in ground effect, with full flaps, at the most forward c.g. location. Or is there any point in your flight envelope where the AOA would cause the tail to stall, with our without elevator deflection?
 

TFF

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The stab angle is there because your plane is designed to fly nose heavy. Nose heavy is stable. The designer chose how he wanted to handle the CG. I’m sure there is a very large static margin. All that was designed in. Airfoil, seating arrangement, engine placement, total size of aircraft. Each decision is plus and minus. To fly level all the forces have to equal out. They don’t have to be in line, just equal out.

To flatten out the Stab angle requires the kind of changes that will push the CG back and some sort of combination of longer tail moment, larger tail size, higher speeds to keep stuff flying. Balance will have to be more precise. That will come with a much touchier elevator, maybe bad stall manners, higher speeds to stay safe.

Its not a decision to change tail size or angle, it is a decision to design to XYZ and the parts fall into place to make it happen.
 

Eugene

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When you don't have sufficient control authority in your most critical condition. For many aircraft, this is flaring
Screen Shot 2017-11-24 at 21.51.43.jpeg

I am confused a little again and need someone to straighten me out.

The way I see it, during stalling or flaring with passenger I am running out of elevator all the time. Stick is all the way back and you just sitting there and waiting for touchdown. Stabilizer is not helping at this point and maybe working against elevator, looking at my picture above. So I am making conclusion that elevator too small or tail arm too short.

Screen Shot 2019-08-09 at 21.57.16.jpeg IMG_5600 2.jpeg

So, this is standard Skyboy in level flight and 2 people. As I see it, stabilizer is doing all needed work and elevator is trying to help. Stabilizer is already installed at neg. - 5.7°and it's not enough. So, what is right thing to do here? Drop stabilizer down to neg. - 6.5°and try? Or should we start thinking about larger stabilizer? Maybe horizontal tail volume of 0.34 not really working here? And when is that cut out line? Is it 5°, 7°, 9°? Or should designer get this message as soon as he needed to go below 4° mark? Do we have airplanes we know with stabilizer set at 6-8°?
 

fly2kads

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I am confused a little again and need someone to straighten me out.

The way I see it, during stalling or flaring with passenger I am running out of elevator all the time. Stick is all the way back and you just sitting there and waiting for touchdown. Stabilizer is not helping at this point and maybe working against elevator, looking at my picture above. So I am making conclusion that elevator too small or tail arm too short.
I don't think you're confused on this point! That sounds like the correct conclusion.

So, this is standard Skyboy in level flight and 2 people. As I see it, stabilizer is doing all needed work and elevator is trying to help. Stabilizer is already installed at neg. - 5.7°and it's not enough. So, what is right thing to do here? Drop stabilizer down to neg. - 6.5°and try? Or should we start thinking about larger stabilizer? Maybe horizontal tail volume of 0.34 not really working here? And when is that cut out line? Is it 5°, 7°, 9°? Or should designer get this message as soon as he needed to go below 4° mark? Do we have airplanes we know with stabilizer set at 6-8°?
The tail volume does appear to be too small. The challenge, as you're finding out, is what to do about it now.

The easiest thing to do would be to revise your forward c.g. limit, found by doing test flights with varying loads and finding the point at which you run out of control authority. In other words, flip the problem around. Given that the tail is too small, what is the point at which you feel you have full control? It would not require any modifications to the airplane, but would limit it's utility. You're clearly not afraid of putting in some work to try new things (I think this is great thing, BTW), so this may not be a very satisfactory solution for you.

I am not an expert, but I am slowly wading through topics related to stability and control, so I may be only a step or two ahead of you here. That said, I have not seen anything yet that provides any guidance on "how much is too much" when it comes to tail incidence settings. There is no "cut out line," as a rule of thumb, that I have seen. I have some thoughts on this, though, that I will type up later when I have a little more time.
 

