Stol ch 801 /Sig senior kit bash
02-19-2018, 06:21 PM
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Stol ch 801 /Sig senior kit bash
Trying to make a stand off scale ch801 out of a senior kit .. going for fixed slots with junkers flaps . Debating on raising rear empennage to level with wing and making horizontal stab an inverted airfoil .. will there be any massive adverse effects ?
02-20-2018, 01:00 PM
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Full size planes use the inverted airfoil where they need a lot of pitching authority. But the full size planes tend to run more forward and stronger pitch stability CG locations. On our models we can get away with moving the CG back which reduces the need for a strong lift from the tail surfaces. So I'd say you don't need the inverted lifting section.
But using a thicker symmetrical airfoil would probably not be a bad idea. Or perhaps you could use a thinner "Randy Reynolds Nearly Symmetrical" airfoil with the flat lower rear area to make for easier building and install it upside down. A sketch of this style I did some years back and a thinner "tail section" version added just now because it's easy to scale and flip is shown below. That might give you the best of both.
I'd also suggest using a wider elevator than the original. That was set up to use as a trainer so they didn't want a strong elevator action. But at lower speeds you will want the more positive response when the controls tend to get mushy otherwise. So I'd suggest a 30% wide elevator as also shown in the "inverted" suggestion.
For flaps you might want to consider a simple slotted flap setup as shown in the second sketch. This would use scissors style hinges made from metal or something like G10 fiberglass board or heavy duty Robart Giant Scale hinge pins partially inserted so the hinging occurs below the wing. A discussion on flaps like this on another forum has posts from someone that is flying a model with such flaps. He reports that they are very effective. So much so that if used at more than 45 to 50° during a landing approach that he has to add some power to avoid the model flying so slowly that the controls become too mushy and ineffective. And it should not take a whole lot do include them as shown by the upper right option.
Another thing if you WERE to use the same style hinging for the ailerons is that when the aileron on the down moving wing goes up the nose would stick down on the lower side and produce a little drag. And that could be a great thing to aid with killing off the adverse yaw you can have when flying at high angles of attack and slow speeds. So using this same knife style hinging arrangement for the ailerons could have a double advantage.
I'd almost say that it might be nice to hinge the ailerons in the same manner for a full span arrangement. And with mixing and trigger points if you have them available set up the flaps and aileron mixing so the ailerons droop by a lower amount mixed in through trial and error to give you full span camber changing for super short takeoffs and super slow landings.
Just keep in mind though that all this other stuff does not avoid the need for a light wing loading. The first and primary requirement of slow flying is a light wing loading. You can't cheat this by using flaps and a lot of power. So watch that weight more than anything.
But using a thicker symmetrical airfoil would probably not be a bad idea. Or perhaps you could use a thinner "Randy Reynolds Nearly Symmetrical" airfoil with the flat lower rear area to make for easier building and install it upside down. A sketch of this style I did some years back and a thinner "tail section" version added just now because it's easy to scale and flip is shown below. That might give you the best of both.
I'd also suggest using a wider elevator than the original. That was set up to use as a trainer so they didn't want a strong elevator action. But at lower speeds you will want the more positive response when the controls tend to get mushy otherwise. So I'd suggest a 30% wide elevator as also shown in the "inverted" suggestion.
For flaps you might want to consider a simple slotted flap setup as shown in the second sketch. This would use scissors style hinges made from metal or something like G10 fiberglass board or heavy duty Robart Giant Scale hinge pins partially inserted so the hinging occurs below the wing. A discussion on flaps like this on another forum has posts from someone that is flying a model with such flaps. He reports that they are very effective. So much so that if used at more than 45 to 50° during a landing approach that he has to add some power to avoid the model flying so slowly that the controls become too mushy and ineffective. And it should not take a whole lot do include them as shown by the upper right option.
Another thing if you WERE to use the same style hinging for the ailerons is that when the aileron on the down moving wing goes up the nose would stick down on the lower side and produce a little drag. And that could be a great thing to aid with killing off the adverse yaw you can have when flying at high angles of attack and slow speeds. So using this same knife style hinging arrangement for the ailerons could have a double advantage.
I'd almost say that it might be nice to hinge the ailerons in the same manner for a full span arrangement. And with mixing and trigger points if you have them available set up the flaps and aileron mixing so the ailerons droop by a lower amount mixed in through trial and error to give you full span camber changing for super short takeoffs and super slow landings.
Just keep in mind though that all this other stuff does not avoid the need for a light wing loading. The first and primary requirement of slow flying is a light wing loading. You can't cheat this by using flaps and a lot of power. So watch that weight more than anything.
Last edited by BMatthews; 02-20-2018 at 01:04 PM.
