Flight Trimming

leetx
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Re: Flight Trimming

Postby leetx » February 19th, 2010, 10:07 am

Reading the Indoor Duration clip posted by leetx,
The spar is blended into the surrounding blade, and the hub area
is lightened.


I understand that I am going to be sanding down the gray spar and the center hub as well as the white blade.
I would focus primarily on the blade and secondarily on the spar. As I mentioned earlier, to my touch, the new blade is significantly thicker than the older one that I have used. I have some doubt that the blade will flare as it's supposed to.

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Re: Flight Trimming

Postby WrightStuffMonster » February 19th, 2010, 1:49 pm

Could someone explain to me the method of creating wing wash? Am I correct in assuming that the left wing (by left, looking at the plane head on) should be tipped so that the leading edge is about 1/8th and inch higher than the trailing edge . . . while the right side of the wing is level. Is that correct? I just don't know how to teach my students how to create it.
I think you have it backwards. If you are looking at the plane head on the right panel should be the one with wash. The reason is that it is the inboard wing in turns and is moving slower than the other panel of the wing so it creates less lift. Without wing wash your plane will not fly flat and not all of your lift goes to keeping your plane in the air. If you do wing wash correctly you should find your plane flies much flatter. The traditional way to create wingwash is by creating a diagonal cut in the center of your front spar, propping one corner of the wing up, weighing the rest of the wing down with coins, and gluing the angle in the spar. When you get more advanced there is a way to adjust the amount of wing wash that you have to suit a particular turn if you are using tissue tubes with your wing posts, but concentrating on trimming the other aspects (rubber wing tail ect) before worrying too much about adjusting wing wash for a particular radius of turn.
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Re: Flight Trimming

Postby SOCoach » February 20th, 2010, 5:31 am

Thanks for the reply - our kids have never attempted wing wash, they have gotten turn by angling the tailboom to the right by looking head on . . . I have a feeling that is causing excess drag.

So I am clear then . . . looking the front of the plane it is the RIGHT side that needs wash. So the leading edge on the needs to be raised by 1/8 inch, keeping tailing edge flat. The left side needs to be flat (The left side of the wing should be a few cm longer correct?)

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Re: Flight Trimming

Postby jander14indoor » February 20th, 2010, 7:09 pm

I think you have the longer wing backwards too. The inside wing is longer and has wash in. Note as described above, the purpose of wash in and longer inside wing is to fly flat, NOT banked like a piloted plane.

Your left turn is NOT doe to wing lift inward like a piloted plane. Turn comes from differential drag, OUTWARD lift on the tilted tail, and prop offset.

Note, you still need the tail offset as you have described it, but not so much to induce turn, but to reduce drag. You want the tail fin to be on the circumference of the turn and the circle in most sites is small enough to make noticeable bend in the tail to implement this.

Because of the widely varying torque from a rubber band, you actually need a balanced combination of all these adjustments to keep the turn radius at minimum drag.

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Re: Flight Trimming

Postby SOCoach » February 20th, 2010, 7:48 pm

:)

I am glad my kids have managed to get a plane to turn DESPITE my help . . . .

So what I said earlier . . . except the RIGHT wing should be a few cm longer . . Thanks!

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Re: Flight Trimming

Postby jander14indoor » February 21st, 2010, 3:26 pm

Nah, pretty easy to get the planes to turn, STRAIGHT is hard. Trick is, turning the way you want, radius you want, and doing it consistently and efficiently. Not real critical when all you are trying to do is get it to fly at all. As the times go up, these things become more important. You are just starting to get to that point is all.

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Re: Flight Trimming

Postby SOCoach » February 25th, 2010, 7:28 am

Okay Jeff you jinxed us . . . . we now have a plane that flies straight!

My kids created a new tail boom and now the plane acts like it is fighting itself. It looks like the propeller is trying to turn the plane left (counter clock wise), but I believe the tail boom is fighting it . . .

So . . . the kids need to angle the tailboom . . . let me see if I can get this correct . . . . it should be offset to the right when looking at the plane head on. The haven't created a new wing with wing wash, so I am assuming it it strictly the new tailboom causing the problem right?