Dana

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If in level flight the elevator is trailing straight behind the stabilizer but you still run out of elevator authority at forward CG loadings, it's either insufficient tail volume or insufficient elevator deflection. Increasing the (negative) stab incidence means you'd need down elevator for level flight, and you don't want that.

Installing VGs on the stabilizer (they go on the underside when you put them on a tail) might be a simple thing to test, a lot easier than building a new tail.
 

TFF

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You are running out of elevator because of the CG being forward. In simple terms how much elevator do you have to lift the nose. The fulcrum point is the neutral point and the weight you are lifting is the CG point. Your counterbalance is the tail. It’s the way that plane was designed. It’s just what it is. You can rearrange the parts. Put people at CG so CG change is small. Balance it out with engine in front. With he CG range tight, you don’t have to over compensate with extra angles. Other options are longer tail moment or bigger tail, but even then it can only do so much.

RV7 vs RV8. The CG range of the 7 makes loading easy and handling almost always the same. Everyone I know at my airport with 8s fly with about 50 lb of ballast in the back seat if most of the flying is solo. 8 is much cooler looking, no denying. I can’t even get a ride in a RV4 because I throw the CG out if the n the back.

It’s back to configuration. You want to carry two people with 100 hp how fast, how slow, how high, what style if it has to be part of it, with material to build. Things start making configuration concrete when requirements forced.
 

Eugene

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I see this whole thing a little different. But before I wanna say that this is just theoretical brainstorming, without any attempts to call anybody stupid or so. If anybody stupid here, that would be me, trying to understand very basic staff.

I see all kinds of problems at work and they all have an explanation. If you know what you doing, of coarse. People sometimes asking you to do some challenging things and they ask if this can be done at all. I always tell them that with right amount of money and time = we can do anything. And if they willing to pay, they will get what they want.

Going back to my Skyboy -
"You are running out of elevator because of the CG being forward" - This would be like me telling my customers that reason they couldn't get there house in January above 59°F, because they installed to many windows in the living room. And absolutely nothing wrong with my heating system. No, it is my job to find solutions and give them 72° on -25° below.

So, if you decided to place pilot and passenger 6 inches in front of leading edge = your job is to build correct tail to balance this forward CG situation. If you installing engine above the wing = need bigger tail. Sure there will be big price to pay in so many ways and aircraft will get slower and heavier, but do we really have another option? Is to make tail exactly 2 times smaller then any book out there recommending is good option?

Little reminder that this forward CG we are talking about is only 24-23% MAC. Solo at 34-36%. With my wife we are at 25-26% only. Is this really forward CG situation???

"If in level flight the elevator is trailing straight behind the stabilizer" = it's only true when I am flying solo. With passenger, you can see from my pictures, that elevator is up a little and counter weights down. Not very much, I agree, but that is telling me that stabilizer is not doing its job at already neg. - 5.7°angle and need help from elevator.

So, how did this happened? First time I mention to my designer that tail is too small, he call me crazy!!! "....do you think that you smarter than team of my engineers!..."

Of coarse not. I didn't go to school for this. But my books telling me that tail needs to be 2 times bigger. Maybe wrong books...
 

