Wright Stuff C

[CrayolaCrayon]

I am not sure what the angle is for lechassin's 3-bladed prop at the ¾ R position. Assuming his prop has blades with a true helical shape I calculate that the P/D of lechassin's 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.

Is P/D applicable for props that aren't perfectly helical?

[lechassin]

Here's the info on our props, you guys can see what comes up: the angle at the tip is 30 degrees and the angle at the hub is 45 degrees. My guess is that at 3/4 R the pitch is 75% of the way between 45 and 30, so about 34 degrees.

I did some drawings on paper to see how far those angles would go if, instead of traveling through their respective circumferences, they projected in a straight line: our tip pitch is 5" and the root pitch is 4". It turns out that that no matter how the blades are drawn on the bottle, we can't get more helix. We could deform the blades, but we decided not to. Testing showed that the best tip pitch for us is 5", and for the root to have 5" pitch, the root angle would be so high it would just add drag, not thrust.

Like everything else, our prop is a compromise, I'm curious to see what the math tells us.

Tomorrow we have a practice session and we need to change things up. We watched video of last year and realized the venue is a basketball court, so we need a tighter radius. I'm guessing more rudder and more decalage, so shorter flights. We'll report back with numbers.

Edit: Luke just got home and calculated the ratio is 1.65. To get 1.8 we would need to add 2.5 degrees of pitch, which hopefully is how much the blades flex as they load, lol.

[calgoddard]

P/D can be used as an indicator of the performance of: 1) props having blades with a true helical shape; and 2) props having blades that approximate a true helical shape. The later are referred to as "bucket props." In the Penny Plane class, as an example, very thin 5 1/2-inch long sheet balsa wood blades are soaked in water and then formed around a 5-inch diameter cylinder at a 17 degree angle relative to the central axis of the cylinder. The blades are tightly wrapped with an Ace bandage and allowed to dry. The finished blades are glued to spars that are in turn glued in the opposite ends of a tubular hub using a pitch gauge to achieve a suitable P/D, e.g. 1.8.

The term "bucket prop" was coined because some blades used to be formed on a bucket having a frusto-conical shape.

True helical prop blade forms used to be hard to fabricate out of wood, fiberglass bonded with epoxy, or foam. They are now easily produced with a 3-D printer.

A variation of the bucket prop uses blades cut from a plastic soda bottle or platic yogurt cup. This is not appropriate in the Penny Plane class where a 12-inch diameter prop typically weighs around 0.80 grams.

I estimate that a well-made bucket prop can achieve 95% of the efficiency of a similar configuration prop with truly helical blades.

Some expert free flight modelers use props with blades that depart from a true helical shape. One example is the Larabee prop. I am not certain of the spelling of that name.

[lechassin]

It turns out the venue we thought was the venue may not be the venue after all, and the one that may be the venue hasn't released any room dimensions, so we didn't know what to do...

So here's what we did with our three planes:

For a high room with a large floor, we kept last week's centered-wings plane as is, and if that's the room we get, we can hope for 1'45' to 2'01" on an 80" strand of 0.060 rubber launched at 0.2-0.25 in.oz.

For a room with a small floor, we tried to use the other centered-wings plane with extra rudder and incidence. To the left we had steep banking, and adding incidence was no good (the plane almost rolled over once). To reduce banking, we switched to the offset-wings plane: 15 foot radius both ways, 1'45 to the right, 1'30" to the left at 25 feet. 67" strand of 0.0625 rubber at 0.20-0.25 in.oz torque but HVAC was on so that number will need tweaking on site.

For a room below 25 feet with a large floor, we put the last plane back to the setup we've had in recent videos with a 67" inch strand of 0.0625 rubber, 1'35" to 1'45" at 25 feet.

Luke now has no backup plane for any given scenario, so he will need to forego some height at the expense of flight time; he cannot afford to get one plane stuck and scramble to get another one going. We have one more session next Sunday for him to practice no-touch flying, and today he practiced a salvage plan, launching all of the planes at 0.25 in.oz (initial and launch, no de-winding), which get s a 1'05" to 1'15" flight both ways at fifteen feet.

[coachchuckaahs]

Unfortunately, not getting detailed site measurements is an ongoing problem. The rules say this will be announced "before the event", but some tourney's consider that morning to be "before". Ill I can say is have lots of data. Also, DO NOT trust the height given, it is often an estimate. Buy a laser measurement tool off Amazon for about $12 and measure the ceiling when you get there.

Not sure why you would only wind up to 0.22 oz-in. Always wind to max torque (near breaking) and then back off to desired launch torque.

Coach Chuck

[lechassin]

Ref winding to 0.25: That's just the abort plan. It's only to be instituted if he finds himself in a hurry.

Edit: I searched "Larrabee propeller" and quickly back-clicked...

[jander14indoor]

Good backtracking on the Larabee props not being helical. They were designed to be the 'optimal' shape for low induced drag. The can certainly be helical pitch props and there are some good articles out there where that is assumed to do the calculations for free flight props. Shape tuned to a particular P/D, and you can plan with more or less area before or after the spar. I've always liked them, but can't say if they are as good as advertised for our purposes.

Jeff Anderson
Livonia, MI

[nobodynobody]

How do you stop a plane from barreling? Are there anyways to keep the tightness of the turn more consistent and not have the plane crash inward when "climbing"?

[lechassin]

If your radius is tighter than it needs to be, try widening it, and make sure you have enough incidence to compensate for the banking.

If you need the tight radius, it's a problem. Yesterday we almost went upside down trying to tighten the radius on the plane that won the last invitational, and the only way around it was to use the plane with offset the wings that eliminate banking.

Offset is a easy to do, something to consider because it needs to be reversed reliably in the 8 minute 2-launch window, but some people prefer warping the wings. If you want to use offset, watch this video: https://www.youtube.com/watch?v=fqAXTjMOl8U "(thumbs-downs" noted, not sure why that is, we're trying to help )

Luke's plane now circles easily in the width of a basketball court, with 1/2" wing offset to the left when circling right, and 9/16"+ offset to the right when circling left (a little more because of the prop torque). I think I say it backwards in the video.

[xiangyu]

Alright, some updates here on my progress.

I have finally broken out of my 1:30 trap LOL, reached 1:40 today both ways. Here's the funny thing, after testing bi planes and tri-blade propellers for so long, I still can't build them perfectly for them to be efficient. Thus, after some redesigning, I went back to mono plane with 2 bladed no flare props and is able to get 1:40

I was gonna take a video for you guys but I realized that I ran out of the correct size rubber bands to fly again. This is kinda problematic as I don't quite have the left turn perfectly trimmed AND I have an invite coming up this Saturday. Anyone knows how fast Freedom Flight Ships rubber? (Hopefully 2 days?)

Xiangyu