Whats the best propeller to use?

[coachchuckaahs]

Is there scaling factor to see how excess weight affects those benchmark times? My two Div B teams split up the light parts of their kits (and I'm sure some unscrupulous glue usage was a factor) and one plane is about 9.5 g and another is in the mid to high 8s. We plan to use all the lightest parts of our next kit for our state plane and hopefully be just above 8.00, but it would be nice to have a sense of where we are with our excess weight. I mentioned in another thread one of the planes is doing 2:10 and you and others mentioned it should be doing better with just trim adjustments and without substantially denser rubber or different props, but it would be nice to know how much the weight is depressing its max time. Thanks!

I don't have a scaling factor for mass, but it is fairly strong. You want to build at minimum weight. The SO planes should be relatively easy to build at 6g, adn ballast up to 8. If using very heavy Hobby Lobby wood, you should still be able to build close to 8. See the NFFS video I did on weight management through glue control. While you cannot make up for really bad wood selection, most wood in kits is going to be in the 6-10# range. Coach Brian's video on super simple plane used much heavier wood and came out on weight.

Glue is a VERY SURPRISING weight addition. Most kids are applying glue from the bottle, whether using a fine tip or not, it is too much glue. You can easily add 2-3g from glue. A team that asked for my help last year had a J&H kit that built out at 19g. We made it fly for 1:50 (B Div) with very thick rubber (3/16" wide!). They then built again after some glue training, and were able to do 4 minutes with a plane they built to 6g and ballasted up to 8. The heavy plane was caked with CA glue. Virtually all the difference in mass was glue (as the new plane was carbon spars).

Another point of reference was the 2016 season, when there was a bonus for carrying dimes. Each dime is 2.27g, and they offered a 50% bonus for each dime up to 3. Halfway through the season this looked like an even break. At Nationals, most top teams (top 7) used two dimes. We used none because our early tests said it was too much a risk, and we came in 8th. Our time was around 2:54. I think top 2-dime teams were around 1:30-1:40, but got a 100% bonus.

I mentioned trim as a key property (decalage) that must be right, as it has a huge impact on your time. However, the next largest impact is matching of prop and rubber. You cannot match prop to rubber with a plane that is not trimmed right, but once trimmed the huge low-hanging fruit is matching prop to rubber. This means measuring your density, and adjusting which loop you use to use up most of your turns when flying to near the rafters (correct launch torque). Without stripping, you will need to bracket it between two different "widths" that you have, and then tweak the prop pitch (not change prop, just change pitch a little) to match the rubber. A little pitch change goes a long way. Ideally you would be able to adjust pitch and then cut rubber to optimize to that pitch, to home in on a two-variable optimum. But that is secondary to having basic prop-rubber combo close.

After basic trim, then prop-rubber matching, winding technique is important. See Coach Brian's winding video on NFFS. Poor winding can become more of an influence than prop/rubber matching, so both must be worked on.

So, I want to be clear, I am not saying you need different props, but you do need to have your rubber match your prop, at least first order. If it needs "substantially" thicker rubber (leaving many turns), then you absolutely need to change rubber. You can make small adjustments to prop pitch, but if substantially off on the rubber then you would have to pitch down to the point the prop is too inefficient.

My original point, if not stated clearly, is that these three main factors (trim, prop/rubber matching, winding technique) are the low hanging fruit, and should be accomplished before pursuing an increase through the use of custom props.

Coach Chuck

[jgrischow1]

Is there scaling factor to see how excess weight affects those benchmark times? My two Div B teams split up the light parts of their kits (and I'm sure some unscrupulous glue usage was a factor) and one plane is about 9.5 g and another is in the mid to high 8s. We plan to use all the lightest parts of our next kit for our state plane and hopefully be just above 8.00, but it would be nice to have a sense of where we are with our excess weight. I mentioned in another thread one of the planes is doing 2:10 and you and others mentioned it should be doing better with just trim adjustments and without substantially denser rubber or different props, but it would be nice to know how much the weight is depressing its max time. Thanks!

