Flight Trimming

[jander14indoor]

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

[leetx]

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.

[carneyf1d]

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

[WrightStuffMonster]

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.

[jander14indoor]

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
Livonia, MI

[leetx]

Thanks jander, carney, WSM for your inputs. I did a little more poking around the internet, especially at the Yahoo Indoor Construction Group.

I noted that there are differences in opinion among the "experts". Some like flying nose up, with thrust in line with motorstick and hence up. Others don't like this and prefer nose level and thrust level during flight.

There were a couple of reasons cited by the proponents of nose up flying. First, when the nose is up during cruise and descent, the propeller slows down, and this contributes to a longer flight. Second, others have noted that if the nose is not up during cruise, the plane will take a small dive and lose altitude when cruise power is used up. The common static set up to achieve nose up flying is to set the wing level with motor or slightly positive and have the stab slightly negative wrt the motor stick.

In two of our planes, even though the wings and stab incidences are both positive relative to the motor stick, they cruise slightly nose up. These two fly nose up and have very smooth transitions from cruise to descent.

In a third plane, the wing and stab incidences are more significantly positive, and the plane cruises with the motor stick level. I have noticed that this plane suffers a 3' level drop (a small dive) at the end of cruise.

However, the proponent of level thrust, level motor stick flying is a current world record holder. (The proponents of the nose-up school are also world record holders.)

What's to make all this? One, use your stop watch and measure your trim by time. Discussing theory and other people's opinions/experiences is useful, but make enough measurements yourself so that you can better understand the theory. Continue to question, experiment, and analyze.

Have fun.

leetx
WS coach

[jcollier]

What makes this event so interesting is that every plane / wing / etc. is an entity of its own. My son's best time last year was with a plane that flew nose up and was fighting for altitude every second. Our high school coach likes planes to fly like that. When I help John Clapp at states, he often comments on planes flying and clearly favors more level flight. Yesterday my son was doing trim flights and the plane looked like it was low on power. He lowered wing incidence 1mm at a time and the height increased and trim times went from less than a min. to 1.25. The other plane was literally just the opposite! As jander said a few times, the event just begins when the student finishes building the plane.

[StampingKid]

Regional tomorrow. Flying Freedom Flights with broad blade which has large rear stab and works best on a rearward cg. Pea-sized mass on end of tail stick gets good cg. Placing that on top of rudder should move the cg back a bit but hard to tell as I have only been able to get good reading on cg by balancing plane upside down. I have gym time tonight. Any thoughts on whether placing it that high will adversely effect flight? It would be more prominent there--larger mass as opposed to sandwiched on tailstick and would also be more offset to left.

[StampingKid]

J.Collier write

What makes this event so interesting is that every plane / wing / etc. is an entity of its own.

That makes me think that I still have both my planes from last year. And they are built to this year's rules. I am going to get them out of moth balls and check their static set up and bring them to my next practice. I will not be nearly as nervous about banging them against the rafters. It will be neat to see if I have gotten better as a flyer. I may even see about building a wing to the state bonus, though I doubt it.

[illusionist]

What is an ideal location for the cg?