What is wrong with high thrust line???

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wsimpso1

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Lots of folks are after "perfect". Define your mission, and go for what you want the bird to do.

Excellent view forward, to the sides, and down;
Easy ingress/egress;
50 knot touchdown speed;
Good roll authority with nice control harmony;

Slide the engine and the wing up and aft and the people forward, bingo. Hmm. Prop efficiency is not so good, and trim drag is a little high, but the other missions are met. Clean up the engine, make the tail plane volumes adequate and the aft fuselage stiffer, and it behaves like a real airplane. Away you go.

What was that? Slower than a C150? Did I see anything in that list about blazing speed? Good trade for that other stuff!

Onward.

Billski
 

Pops

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Sort of like trying to make a race horse out of a plow horse.
 

Dan Thomas

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I should've phrased my initial question differently. My fault. Happens to me a lot. Even my own wife sometimes doesn't understand me. That should tell you something.
Your question was "What is wrong with high thrust line?" We answered that question.
 

Eugene

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Sort of like trying to make a race horse out of a plow horse.
No, this was not a race for speed. Not at all. Speed was only one parameter I was able to understand five years ago. This was airplane with a German certification and I was in shock when my instructor walked away on me after first flight and didn't want anything to do with it. Until that day I was thinking that all airplanes created equal. He complained about many things, but speed was not his concern.
 

Pops

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I'll try again.

Can't make a silk purse out of a pigs ear.

Added-- This is going to be interesting :)
 

Eugene

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I'll try again.

Can't make a silk purse out of a pigs ear.

Added-- This is going to be interesting :)
I am really not trying to make apple from orange. Just trying to make my apple little bit more presentable and acceptable, or may be more manageable. And from what I learned in the last five years this is a very manageable task. Yes, of course you need correct knowledge to do it.

I tell my customers that with the right amount of money and time we can do anything. Tell me what you want and I will make it happen. Like to think that I don't have one unhappy customer in 15 years. And I am not telling them that their problem is that they want to have this Office faced to the north with too many windows and that is why temperature in this room couldn't get above 65°F. If I don't know how to make it happen, it's on me, it's not because they crazy.
 

TFF

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With the right amount of money, you can start over. Is that a reasonable? That is what you you are saying with your mantra. You are saying you would put $100k into this plane to fix it?

It’s why I tend to not like the idea of doing all this work to a flyer. What you want needs a new airplane. Much simpler to design in what you want, than fix something already built. To do all the work you want to do, how much are you willing to scrap? We are not trying to sell you something, we are trying to tell a friend he needs an intervention.

That is where you miss. It’s not that you can’t do what you want, but we are not buying your drugs for you. You are trying to convince us, but you are the only one that believes and doesn’t see reality.
 

Eugene

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With the right amount of money, you can start over. Is that a reasonable? That is what you you are saying with your mantra. You are saying you would put $100k into this plane to fix it?

It’s why I tend to not like the idea of doing all this work to a flyer. What you want needs a new airplane. Much simpler to design in what you want, than fix something already built. To do all the work you want to do, how much are you willing to scrap? We are not trying to sell you something, we are trying to tell a friend he needs an intervention.

That is where you miss. It’s not that you can’t do what you want, but we are not buying your drugs for you. You are trying to convince us, but you are the only one that believes and doesn’t see reality.
Sorry, I have a hard time to follow your logic. I'm sure it's there, I just don't see it.

With correct ballast on the passenger side you can fly solo with CG around 30% MAC. Mini Skyboy owners do just that. Airplane behaves absolutely perfect with power setting approximately 65%. But without ballast with CG at 36%, or with passenger at 23% is another story. Tail is too small.

Conclusion that was made by Sonja is that we need to design new tail which is approximately (In her words) redesigning 30% of airplane. And this is what we doing.

Why would it be easier to design 100% of airplane versus 30% I don't understand. Rest of airplane 70% can stay right where it is and I am very happy with it. I don't need to redesign it.

I like it already. It's working for me and that is why I purchased this airplane. 30% of engineering fees is much less than 100% of engineering fees.

I agree that sometimes in our lives we have situations were much easier to start from the beginning. Some old houses out there need only one thing = big bulldozer. But my situation is far from it. Or at least I see it this way.
 

Dana

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I would like to understand why thrust line above the wing is such a big problem for everybody? Is it true that every problem has a solution?
Who is "everybody"?

I haven't heard anybody say that having the thrust line above the wing is a "problem"; it's a design choice that allows other design choices (like pilot in front of the wing and good visibility), and it also requires other design compromises (like living with power related pitch changes and CG changes depending on passenger weight, perhaps requiring ballast when flying solo). Now, it may be that the designer made a poor choice and made the tail too small-- a not uncommon mistake, many planes have had the tail size enlarged after the prototype flew or after a larger engine was installed-- and perhaps a larger tail and/or other changes would make for a more pleasant handling aircraft... and perhaps not. But the can of worms you open up by making a major change like that may or may not be worth it, and may or may not lead to other unacceptable compromises.

