Can you accurately calculate thrustlines - RC Groups
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Jan 23, 2013, 05:38 PM
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Joel K. Scholz's Avatar

Can you accurately calculate thrustlines

I am into a build that will make it difficult to alter the thrust lines once completed is there an accurate way to calculate thrust lines or is it only a guess?
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Jan 23, 2013, 09:11 PM
B for Bruce
BMatthews's Avatar
Strictly speaking it's a guess. It's also complicated by the style of model. For example a high wing trainer will tend to have a more forward CG location so the wing to stabilizer is set up with more angle to generate the sort of decalage or "longitudinal dihedral" needed to be very pitch stable. But with that comes a stronger tendency to nose up when adding power or during a dive. This being counteracted with a generous amount of downthrust. But that very same model if set up with a more rearward CG can be re-trimmed with far less decalage and much less downthrust.

So what you're making and how close to neutrally stable you plan on setting the balance point all comes into play when guessing the amount of downthrust you need.
Jan 23, 2013, 11:50 PM
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Joel K. Scholz's Avatar
Care to guess this one?
Jan 24, 2013, 12:26 AM
vespa's Avatar
Ideally the thrustline should be horizontal in cruise for optimum efficiency. This you can calculate thru the lift equation, knowing the weight and the speed you wish to cruise, and it will likely be around 2-10 degrees AOA thru your useful speed range. As Bruce describes, it's sometimes useful to skew the thrustline downward such that it counteracts some of your pitch stability.

Note that dihedral is very difficult to implement successfully on flying wings -- you'll want a lot of vertical tail to balance it and almost certainly will need a rudder mixed with ailerons.
Jan 24, 2013, 12:42 AM
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Joel K. Scholz's Avatar
Vespa, I am glad you pointed this out. Please explain why dihedral is a problem. It is not too late to change and served to actually weaken the joint.
Jan 24, 2013, 12:46 AM
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Joel K. Scholz's Avatar
Vespa, thanks for pointing this out. it is not too late to change the dihedral. Can you explain why the dihedral presents problems?Sorry for the double post.
Jan 24, 2013, 02:54 AM
B for Bruce
BMatthews's Avatar
Since it appears that you're using ailerons why do you want that much dihedral? Maybe something like 2 or 3 degrees to encourage a degree of self leveling. But you sure don't need the polyhedral arrangement you show. Not to mention that you'd need a larger equivalent vertical fin area to boot.

On a setup like this and given that you'll be using a relatively forward CG location I'd suggest that you go with around 3 degrees of down angle on the motor with relation to the wing sections center line. That should be enough of a downforce to get by. It may benefit from a degree or two more but you won't know until it's into the first few test flights.

Also SOME amount of nose up pitching with power is considered normal by full size pilots. It's only some modelers that seem to think that the throttle control is supposed to only make the plane fly faster.

Vespa's note about the thrust line being ideally straight ahead when in cruise might well be right for aircraft that fly a long way at a fixed speed. But that simply doesn't apply to our models. We seldom fly for more than a few seconds in any one direction and attitude. So instead of worrying about cruise and angles of attack at cruise it's simpler to see the downthrust effect as a way to reduce by some amount the tendency to pitch the nose up as power is added. Another way to look at it is that downthrust avoids the need to tinker with the elevator trim to avoid a strong pitch up with power addition or removal of power. Meanwhile full size aircraft pilots accept the need to alter the elevator trim in association with power changes as a primary flight control.

