Wright Stuff C

[madhavaniyengar]

Hey guys,

I was testing my plane today (FFM, monoplane), and it was flying to the left pretty decently, and then when I switched it to the right, it started stalling and bobbing up and down, unable to climb at all. I only had enough time to run it twice to the right before I got kicked out by sports, so I don't have any videos rn. It was also hard for me to get it to fly to the right at all, I had to turn the stab all the way to the right on the protractor, and even then, it was a bigger circle than leftwards, It was with a pretty low torque btw, about 120 x 15 winds with one de-wind.

Any idea what could be causing this? I'm just looking for some solutions before I test again because I don't have many days left to test before invitational.

Thanks!

My plane behaves similarly in terms of the turn. (ish) Yes, the right circle is much harder to turn as the torque is fighting the turn as the way the rubber unwinds favors it to turn left (someone correct me if I don't have the exact reason down, but I know it was something along those lines) . For my plane, when I put the rudder all the way to the right, the circle, while still big, is only the diamter of a basketball court which would allow me to fly in most competitions.

For you problem, have you tried adding shims to the wing to cause intentional slight wing warp to help your plane better turn? Also, make sure your wing is perfectly straight without warp before you add it to the plane. I'm not entirely sure why your plane would stall, but I bet others on the forums who are much more experience probably knows why.

Hope this helps.
Xiangyu

Thanks bro
I added a lot of shim, and I'm thinking it might be too much. I added like 5 layers of the sticky paper and also a sliver of post-it note, which still let it fly while turning left. Idk the thickness of the post-it but it could be too much for turning right.
I made the wings as parallel as possible while building, so I hope that isn't the issue.
If anyone has any other ideas/suggestions, I would appreciate it!
Thanks again

[lechassin]

I know our planes as well as the Senior flyer need decreased decalage going right or else they stall, especially while trying to climb.

In our case we need to decrease the wing incidence about 1mm going to the right. IDK how much the senior flyer needs but there's a shim under the leading edge of the stabilizer going right (different way of decreasing decalage).

Maybe try something like that.

[xiangyu]

I know our planes as well as the Senior flyer need decreased decalage going right or else they stall, especially while trying to climb.

In our case we need to decrease the wing incidence about 1mm going to the right. IDK how much the senior flyer needs but there's a shim under the leading edge of the stabilizer going right (different way of decreasing decalage).

Maybe try something like that.

Hmm that's interesting. While I've never had this problem (yet), I'm curious to know why that might be. Is it because the inward wing stalls less so the plane climbs easier?

Xiangyu

[lechassin]

When our planes fly well one way but not the other and we know the plane is straight, we figure we're close to a decent setup (prop, motor, CG, etc...). A little change can help with a problem going the other way.

In our case, stalling to the right implied our decalage was too high, but the "problem" is actually when we go left: the nice flight profile we have going left is misleading and is the result of an inefficiency that we're accepting for now: prop torque causes a slight left bank and reduction in lift that we fix with decalage, but that decalage setting is too much going right, because the prop torque keeps the wings flat going right ("too much" lift).

Our approach for now is reduce decalage going right. It works but admittedly we're adding in the same degree of inefficiency we have going left. That's fine as a quickie-fix if a contest looms close, but it's not ideal.

Our goal is to keep the wings level both ways and eliminate that inefficiency, and wash-in or offset wings will achieve that. But in our case we then get numerous ceiling hits unless we unwind a LOT. We have thinner rubber to try but we need to get into the gym...

[lechassin]

I just noticed a question from a month ago on the Youtube DIY prop video, on how we determined the prop helix. Here is a copy of the belated response:

We determined the pitch helix empirically: we flew with rigid, non-helical props first and determined that 30 degrees at the tip was best for us. We then simulated one turn of the prop by drawing a 30 degree line on paper that projected straight the same distance as the circumference at the tip of the blade. The line reached 5" from the horizontal, so our tip pitch is 5". We then traced 5" lines for smaller radii and determined that 5" pitch at the root would be an excessive angle (>60 degrees IIRC). We settled for 45 degrees near the root which measures out to 4" pitch. We cut blades till we got one with the right helix, and we keep that blade as a template. Not scientific, really...

