extra long run trouble

[ujjawal]

I just made an airplane which is all self designed. I am more into designing than in flying so I literally know nothing about piloting the thing.
The aircraft has not flown yet mainly because it is supposed to generate suffice lift at about 40kph windspeed which I have not managed mainly due to the poor control I have and thin take off road. The throttle cannot be increased quickly as it is much above CG and the downforce producing tail is needed to balance the torque.
I have seen airplanes getting off without any significant runway, so can it be a CG problem that I am facing(like nose heavy). I have not yet tried hand launches as I am not sure what to expect and my basic flying skills will surely end things to a crash.
please help.

 

[johnmw]

any picture of the plane?
what are the setup?

 

[joshuabardwell]

40 kph / 25 mph is a pretty high stall speed for an RC plane. Are your wings properly sized?

If your thrust line is far above your CG, it needs to be tilted down so that it pushes through the CG, such as in the Bixler, or even if you get the thing into the air, it is going to fly terribly.

 

[quorneng]

A picture would certainly help diagnose what might be wrong however if it really needs 40 kph to fly (how have you determined this?) and as an untested design I have serious doubts you will have the skill to fly it - even if it does.
This may sound harsh but until you have sufficient design experience you really should stick to well tried and tested plane designs that are suitable for your level of piloting skill.
I agree designing planes is fascinating but I can assure you that even after doing it for a great many years not every one flies properly straight off the drawing board!

 

[ujjawal]

The aircraft's wings were designed using foil sim 3 which is a freeware by nasa. Some modifications were made but I have sufficient understanding to guarantee that those wont change the things much. I use special techniques that I designed to make very accurate variable cross-section variable airfoil wings. The aircraft was designed for a study that I wanted to conduct regarding its behavior very close to the ground. Although this is the first powered craft I have made, I have made many gliders so I think that I have complete understanding regarding aerodynamics.

I basically need tips so that I could fly the thing for indeed it is a tough challenge to have an experimental aircraft as your first craft.

 

[ujjawal]

The photos have the front landing gear broken. I have to fix it but comments regarding the front landing gear longer or shorter than the back one might be helpful.

 

[ujjawal]

The tail is somewhat different as the elevator has been replaced by a fully moving tail . I have taken care that this movement does not cause the tail to stall.

 

[quorneng]

Hmmmm!
I would comment that your design has several features as a plane that might cause some serious issues.
1 A very high thrust line
2 Inadequate tail 'volume'.
3 Massive longitudinal inertia.
4 The rear undercarriage location means it can only 'fly off'. The elevator will not be able to lift the nose on take off to alter the angle of attack.
5 Does it have any form of roll control?
6 How much power (Watts) is available and what does it weigh?

Finally
I assume this is intended to be a ground effect plane. If so does it have the aerodynamic features required?

 

[ujjawal]

1) the tail produces downforce unlike normal neutral tails( I have taken care of the affected correct position of CG) so as the thrust increases the wind from thrust goes over tail that reduces (does not end it) the effect of high thrust line (see that the tail is aligned with the thrust line.
2) down-force on tail is what I believe is also used on real aircrafts and it theoretically is much more stable than CG under centre of lift and neutral tail configuration.
3) The thing looks heavy but it is nearly a 600gm craft with the electronics mostly in the centre. The thing is not required to do extreme maneuvers.
4) The rear wheel is planned to be moved forward and a small castor (connected to rudder) wheel is planned so that when the craft is kept then it is nose up which shall be of some help.
5) I plan to install an aileron soon.
6) It is around 700gm thrust at max throttle. but the drag is negligible. I tried running with the thing at around 60 percent throttle and it felt like it wanted to go faster and lift off. But i dont want the first flight to be a hand-launch as if it has a nose heavy setting then I might not be able to control it and crash! I can be patient but I dont wanna crash before it flies!!

The thing is a design project and is specially designed to do both ground effect and free flight with minimal trim.

 

[quorneng]

My comment about the high thrust line is in relation to the centre of drag. A significant proportion of the drag comes from the wings and your thrust line is well above them so it will tend to pitch nose down under power.
For stability the centre of gravity must be in front of the aerodynamic centre of pressure so that produces a nose down pitch as well.
The only thing holding the nose up is a downward force from the tailplane. Is your tailplane big enough to do this without resorting to an extreme angle?
Aligning the tail plane directly with the prop wash is a mixed blessing as it only sees airflow directly from ahead regardless of the direction the plane is actually travelling. This means the tail plane will provide very little automatic stability and the plane will be harder to control.
Your current layout (a low swept forward wing set well below the centre of gravity) will have very little lateral stability so it is unlikely it could be flown successfully on rudder only and even with ailerons it will still be a handful.