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Re: Flight Trimming

Postby Greg Doe » February 25th, 2010, 11:44 am

SOCoach,
Jeff and others may respond with help, but let's all get on the same page. Right and left
on any airplane is referenced the same as a car or boat. If you are sitting in the airplane
(or car or boat) facing forward, the right side of the airplane is to the "drivers" right.
To answere your question, yes the fuselage should be offset slightly to the right if "looking
at the plane head on". SLIGHTLY can mean a lot of things! For a 12 inch tail boom, probably
no more that 3/16 inch from front to back. Remember there are several trim factors that all
work together to keep your airplane circling. Too much of any one adjustment adds excessive
drag. All of these adjustments have been discussed in this forum. The prop should be angled
slightely to the left. The tail boom should be off set slightely to the left. The wing should have
a very slight right turn twist. (washout of the right wing panel is prefered over washin of the
left wing panel). Washout means the trailing edge of the wing is twisted up at the tip when
compared to the root (center of the wing). Washin means the trailing edge of the wing is
drooping when compared to the root. You also want a slight off set of the wing to the left. That
means that if you measure from the fuselage the left wing should be about 1/4 longer than
the right wing. Here is the thing to keep in mind about the wing twist (washin and washout).
If your airplane had none of the left trim adjustments (thrust, tail boom, or wing off set) the
wing twist would make it turn to the right. The right wing twist is extra drag, but there is a
more important benefit in that the airplane won't screw into the ground from torque when the
rubber motor is fully wound, and your airplane will circle in a flat turn, instead of a banking turn,
which is more efficient.
Now all of this is great if you build a perfect airplane, but nobody does, so sometimes you
will have to use a little more or less of any or all of the trim adjustments. Good luck
Greg Doe
Smyrna, TN

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Re: Flight Trimming

Postby Greg Doe » February 25th, 2010, 12:08 pm

SOCoach
I failed to mention one other important adjustment which is stab tilt. When viewed from the
back of the airplane the right stab tip should be lower then the left tip. Hold the airplane with
the nose pointing away from you. Sight across the trailing edge of the stab and compare it to
the trailing edge of the wing. Instead of everying being SQUARED up, the stab should be
angled slightely. From tip to tip about 3/8 of an inch; or each side about 3/16 of an inch.
Greg Doe
Smyrna, TN

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Re: Flight Trimming

Postby leetx » February 26th, 2010, 9:48 pm

I would like to ask how the plane flies at cruise when it is efficient. I would like to focus on the lateral (up-down) characterisitcs of the flight.

Assuming that the static set up for the plane is as these:
Balance point should be at rear wing post for say a 10 cm chord wing to last years rules. 1-2 cm if front of rear post for a 12-15 cm wing. 2-4 cm behind wing for a 5 cm bonus wing.

Wing level with motor stick (I use top of motor stick for my consistent reference on angle of attack) fore and aft.

Prop should be pointing SLIGHTLY (2 degrees or so) left and down.

Leading edge of tail should be 2-4 mm lower than trailing edge with respect to wing level (or again, top of motor stick).

Jeff Anderson
Livonia, MI
During level flight, I would expect the wing's angle of attack to be positive, the stab's angle of attack to be slightly positive. This means that this plane flies nose up at the same angle as the wing's angle of attack. I also assume that the prop's thrust points up, less than the nose but still up.

I ask because I see different static set up in the plans of different designs of indoor planes. Here are some that I see:

1 prop thrust level, wing incidence positive, stab incidence slightly positive (as in the Freedom Flight kit)
2 prop thrust negative, wing incidence level, stab incidence negative (as jander's above)
3 prop thrust level, wing incidence level, stab incidence negative

Do these different set ups basically lead to the same flight characteristics? One set up that confuses me is the down thrust of the prop. What is its significance?

In the planes we fly, #1 above is the static set up. During cruise, the nose is slightly up. One thing I notice is that during flight, the tail boom tends to bend a little up so that the stab is almost level during flight. The wing's angle of attack during flight is slightly higher than its static incidence.

Answers and comments welcome!