wsimpso1

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I have a few possibilities for you.
  • Document current airplane:
    • Weigh the airplane, determine and record its exact empty CG, figure out the loaded CG. You will want to know what it was, and how much you change it;
    • Record speed at which it runs out of elevator at both low and high seat weights, both aloft and in ground effect, and at idle speed and high power - that is eight conditions;
  • Make the tail more effective;
    • VG's on the underside of the tail will increase tail authority a noticable amount;
    • Add gap seals on the horizontal tail;
  • Shift CG:
    • Where is the battery? If forward, shift it aft;
    • Is the engine mount a separate assembly? If it is, you can build another that shifts the engine aft an inch or two;
    • This thing of strengthening the tail boom will add weight aft - the stuff that is added, keep track of its weight and position;
  • Test fly each change:
    • Read up on test flying and looking for pitch stability issues, how to spot them, what to do about them, etc. Neither I nor anyone else on here can educate you adequately as a test pilot;
    • If you are shifting CG aft it can be a sensitive thing. Aft CG is nothing to play with. While it sounds like you have a forward CG and can adjust it quite a bit, move the CG aft in small increments. Some folks talk about having a bag of shot aft and way to pull a handle and dump it. Done properly, that lets you adjust in small increments and go back to a known good CG with the tug of a handle;
    • Calculate the new flight CG position and record it;
    • Taxi test, first slow, then at increasing speeds. You should be able to set the stick and have the nose settle on a pitch attitude, nice and steady with the nose wheel off the ground. If it wants to bang the tail skid or bang the nose wheel on, the CG got too far aft - record that CG and resolve yourself to not flying that far aft again;
    • If you get any indications, either on the ground or in flight of it not wanting to set a steady pitch attitude based on a fixed stick, land as soon as practicable. If it does it right after breaking ground, close the throttle and land - record that CG and resolve yourself to not flying that far aft again;
    • Carefully test fly looking to see if you raised or lowered the speed at which you run out of elevator in all eight cases;
    • Check out the rest of the envelope - if it gets touchy, record that CG and resolve yourself to not flying that far aft again;
    • Read up on test flying and looking for pitch stability issues, how to spot them, what to do about them, etc. Neither I nor anyone else on here can educate you adequately as a test pilot. Yeah, I said it twice. That is selective redundancy, and we do that because the repeated topic is really important.
Billski
 

Eugene

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I have a few possibilities for you.
  • Document current airplane:
    • Weigh the airplane, determine and record its exact empty CG, figure out the loaded CG. You will want to know what it was, and how much you change it;
    • Record speed at which it runs out of elevator at both low and high seat weights, both aloft and in ground effect, and at idle speed and high power - that is eight conditions;
  • Make the tail more effective;
    • VG's on the underside of the tail will increase tail authority a noticable amount;
    • Add gap seals on the horizontal tail;
  • Shift CG:
    • Where is the battery? If forward, shift it aft;
    • Is the engine mount a separate assembly? If it is, you can build another that shifts the engine aft an inch or two;
    • This thing of strengthening the tail boom will add weight aft - the stuff that is added, keep track of its weight and position;
  • Test fly each change:
    • Read up on test flying and looking for pitch stability issues, how to spot them, what to do about them, etc. Neither I nor anyone else on here can educate you adequately as a test pilot;
    • If you are shifting CG aft it can be a sensitive thing. Aft CG is nothing to play with. While it sounds like you have a forward CG and can adjust it quite a bit, move the CG aft in small increments. Some folks talk about having a bag of shot aft and way to pull a handle and dump it. Done properly, that lets you adjust in small increments and go back to a known good CG with the tug of a handle;
    • Calculate the new flight CG position and record it;
    • Taxi test, first slow, then at increasing speeds. You should be able to set the stick and have the nose settle on a pitch attitude, nice and steady with the nose wheel off the ground. If it wants to bang the tail skid or bang the nose wheel on, the CG got too far aft - record that CG and resolve yourself to not flying that far aft again;
    • If you get any indications, either on the ground or in flight of it not wanting to set a steady pitch attitude based on a fixed stick, land as soon as practicable. If it does it right after breaking ground, close the throttle and land - record that CG and resolve yourself to not flying that far aft again;
    • Carefully test fly looking to see if you raised or lowered the speed at which you run out of elevator in all eight cases;
    • Check out the rest of the envelope - if it gets touchy, record that CG and resolve yourself to not flying that far aft again;
    • Read up on test flying and looking for pitch stability issues, how to spot them, what to do about them, etc. Neither I nor anyone else on here can educate you adequately as a test pilot. Yeah, I said it twice. That is selective redundancy, and we do that because the repeated topic is really important.
Billski

Thank you very much for detailed message.