With any radical 'own design' you are likely to crash (several times) before you achieve a reasonable flight which is why it is best to get to where you want to be in several gradual steps testing each feature as you go along but starting either from a conventional layout plane (like a powered glider?) that flies well or starting from a proven ground effect design that you intend to make fly.

I hope it all works out for you in the end

 

[joshuabardwell]

Your current layout (a low swept forward wing set well below the centre of gravity) will have very little lateral stability so it is unlikely it could be flown successfully on rudder only and even with ailerons it will still be a handful.

In order to fly rudder only, the plane needs to have dihedral in its wings, and have its CG somewhat below the center of lift, right? Otherwise, adverse yaw won't cause it to turn and self-right.

 

[dflash]

this guy has to be trolling./>?! Impressed with futuristic design! - LOVE the flip-flops BTW

 

[pgerts]

My first impression was " What a funny canard".

 

[joshuabardwell]

this guy has to be trolling.

I think he's serious, but (with all due respect), his skills as a designer are lacking--in no small part due to the fact that he is "more into designing than flying". As soon as you get the model in the air, you find out how good or bad your design really is. Until then, it's all conjecture.

 

[ujjawal]

the winglets are angled and so they do the same thing that dihedral does although slightly less efficient.

And the position of wings below or above the CG does not cause any diff to lateral stability as often confused. Just analyse and you will find clear difference between a pendulum and an aircrft.

 

[quorneng]

ujjawal
If a plane is 'disturbed' from straight and level flight the position of the CofG above or below the wing does indeed make a difference to the way the plane subsequently responds.
Trainers tend to be high wing, competition aerobatic tend to be mid or low wing - I wonder why that is?

 

[FAI-F1D]

I have seen airplanes getting off without any significant runway, so can it be a CG problem that I am facing(like nose heavy). I have not yet tried hand launches as I am not sure what to expect and my basic flying skills will surely end things to a crash.
please help.

First off, heed the advice that's been given you. The airplane does not look to have enough wing or in the right place, and the undercarriage won't allow you to take off, especially as long as you refuse to give it full throttle.

Now all that said, go find a field with some tall grass and throw the airplane. Taking off from the ground is actually more, not less, risky than hand launching. Trust me on this, I've got a lot of experience in this area. If you fly over tall grass, the impacts are less damaging should you mess up.

 

[joshuabardwell]

the winglets are angled and so they do the same thing that dihedral does although slightly less efficient.

With all due respect, you asked a question, and people are taking the time to give you good advice and answers, and you are arguing with them. It's rude. If you don't like the advice you're given, best to say, "thank you," and ignore it.

And the position of wings below or above the CG does not cause any diff to lateral stability as often confused. Just analyse and you will find clear difference between a pendulum and an aircrft.

Please check your ego. If you knew about aircraft design, your plane would fly, and you wouldn't be asking for advice here. You have a lot of strong opinions about aircraft design, for someone who has never flown, and whose craft has so many fundamental flaws. Your enthusiasm is admirable, and I understand that you are working with what you have, as best you can. But the first step to learning is an open mind and a closed mouth. I would never say that to anybody unbidden, but you did come here asking questions, so that leads me to believe that you want to learn. If so, stop arguing with the people here who know more than you and who are trying to help you, and try implementing some of their suggestions and see how they work.

One of the things that I have learned as I have gotten older (okay, okay--shut up, grandpa) is that people who know more than you are often happy to help you, but they have better things to do with their time than offer help to people who will be ungrateful or argumentative about it. When somebody helps you out, they're doing you a favor, and a grateful, humble, and attentive attitude is appropriate.

 

[ujjawal]

Well I am sorry if I seemed extra argumentative.

You are right That I have a bit extra confidence in what I know moreover because I have read lot of applied Physics and try using the theory to clear the doubts I generate.
I posted my thoughts because I wanted someone to reason where I went wrong.

I am sorry for the behavior once again! I didnt mean to be rude.

By the way i am just 18 so I really do lack experience.

 

[ujjawal]

First off, heed the advice that's been given you. The airplane does not look to have enough wing or in the right place, and the undercarriage won't allow you to take off, especially as long as you refuse to give it full throttle.

Now all that said, go find a field with some tall grass and throw the airplane. Taking off from the ground is actually more, not less, risky than hand launching. Trust me on this, I've got a lot of experience in this area. If you fly over tall grass, the impacts are less damaging should you mess up.

Thank you for that! I will try the same.