I don't have a scaling factor for mass, but it is fairly strong. You want to build at minimum weight. The SO planes should be relatively easy to build at 6g, adn ballast up to 8. If using very heavy Hobby Lobby wood, you should still be able to build close to 8. See the NFFS video I did on weight management through glue control. While you cannot make up for really bad wood selection, most wood in kits is going to be in the 6-10# range. Coach Brian's video on super simple plane used much heavier wood and came out on weight.

Glue is a VERY SURPRISING weight addition. Most kids are applying glue from the bottle, whether using a fine tip or not, it is too much glue. You can easily add 2-3g from glue. A team that asked for my help last year had a J&H kit that built out at 19g. We made it fly for 1:50 (B Div) with very thick rubber (3/16" wide!). They then built again after some glue training, and were able to do 4 minutes with a plane they built to 6g and ballasted up to 8. The heavy plane was caked with CA glue. Virtually all the difference in mass was glue (as the new plane was carbon spars).

Another point of reference was the 2016 season, when there was a bonus for carrying dimes. Each dime is 2.27g, and they offered a 50% bonus for each dime up to 3. Halfway through the season this looked like an even break. At Nationals, most top teams (top 7) used two dimes. We used none because our early tests said it was too much a risk, and we came in 8th. Our time was around 2:54. I think top 2-dime teams were around 1:30-1:40, but got a 100% bonus.

I mentioned trim as a key property (decalage) that must be right, as it has a huge impact on your time. However, the next largest impact is matching of prop and rubber. You cannot match prop to rubber with a plane that is not trimmed right, but once trimmed the huge low-hanging fruit is matching prop to rubber. This means measuring your density, and adjusting which loop you use to use up most of your turns when flying to near the rafters (correct launch torque). Without stripping, you will need to bracket it between two different "widths" that you have, and then tweak the prop pitch (not change prop, just change pitch a little) to match the rubber. A little pitch change goes a long way. Ideally you would be able to adjust pitch and then cut rubber to optimize to that pitch, to home in on a two-variable optimum. But that is secondary to having basic prop-rubber combo close.

After basic trim, then prop-rubber matching, winding technique is important. See Coach Brian's winding video on NFFS. Poor winding can become more of an influence than prop/rubber matching, so both must be worked on.

So, I want to be clear, I am not saying you need different props, but you do need to have your rubber match your prop, at least first order. If it needs "substantially" thicker rubber (leaving many turns), then you absolutely need to change rubber. You can make small adjustments to prop pitch, but if substantially off on the rubber then you would have to pitch down to the point the prop is too inefficient.

My original point, if not stated clearly, is that these three main factors (trim, prop/rubber matching, winding technique) are the low hanging fruit, and should be accomplished before pursuing an increase through the use of custom props.

Coach Chuck

Thanks for the quick reply. I suppose in addition to weighing all the parts when we first open a kit we should have a better sense of "OK, the parts add up to X, so once we know our total weight we will know how much glue we are using." My kids do use a pin applicator and not the bottle but they still probably use too much.

Another factor might be prop weight...On the 8.46 gram plane, the prop weighs 2.25 g...ours range from our lightest at 1.7 g to our heaviest at 2.64 g...quite a difference. If we swapped it out for something in the 1.7s we could get down close to the minimum weight.

In terms of rubber, we are using 3/32 and the next widest we have is 1/8 which seems too wide to my uneducated mind. So we might just keep making motors until we find denser 3/32 varieties.

Once we have the time we will work on matching prop pitch to rubber.

[coachchuckaahs]

That is far too wide a variation of rubber. You can order custom "widths", but not custom densities, through IFFS, J&H, or FFM.

Generally I think people are tending to between 0.094 and 0.087 (even that is a wide variation). If you need heavier than 0.094 (3/32), yo may want to adjust prop pitch downward (though the actual optimum may lie above 0.094, I don't know). I am working with B students with a 12" prop so we are substantially higher than 0.094".