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Re: Flight Trimming

Postby jander14indoor » February 27th, 2010, 4:35 am

If you think about it, 1 and 2 are the same (I'll come back to 3), the motor stick is not a lifting surface, so it just takes the angle the wing and tail take it to. If you reference everything to the wing, the prop and the tail are the same on those two options. The only difference is the angle of the motor stick, which is very visible, but not real important (within reason) to controlling flight.

For maximum flight efficiency, the wing must fly at its maximum lift over drag point, which will be at a fairly high angle of attack, but this angle of attack is with respect to the local air flow, NOT the motor stick. This is controlled by the center of gravity and the horizontal stabilizer's angle of attack. For these indoor planes where you want some lift from the tail (limited wing size) this means a very rearward cg as described, and stab at a slight negative angle of attack with respect to to the wing. Note, it will probably be POSITIVE with respect to the local air flow, if you have cg back far enough. Both 1, 2, and 3 have this. And all will fly apparently nose high in the cruise portion of the flight. Frankly, near to stall.

Prop angle. This has NOTHING to do with steady flight, but everything to do with taming the very non-linear torque curve of a rubber motor.
- When fully wound, the motor is at maximum torque, will spin the prop at maximum rpm, producing maximum thrust. But this drops very quickly.
- For steady flight though, you must balance lift, gravity, thrust and drag. But on a free flight plane you can't change ANY of those things directly during flight.
- So, what do you do, you compromise! Trim the wing and tail for optimal flight during the cruise portion since it lasts the longest.
- This leaves you with a problem. During max winds, you have too much power, and the plane wants to climb. Too much! It can do something known as a power stall. Plane climbs, raises nose, climbs, raises nose, whoops, power dropping, STALL, drops nose severely and dives, hitting the floor and stopping the flight. You pick it up, wondering what's wrong, relaunch and it flies wonderfully??
- THIS is why you have negative prop angle. During the high torque part of the flight you get down thrust to keep the nose down and stay out of power stall. As torque drops, the down thrust stops and the wing and tail take over to control flight attitude.

Now, back to trim option 3. If you only fly in low sites, you have to back off winds from the torque peak to keep out of the ceiling rafters and never see the above behavior. As a result, you don't need down thrust. Of course when you get to this years national site with its hundred foot ceiling and wind to the max to reach it WHOOPS, you see power stall for the first time!

Hope that helps,

Jeff Anderson
Livonia, MI

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Re: Flight Trimming

Postby leetx » February 27th, 2010, 12:24 pm

If you think about it, 1 and 2 are the same (I'll come back to 3), the motor stick is not a lifting surface, so it just takes the angle the wing and tail take it to. If you reference everything to the wing, the prop and the tail are the same on those two options. The only difference is the angle of the motor stick, which is very visible, but not real important (within reason) to controlling flight.

..

Prop angle. This has NOTHING to do with steady flight, but everything to do with taming the very non-linear torque curve of a rubber motor.
- When fully wound, the motor is at maximum torque, will spin the prop at maximum rpm, producing maximum thrust. But this drops very quickly.
- For steady flight though, you must balance lift, gravity, thrust and drag. But on a free flight plane you can't change ANY of those things directly during flight.
- So, what do you do, you compromise! Trim the wing and tail for optimal flight during the cruise portion since it lasts the longest.
- This leaves you with a problem. During max winds, you have too much power, and the plane wants to climb. Too much! It can do something known as a power stall. Plane climbs, raises nose, climbs, raises nose, whoops, power dropping, STALL, drops nose severely and dives, hitting the floor and stopping the flight. You pick it up, wondering what's wrong, relaunch and it flies wonderfully??
- THIS is why you have negative prop angle. During the high torque part of the flight you get down thrust to keep the nose down and stay out of power stall. As torque drops, the down thrust stops and the wing and tail take over to control flight attitude.

...

Hope that helps,

Jeff Anderson
Livonia, MI
That was very helpful. I have a few related follow up questions.

1. Assume that the plane cruises near stall: wing has high angle of attack, stab has less but still positive angle, nose is up, prop thrust also up. I want to ask about the vertical component of the prop thrust. Is the vertical thrust wasted? I imagine that the prop thrust would be most efficient if it were strictly horizontal. Or does some vertical thrust in fact add stability and/or efficiency to the cruise? Another way to view this is why not have a static set up for the prop thrust, wing, and stab that matches the desired cruise conditions: prop thrust level, wing incidence large, stab incidence smaller but still large.