[xiangyu]

When our planes fly well one way but not the other and we know the plane is straight, we figure we're close to a decent setup (prop, motor, CG, etc...). A little change can help with a problem going the other way.

In our case, stalling to the right implied our decalage was too high, but the "problem" is actually when we go left: the nice flight profile we have going left is misleading and is the result of an inefficiency that we're accepting for now: prop torque causes a slight left bank and reduction in lift that we fix with decalage, but that decalage setting is too much going right, because the prop torque keeps the wings flat going right ("too much" lift).

Our approach for now is reduce decalage going right. It works but admittedly we're adding in the same degree of inefficiency we have going left. That's fine as a quickie-fix if a contest looms close, but it's not ideal.

Our goal is to keep the wings level both ways and eliminate that inefficiency, and wash-in or offset wings will achieve that. But in our case we then get numerous ceiling hits unless we unwind a LOT. We have thinner rubber to try but we need to get into the gym...

Hmmm interesting. What if you build your plane with the propeller mount turned 1-2 degrees to the right to try to counteract that torque? That's what I tried to do and I wonder if that is why my plane flys similarly both ways... That could be something interesting to explore, I'll play around with that after the invite.

[jander14indoor]

Prop torque causes roll which causes the plane to bank left and turn left. To control roll, you can offset wings or change angle of attack left to right (wash-in or -out). Prop offset can only change pitch or yaw (do the force vectors) directly. Changing yaw to MIGHT affect roll because it pushes one wing faster than the other (slightly) but is probably not the best way to do it. But changing yaw will change turn.

Jeff Anderson
Livonia, MI

[CookiePie1]

So, we're going to start testing our planes, we have a ff and a guru. Just a few things i'm confused about?
How do you measure decalage and what effect does it have in flight?
How does changing cg affect the performance of an aircraft? What's the best place to have it depending on the circumstance?
What does prop pitch do?

[lechassin]

I hope this helps: I answered in general (practical) terms within your quoted text. Experts will hopefully add as needed.

1)How do you measure decalage and what effect does it have in flight? Decalage is the difference in angle (seen from the side) between the wing and the stabilizer (incidence is the angle of one or the other with respect to the thrust line). It's a balance between lift and drag that needs to be optimized, and it also contributes to the plane's ability to recover from a dive (like after a hit or flying under an HVAC vent). The leading edge of our wing is about 4mm higher than the trailing edge, and our stabilizer is set at zero, so for us, decalage is about 4mm. If the wing were at 2mm and the stab were at -2mm, decalage would also be 4mm. We put all of the decalage in the wing to create the effect of down thrust, which helps control the climb rate and prevent stalls at highest power (right after launch). As the power bleeds off, the down thrust effect is diminished and the plane cruises flat.

2)How does changing cg affect the performance of an aircraft? What's the best place to have it depending on the circumstance? "A nose heavy plane flies badly, a tail heavy plane flies once". Err on the nose heavy side with decalage to hold the nose up. However: the more aft you make the CG, the more efficient the plane is (less decalage needed to hold the nose up, so less drag), but stability and recovery characteristics suffer, so it's another compromise to be optimized. For example, a huge space with a high ceiling and no drafts will accomodate low decalage/aft CG since the flight path is unlikely to be disturbed, whereas a confined space with a low ceiling and HVAC will demand a fore CG and more decalage, since any disturbance can otherwise result in a crash. This year a good place to start would be decalage 5mm, then adjust the CG till the plane flies flat under moderate power (1/2 wind).