With those suppliers, at least with IFFS, you can specify any width you want. Be aware, as other conversations here show, that this does not mean your density is specified, and the rubber stock varies in "linear density", which is really a thickness variation. The width is cut fairly accurately, but the thickness varies along the length.

Yes, out props for a 6g plane tend to be under 1.5g. However, we are putting all the ballast (2g) at the front, so a heavier prop is not end of world if you can get the mass down on the plane.

Coach Chuck

[jgrischow1]

That is far too wide a variation of rubber. You can order custom "widths", but not custom densities, through IFFS, J&H, or FFM.

Generally I think people are tending to between 0.094 and 0.087 (even that is a wide variation). If you need heavier than 0.094 (3/32), yo may want to adjust prop pitch downward (though the actual optimum may lie above 0.094, I don't know). I am working with B students with a 12" prop so we are substantially higher than 0.094".

With those suppliers, at least with IFFS, you can specify any width you want. Be aware, as other conversations here show, that this does not mean your density is specified, and the rubber stock varies in "linear density", which is really a thickness variation. The width is cut fairly accurately, but the thickness varies along the length.

Yes, out props for a 6g plane tend to be under 1.5g. However, we are putting all the ballast (2g) at the front, so a heavier prop is not end of world if you can get the mass down on the plane.

Coach Chuck

Understood. The best flight I mentioned in another thread a few weeks ago that I thought was "fairly well trimmed" was 2:10 in a 22 ft ceiling close to the rafters, winding 90% or so of max turns then backing off to launch torque. My kid backed off about 150 turns but turns remaining was 340, so that's why I brought up in that thread the question if we needed lower pitched props. With our newly retrofitted pitch gauge that measures pitch angle at 3.5" radius, that particular prop was coming in at 31 deg. Plane is 8.46 grams. So I'm thinking we try again to get the trim settings right and get closer to 2:30, then go with denser 3/32 rubber, then work on changing pitch or swapping out for a different pitched prop.

[coachchuckaahs]

At3.5" radius and 31 degrees, that is a pitch just over 13 inches.

With a 9.5" diameter prop, that is a P/D of 1.37, which is rather low. I would be going up in pitch if you make a change.

Of course that will result in the need for denser rubber. Is this the Ikara flaring or symmetric prop?

If your pitch is too low, especially for fast moving SO planes, then the prop will no longer have enough angle of attack to be effective. Generally we like P/D between 1.5 and 2.0 for indoor duration, Many seem to be preferring the lower end of this for this year.

Coach Chuck

[jgrischow1]

At3.5" radius and 31 degrees, that is a pitch just over 13 inches.

With a 9.5" diameter prop, that is a P/D of 1.37, which is rather low. I would be going up in pitch if you make a change.

Of course that will result in the need for denser rubber. Is this the Ikara flaring or symmetric prop?

If your pitch is too low, especially for fast moving SO planes, then the prop will no longer have enough angle of attack to be effective. Generally we like P/D between 1.5 and 2.0 for indoor duration, Many seem to be preferring the lower end of this for this year.

Coach Chuck

Ikara symmetric. We will try higher pitches. Thanks!

[jgrischow1]

At3.5" radius and 31 degrees, that is a pitch just over 13 inches.

With a 9.5" diameter prop, that is a P/D of 1.37, which is rather low. I would be going up in pitch if you make a change.

Of course that will result in the need for denser rubber. Is this the Ikara flaring or symmetric prop?

If your pitch is too low, especially for fast moving SO planes, then the prop will no longer have enough angle of attack to be effective. Generally we like P/D between 1.5 and 2.0 for indoor duration, Many seem to be preferring the lower end of this for this year.

Coach Chuck

I have seen it mentioned this year a few times but nothing so comprhensive as this post from a few years back... viewtopic.php?p=269641#p269641 ...the topic of sanding flaring props to actually get them to flare. In that post, Coach Brian mentions his kids sand their Ikara from .010" down to .007" progressively...we have both some Ikara flaring and some IFFS flaring...the Ikara stock thickness is indeed around .010" so we will try that but the IFFS are already .006...do they need sanding or is that thin enough to provide sufficient flare?