2. One of the reasons I ask these questions is because I have heard from a very experienced indoor flyer that it is best to trim the plane to fly with the lifting surfaces (wing and stab) as flat as possible ( along with as far back CG as long as plane is stable). This advice seems to be in conflict with your explanation as well as my own observations.

3. Here are the trimming notes from Cezar Bank's Leading Edge plan: "you goal is a smooth, slow 'cruise' flight speed [and a left circle of about 25 ft diameter]. Tweak (coerce) boom up or down as needed to achieve slowest speed w/o 'mushing'. Fine tune by adjusting posts in socket." What is "mushing"?

Thank you again.

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Re: Flight Trimming

Postby carneyf1d » February 27th, 2010, 12:56 pm

i think "mushing" refers to stop and go flight where the planes stalls, dips, stalls, and repeats.

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Re: Flight Trimming

Postby WrightStuffMonster » February 28th, 2010, 12:33 am

1. Assume that the plane cruises near stall: wing has high angle of attack, stab has less but still positive angle, nose is up, prop thrust also up. I want to ask about the vertical component of the prop thrust. Is the vertical thrust wasted? I imagine that the prop thrust would be most efficient if it were strictly horizontal. Or does some vertical thrust in fact add stability and/or efficiency to the cruise? Another way to view this is why not have a static set up for the prop thrust, wing, and stab that matches the desired cruise conditions: prop thrust level, wing incidence large, stab incidence smaller but still large.

2. One of the reasons I ask these questions is because I have heard from a very experienced indoor flyer that it is best to trim the plane to fly with the lifting surfaces (wing and stab) as flat as possible ( along with as far back CG as long as plane is stable). This advice seems to be in conflict with your explanation as well as my own observations.

3. Here are the trimming notes from Cezar Bank's Leading Edge plan: "you goal is a smooth, slow 'cruise' flight speed [and a left circle of about 25 ft diameter]. Tweak (coerce) boom up or down as needed to achieve slowest speed w/o 'mushing'. Fine tune by adjusting posts in socket." What is "mushing"?

Thank you again.
1. That was always my theory. I think the reason alot of people use downthrust is because indoor planes tend to fly with the tail down for much of the flight so having a bit of downthrust reduces climb at the beginning of flight and makes cruse and decent more efficient.

2. You are totally right with this. This was the advice I got from the best indoor fliers as well and I used it to good effect. Like you say I would always trim my planes flat and adjust flying characteristics solely with the CG. There is one major disadvantage to flying this way though in that your plane is quite a bit less stable and if the air is particularly bad or if you do alot of hits it might not be the way to go (I have had planes trimmed this way fall 20 feet out of the sky)

3 Mushing is a type of stalling but its pretty subtle. Like a hickup in flight. It does not totally stall so sometimes people miss it.
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Re: Flight Trimming

Postby jander14indoor » February 28th, 2010, 4:15 am

question 2. Careful where you are measuring wing angle of attack from! When the indoor fliers recommend wing and tail as flat as possible, they are generally referring to with respect to the motor stick, NOT the local air flow. This is because its easy to measure wrt the motor stick while local air flow (which is the REAL angle of attack) is hard without a wind tunnel. In flight these 'level' flat wings and tail fly very nose high during level cruise meaning they have very positive angles of attack wrt the air, which is what counts.

question 1. Prop thrust. Yes, any up or down angle contributes to the total lift budget. But this contribution isn't near as efficient as the wing or tail so you want to use the prop for thrust as much as possible, not lift. More important is the up thrust or down thrusts impact on flight attitude throughout the flight. Its very strong when the motor is wound tight early in the flight, dropping of quickly with torque till its fairly insignificant in the end. This lets you use down thrust to manage that early hard climb by pulling the nose down when thrust is high, limiting lift, and letting the noise rise as it drops increasing lift as speed drops.

question 3. already covered by others

Jeff Anderson
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