rcpilotsteve-RCU

My Feedback: (5)

Please excuse what might be heavy stupidity but what gives. I think everyone at sometime wants to design and build there own model so I figured now's my turn. I have started doing some research and I'm learning most self designed crafts have started out by simply modifying someone elses attempt. I want my own design, so I have been testing my knowledge and I think I'm doing or thinking very wrong (lol) so now I'm just flying my high wing trainer..............
but seriously this is what I'm doing
I would like to mount my Stab and wing at 0 degree's to the Datum line. This is what I thought the base for measuring Incidence is on my kits, with the thrustline measured off the Datum. So I have been comparing other Planes and Plans and it just does not work out like this at all.
If I run a string (compensating for down thrust) from the engine to the stab I can't get 0 degrees off of nothin'
If someone wants to enlighten me I would sure appreciate it!!!

rcpilotsteve-RCU

My Feedback: (5)

I'm not going to think I'm stupid yet. No replies means there is still hope
I'm sure someone would have insulted me by now if I was way out to lunch.........

Spadiator

For what you want to do, just make sure the wing and stab are parallel, and the engine is mounted square to that. Trainers have a lot of downthrust, some have right thrust too. Heck it may even have positive incidence on the wing.

Sounds like you want to build a sport plane. As long as you have the incidence of the stab and wing at 0 and no downthrust, it should fly well. The best thing to do is reference everything off one surface, typically the h-stab. When that is at 0, make the wing at 0, then mount your engine square to that.

Hope this helps, good luck designing your first.

Steve Lewin

I'm not exactly sure what you're asking but I think you may have missed the fact that the "Datum line" is just an arbitrary line drawn on the plans by the designer. There's absolutely no guarantee that joining any 2 points on a built plane will give you anything like the datum. In particular the engine, wing or stab can be mounted either above or below the datum. Typically the datum is drawn from the centre of the prop but it can be anywhere.

0 degrees incidence just means they are parallel to the datum line not that they are on it. So decide on the position of wing, stab and engine, draw your own datum line and make sure everything stays parallel to it.

HTH - Steve

Ollie

As Steve says, the datum line is arbitrary. Its functoin is to make it easier to measure the angle between the wing and tail, called decalage, and the angle between the wing and thrust line. In a prototype of a new design, the angles between the wing, tail and thrust line often have to be adjusted through flight testing to get the desired flight characteristics. So, my advise is to make at least two of the three items easily adjustable by incorporating adjustment capability into your prototype.

rcpilotsteve-RCU

My Feedback: (5)

Thanks for the replies guys
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quote "I would like to mount my Stab and wing at 0 degree's to the Datum line" unquote
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I didn't say on the Datum line The problem I am experiencing:
I am tansposing the "Arbitrary line" from the plan onto the plane and it is not parallel.
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quote "I can't get 0 degrees off of nothin' "unquote

Yes, the plans are telling me 0 degrees incidence on both the wing and Stab. I just want to make sure that I am thinking right. I have checked the stabs and wings compared to the Datum line on three or four different crafts from trainer to sport and it is not working for me.
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From your replies I have learned that I am right. Parallel being the key word. I'm going to retry on the built models again. I must be doing something wrong.
Thanks

Ollie

Calculating the angles between the wing, stab and thrust line in order to produce a plane that is trimmed for level flight at a given throttle setting is a fairly complex job. It is plagued with a lack of some some necessary data. You can come close by copying a plane with the same wing location relative to the thrust line and a plane that has a similar airfoil and a plane that has a similar CG and a plane that has a similar percent tail area and a plane that has a similar tail moment arm as a multiple of wing chord. Adjustment is still necessary. That's why new full scale designs undergo flight testing programs. That's why they put trim tabs and decalage adjustments on full scale planes. That's why some of the measurements you have made of existing planes don't agree. With a given configuration there are several combinations of CG location and decalage that will give identical trimed air speed and attitude. Also, the best CG location for a given configuration depends on the purpose of the model and the skill and flying style of the pilot.
The only thing you have to predetermine is that the angles and CG are within an acceptable safe range to begin testing and adjustment!!!!

rcpilotsteve-RCU

My Feedback: (5)

Thanks Ollie,
You really should be awarded a title here at RCU.