[coachchuckaahs]

What is "sufficient"? You can vary the degree of flare by sanding. Which begs the question, how do I measure flare?

We typically will take the prop by the prop shaft. Holding the shaft horizontal with the spar also horizontal, we press the leading edge of the prop at the widest point against our foam block on our scale. We press until the blade is flat (90-degrees pitch angle). This is EXCESSIVE, and may not be a good measure. It would be better to have a fixture to force flare about 10 degrees and measure the load. Especially if you have carbon spars, they can "load up" and a full flex-to-flat is misleading.

I have run flaring from 12g down to about 4g. that covers a wide range, and most useful have been in the 6-8g range, though your mileage may vary. My numbers are for balsa props where the spar does most of the flare, except at the lower end of this range the balsa is A-grain and flexes too.

The softer your flare, the lower you need to set your static pitch, as it will flare even at low torque.

The very soft flares can get you to a point that the prop flares enough that you don't climb at all at high torque! You can help this by reducing static pitch, but then it may spin too fast at letdown.

So you are introducing more variables that all work together. Unless you want to do a LOT of testing, I would stick with a medium flare of 6-10g.

Coach Chuck

[jgrischow1]

What is "sufficient"? You can vary the degree of flare by sanding. Which begs the question, how do I measure flare?

We typically will take the prop by the prop shaft. Holding the shaft horizontal with the spar also horizontal, we press the leading edge of the prop at the widest point against our foam block on our scale. We press until the blade is flat (90-degrees pitch angle). This is EXCESSIVE, and may not be a good measure. It would be better to have a fixture to force flare about 10 degrees and measure the load. Especially if you have carbon spars, they can "load up" and a full flex-to-flat is misleading.

I have run flaring from 12g down to about 4g. that covers a wide range, and most useful have been in the 6-8g range, though your mileage may vary. My numbers are for balsa props where the spar does most of the flare, except at the lower end of this range the balsa is A-grain and flexes too.

The softer your flare, the lower you need to set your static pitch, as it will flare even at low torque.

The very soft flares can get you to a point that the prop flares enough that you don't climb at all at high torque! You can help this by reducing static pitch, but then it may spin too fast at letdown.

So you are introducing more variables that all work together. Unless you want to do a LOT of testing, I would stick with a medium flare of 6-10g.

Coach Chuck

Very helpful. I had always wondered how you measured flare. Thanks!

[jgrischow1]

What is "sufficient"? You can vary the degree of flare by sanding. Which begs the question, how do I measure flare?

We typically will take the prop by the prop shaft. Holding the shaft horizontal with the spar also horizontal, we press the leading edge of the prop at the widest point against our foam block on our scale. We press until the blade is flat (90-degrees pitch angle). This is EXCESSIVE, and may not be a good measure. It would be better to have a fixture to force flare about 10 degrees and measure the load. Especially if you have carbon spars, they can "load up" and a full flex-to-flat is misleading.

I have run flaring from 12g down to about 4g. that covers a wide range, and most useful have been in the 6-8g range, though your mileage may vary. My numbers are for balsa props where the spar does most of the flare, except at the lower end of this range the balsa is A-grain and flexes too.

The softer your flare, the lower you need to set your static pitch, as it will flare even at low torque.

The very soft flares can get you to a point that the prop flares enough that you don't climb at all at high torque! You can help this by reducing static pitch, but then it may spin too fast at letdown.

So you are introducing more variables that all work together. Unless you want to do a LOT of testing, I would stick with a medium flare of 6-10g.

Coach Chuck

So we tried sanding a flaring Ikara prop...it was about .010 in thickness and we got it down to about .005-.006 at the edge...and the pitch has changed...we had previously re-pitched it to 37-ish deg. at 3.5 inch radius but now it's almost 45 deg...we figured we should have sanded first and then re-pitched, but any other advice? Are we sanding too aggressively?

Also, should be be sanding a specific side of the prop, i.e., the leading face or trailing face, or both faces, or does it not matter much?

Thanks!