First question that comes to mind:
- I am flying exactly same wing as Aeronca Champ and have exactly the same CG range 21-36.6%. Yes I need bigger tail because engine above the wing. But from CG stand point why I need to experiment with it? As long as I stay inside of this 21-36.6% window = I am safe! To get my CG in to this window I am changing my wing swept.

I must be missing something here
 

fly2kads

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But from CG stand point why I need to experiment with it? As long as I stay inside of this 21-36.6% window = I am safe!
Why experiment with it? Because you keep making observations about undesirable behavior, like this:
Aircraft can not be trimmed for level flight and hunting up and down by 300-500 ft apart. And aircraft with passenger feels like filled with lead.
And this:
Stick is all the way back and you just sitting there and waiting for touchdown.
These are indications that the aircraft is not fully controllable across the entire c.g. range. Is it safe, but just annoying? Possibly.

Here is an illustration from Perkins and Hage that shows what you're bumping up against:
Perkins_Hage_CG_Limits.jpeg
Don't worry about the absolute values on this graph, just the idea. If flying with a passenger is unpleasantly stiff, then your c.g. is probably too far forward. If you can't fully flare for landing, your c.g. is probably too far forward when in ground effect. If you can't maintain a trimmed flight path when solo, then your c.g. is probably too far aft.

The "useable c.g. range" in the middle of the graph is where you want to be. Testing will tell you where that is in the current configuration and establish a baseline against which you can measure your proposed modifications.

In any case, you have expressed a desire to improve the flight behavior. So there are a couple of categories of approaches:
1. Find a way to live with the current tail. Explore the c.g. range to find where your aircraft is well-behaved.
2. Modify the aircraft to make it behave well across the currently specified c.g. range.
The fact that you opened this thread in the first place would indicate you want to pursue option 2.
 

Eugene

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2. Modify the aircraft to make it behave well across the currently specified c.g. range.
The fact that you opened this thread in the first place would indicate you want to pursue option 2.
Yes! Thank you!

Every Skyboy owner out there knows how to manage CG for solo flights and with passenger. Pilot/passenger CG is about 6 inches in front of the wing. This situation by itself is creating CG spread of 12% solo vs passenger. They all flying with ballast on passenger seat for solo flights. If you light person 150-160 lb your attempt to fly Skyboy will be maybe your last flight with CG at 38-40%.

I changed my wing swept by 1° and that moved CG forward by 2.3% in to safe zone of 34% for solo flight. But flying with passenger my CG now at 23-25% and I have no ballast to remove. Looks very good on paper with max forward CG limit of 21%, but in real life you almost running out of up-trim for level flight with 2 people. And aircraft feels like filled with lead.

I install small extensions on my stabilizer and situation improved somewhat. So, making tail larger will be moving in right direction.

But every time I do something original designer call me crazy, that I will kill myself with this so- called improvements. So, naturally I am nervous and that is why I am asking all this questions. What do I know? Nothing in comparison to them! Team of 12 engineers designed Skyboy and was tested by professional test pilots. And now is this stupid Russian is telling them that they made tail too small!!!???

444DD2B1-2CAC-48FB-BC27-652479AEF23A_1_201_a.jpeg IMG_2523.jpeg
 

TFF

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The distinction is you are flying with a forward CG. I did not say you were flying out of CG. With the incidences you have and the loading you have, you are stuck with this situation. It might not be the correct way to describe it, but the forward weight bias is to tame the plane. The tail with that much incidence is working hard even if in trail. If you had 200 hp, the plane would be looping with those incidences.

I have built a lot of RC planes. I don’t build perfect planes, but I have built a bunch. About two a year. Many planes you get one easy chance to get the incidences right. Without doing surgery, adding nose weight is the fix when incidences off as long as the wing isn’t negative to the tail. I usually add weight over surgery.
 

BBerson

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The larger the tail the larger the CG range.
Those stabilizer extensions should help. I would try the extensions 2° up for some negative camber. The tail provides download and you want more download so add some negative camber like a Zenair 701 tail.
The symmetrical tail is not ideal for this pusher, in my opinion.
 
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