It's been so long, tell us again (briefly!) what the original perceived problem is that you're trying to solve?
 

Pops

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You can do a lot of work and money trying to make a bad design good. With all the effort you will get some small results that will help that should have been done by the designer. But in the end, you will have a pigs ear with a coat of paint. The problem is the basic design and to change that, it's a whole new airplane.
About 4 hangers away from me there is an airplane of similar design ( Genesis ), same problems and has went through about 8 owners, now hasn't been flown for about 5 years.
 

BJC

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It's been so long, tell us again (briefly!) what the original perceived problem is that you're trying to solve?
An early objective stated here.
Please remind me; what, exactly, is the problem that you are trying to solve with the cables?

Thanks,


BJC
Trying to find a way to prevent tailboom from bending down under a load in flight
 

Eugene

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it's a design choice that allows other design choices (like pilot in front of the wing and good visibility), and it also requires other design compromises (like living with power related pitch changes and CG changes depending on passenger weight
Thank you! This is how I feel as well! As far as what we're trying to fix and make it short..... I have a report from Sonya that summarize it all. If I can teach myself how to upload it, you will see it. Thanks again
 

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Eugene

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The problem is the basic design and to change that, it's a whole new airplane.
The problem inside of my head. I am in love with basic design of helicopter like flying where everything is behind you and you smell fresh air. And I am not trying to fix this problem inside of my head. I am trying to fix small tail an airplane that I like. Whatever is going to come out of this should be better than what it is now. And I am willing to spend time and money to do it. Not very practical and not very smart? How many things is very smart and practical an experimental aviation?
 

Eugene

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The problem is the basic design
I believe there's nothing wrong with the basic arrangement of my airplane or airplane you talking about. Reason both airplanes have poor flying characteristics is because designer didn't know what he was doing. That's essentially why I started this thread.
 

ragflyer

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Thank you! This is how I feel as well! As far as what we're trying to fix and make it short..... I have a report from Sonya that summarize it all. If I can teach myself how to upload it, you will see it. Thanks again
Sonya's conclusion on stability (no surprise given her experience) seems reasonable and perhaps even a little optimistic as per her account she does not take into account fuselage flexibility, which of course would only make things worse.

While her prescription of increasing the tail area, span, and tail arm are no doubt the obvious ways to improve stability, in general. In this instance, I am concerned it will be pose a number of design challenges (no surprise she estimates this to be 30% of new design). Specifically my concerns are around flutter given in the current state flutter margins seem to be an issue.

All these changes likely will make flutter margin even worse. The tail arm increase particularly would make the keel more flexible and prone to flutter and also will move the CG aft. Careful design and testing will need to done. Nothing of course a professional, like Sonja, cannot do.

There is another counterintuitive alternative that, at first glance, may be a simpler way to improve stability, reduce design complexity and also not make flutter worse- clip the span of the wing by about 25%.

The benefits of a clipped wing are an increase in the tail volume coefficient of about 25% without changing the area or arm length of the tail. This in turn will improve stability but not change the flutter margins of the tail. The tail can be left as is.

Now the downside are two fold:
1. stall speed will increase by about 12% and
2. climb rate will drop by (in my estimation) about 180 fpm.

The stall rate may not be a big deal- it would increase to about 54MPH which is reasonable. The climb rate, on the other hand could be a concern. One way around the reduced climb rate that would be to replace the rotax 912 with a 914. They have about the same weight, but the extra power will significantly close the climb rate change.

Seems a simpler alternative to me with a reasonable performance trade off, particularly with a 914. Of course I may have missed something here.

Thoughts?
 

Eugene

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Vigilant1

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You are pretty far along on your engine cowling project. If there's any chance at all that the cowling will improve the situation with the tail (reduction of turbulence/ vortices from the engine impinging on the tail?), then I'd suggest you finish the cowling and fly the plane before changing other variables.
 
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Eugene

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You are pretty far along on your engine cowling project. If there's any chance at all that the cowling will improve the situation with the tail (reduction of turbulence/ vortices from the engine impinging on the tail?), then I'd suggest you finish the cowling and fly the plane before changing other variables.
Are you suggesting that I should finish engine cowling and spinner, put horizontal tail back on and go for a test flight and see if 120x2 mm tube will stop flexing in cruise?

Sonja made the conclusion that this tube is not going to work for us with 100 hp engine.

 
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Vigilant1

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Are you suggesting that I should finish engine cowling and spinner, put horizontal tail back on and go for a test flight and see if 120x2 mm tube will stop flexing in cruise?

I'm suggesting that we aren't sure why your tail/control stick is behaving in a way you don't like and that it is at least possible that changes in the flow of air around your engine/over your wing will change the objectionable behavior you've observed. It might make it better, it might make it worse (I think the former is more likely), but if you change a bunch of stuff at once you'll not know what happened. Making one change at a time has a lot of advantages.
 
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