3)What does prop pitch do? It provides thrust, pulling the plane through the air mass. Pitch is the theoretical distance the prop would travel in one turn. Easily visualized by drawing the blade angle on paper, then projecting a line along that angle equal to the circumference of the prop at whatever point along the blade is being considered. Blade angle is higher at the root to achieve same forward motion over a smaller distance traveled (smaller circumference). Pitch is another compromise. For these slow planes, in general you want the lowest pitch that will pull the plane juuuust fast enough to stay aloft, at the lowest possible power input. This year the main problem to overcome is that the small prop can allow the motor to spin down fast. Every solution to slow the prop down is a compromise: higher pitch requires more power, thinner rubber may not have enough power, more rubber to hold more knots is heavier, more aggressive winding nearer to the breaking point of the rubber to hold more knots is riskier, 3 bladed prop is more complicated and heavier, etc... People don't talk much about their props so I don't know what pitch is best this year for the two-bladed props. I suspect it's higher than our three-bladed props, which are 30 degrees at the tip and 45 degrees at the root, which yields 4-5" pitch.

[calgoddard]

Lechassin –

That was a very nice tutorial you just posted about decalage, CG and prop pitch. I would like to add the following comments that relate to prop pitch.

In the hobby of rubber powered airplanes, the pitch-to-diameter ratio (P/D) is a very significant parameter that needs to be optimized. In the WS 2020 rules the maximum allowed diameter of the prop is 8 cm. For reasons I won’t explain here, you should employ the maximum diameter prop that the WS 2020 rules allow. That leaves the optimum pitch to be determined.

In general, for maximum efficiency, the blades of the prop should have a true helical shape. It is conventional to calculate the P/D of a prop with helical blades by measuring the angle of one of the blades at the “point of interest”, which is usually the three-quarter ( ¾ ) radius (R) position. The P/D of a helical blade prop can be calculated with this formula:

P/D = Tan Ɵ (π)(r/R)

Where

R = radius
P = pitch
D = diameter
r = radius at point of interest
Ɵ = angle at point of interest

I am not sure what the angle is for your 3-bladed prop at the ¾ R position. Assuming your prop has blades with a true helical shape I calculate that the P/D of your 3-bladed prop is about 1.8. In my experience, that is a very good P/D for an indoor duration stick model with a conventional 2-bladed prop.

There is an easier way to calculate the P/D of a prop with helical-shaped blades. Use the following formula, which I attribute to the late, great master builder and flyer, John Barker, of the United Kingdom. Measure the angle (Ѳ) of one of the blades at the ¾ R position. Then P/D = tangent (Ѳ) x 2.356.

The blade outline and the area of the blades also need to be optimized, but in my experience, these are less important factors than the P/D of the prop.

Generally, you would like to minimize the RPM of the prop to maximize efficiency. Induced aerodynamic drag increases exponentially with speed, so the faster the blade tips spin, the more drag they encounter. I believe either Brian T or Coach Chuck previously explained how an ever-increasing number of blades on a prop reduces its efficiency. However, the severely limited prop diameter in the WS 2020 rules probably dictates using more than two blades to get enough total blade area to slow down the prop.

Adding blades and/or increasing blade area adds weight, which is not a problem as long as the model only exceeds the minimum allowed weight by a few hundredths of a gram. Adding weight on the front end of a model is usually a good thing in terms of trimming the model so long as decalage and CG location for a given air frame are optimized.

Increasing total blade area and/or pitch of the prop requires that the length of the rubber motor be shortened while maintaining the same optimum rubber motor weight. Indirectly you are increasing the width of the rubber motor, but length and weight are more accurate measurements than width. Increasing the width of the rubber motor can lead to undesirable motor stick bending at high launch torque. The various parameters of a free flight rubber powered model are subject to trade-offs in terms of cost-benefit.

You must diligently use a torque meter and record all your data on flight logs in order to be highly competitive in rubber powered airplane events.

Once an indoor model has been properly trimmed, the key to winning a conventional contest is matching the rubber motor to the prop in terms of length and weight, and proper winding. The WS 2020 rules also dictate that you must be able to adjust trim to fly efficiently in opposite orbits on consecutive flights.

Good luck to all of the students in your WS 2020 competitions.