Since you're going to have a cowling around the motor with a small nose opening there's a trick you can use that allows for some adjustment of thrust angle without needing to reshape the cowl. What you can do is make up an X mount for the motor that has vertical parts of the X so it looks sort of like |X| . Then you make vertically slotted holes in the vertical parts of the mount plate. You can then use these slots when you shim the mount for more or less downthrust to move the back of the motor up or down a little and keep the prop driver at the same vertical point as the motor's angle alters. You likely won't be able to do this over a long angular to offset amount but it should be enough to be able to shift things around by 2 or 3 degrees either side of the nominal setting that you build in.
Jan 24, 2013, 08:42 AM
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richard hanson's Avatar
Looks OK as is - set the "aileron" panels on the wings to go UP but not down.
I presume -it is tended to soar - not be an aerobatic setup
try it
Jan 24, 2013, 10:10 AM
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Joel K. Scholz's Avatar
Thanks for the tips. The bird is intended to be a soaring glider and wingtip polyhedral to give a more lifelike profile . I still am not clear as to why the dihedral is so problematic?
Jan 24, 2013, 11:43 AM
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richard hanson's Avatar
Originally Posted by Joel K. Scholz
Thanks for the tips. The bird is intended to be a soaring glider and wingtip polyhedral to give a more lifelike profile . I still am not clear as to why the dihedral is so problematic?
Your setup is typically very stable in roll.-the dihedral and additional tip angle assure that .
For ailerons to be really effective - roll stability is typically much less - corrections are made using he ailerons - also turns are typically done using the ailerons to establish a bank- Not really necessary in some cases but works well on many designs.
Your setup looks ine - but you may need to fuss with how the relative up n down of the "ailerons" is set. try mostly UP" which will reduce lift on the panel and establish a bank angle too. Should make for nice easy turn
The thrust line - may/may not need fussing with-
as long as th e model does not try to "nose up or nose down" excessively as you change power - leave it alone
The CG will likely require th e most fine tuning to get a flat controllable glide
depending on how wing loading turns out - this will change.
Jan 24, 2013, 01:53 PM
vespa's Avatar
Dihedral causes Dutch roll instability. Yaw damping cures it. Yaw damping is a very strong function of the tail boom length and differs from basic yaw stability (a.k.a. yaw stiffness) which is just a simple function of tail volume (area x distance). So with a typical short-tailed flying wing you often cannot obtain sufficient yaw damping to balance the dihedral without oversizing the vertical tail to the point where spiral instability becomes an issue.

Second, dihedral aggravates adverse yaw as recently discussed here. Being that this is a flying wing, aileron differential cannot be used to combat adverse yaw so you either need to minimize it with low dihedral and high speed flight or combat it with a mixed rudder. The best option would be both. You may notice that of the few examples of flying wings with significant dihedral (e.g. Windfreak, Windlord, Raven) most have large rudders and no ailerons, while other aircraft like the Helios are actively stabilized, as are of course, birds.
Jan 24, 2013, 02:48 PM
B for Bruce
BMatthews's Avatar
Joel, one easy and fun way to test the design for signs of what is being discussed is to make a simple little all sheet balsa test glider. For the sake of it you can make the wing from flat 1/16 stock and simply steam a small amount of reflex into the trailing edge. A saw cut at the polyhedral joint about 1/2 inch long might be needed for clearance to allow the refex to be steamed into the wood. You only need to steam in about 1/32 worth of reflex over about a 1/2 inch of the trailing edge if the span is around 16 inches.

The tail sections you show can be made from 1/32 sheet and the fuselage "body" from some 1/8.

You'll quickly find out if the design you propose is stable in yaw or if it has a nasty Dutch Roll issue.

And when done you can hang it over the bench as a decoration....
Jan 24, 2013, 04:21 PM
An itch?. Scratch build.
eflightray's Avatar
My Eagle has quite a lot of down thrust, but that could be down to the amount of power, CG location, and tail configuration.

But she flies a treat, so I'm not particularly worried about it.
Jan 25, 2013, 03:28 PM
vincent123's Avatar
Most important reason why you need downtrust, is the effect you plane climbs when powering, right?

I bed you are using cambered airfoils, these give (a lot of) increased lift when speeding.
Use symmetric foils, and the effect will be only a fraction. Place the powerline above the cog and up you go.
Jan 25, 2013, 11:20 PM
B for Bruce
BMatthews's Avatar
Vincent, you can fly with the same downthrust angle, or lack of it, on the lifting airfoils as with the symetrical ones. It's not about the airfoil. It's about the CG position and how close it is to the aircraft's neutral point.

It's just that so often the lifting airfoils are used on trainer or trainer like models that it SEEMS like the airfoil is what requires the downthrust. But don't be sucked in by that. Instead try it some time. Move the CG back on a flat bottom wing airplane until it darn near won't pass the dive test for the CG being so far back that it tucks. You'll find that you have a whole other sort of model on your hands compared to the usual trainer like setup.

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