80" strand of 0.060" (2.8 grams), 5400 turns/ 0.6 in.oz launched at 5100 turns/0.2 in.oz., centered wings (no offset), CG 23mm, 6mm decalage left, 3mm right. Rudder set for slight bobbing during the climb to avoid the risk of an initial dive. An entire row of knots remained but it cruised 5 feet too low because of the HVAC, otherwise I think it would have flown 10 seconds longer and used those knots. I think the motor is correct, the problem is the HVAC. To get past 1'45" on any given launch torque in HVAC air, we need to take risks to get good height and it's very random; we get either one hit on the banner or else the plane doesn't even climb get past 15 feet, depending on the extent of HVAC interference.
No video, sorry, but here is the next flight, 1'59" with the same type of performance, also with one hit on the same banner, and some luck after the resulting flight path deviation: https://www.youtube.com/watch?v=5EKOtW7UqQo. That recovery is typical of what we get.
We don't like the risk those hits create so we'll stick with our previous conservative plan(s) for regionals, but it was fun to just see what's possible even in turbulent air.
Edit: we've also tried to do our part to get past regionals (which is in 3 weeks), and we have a large garbage bag full of broken boomilevers ($$$) to show for it...
Last edited by lechassin on February 9th, 2020, 12:48 pm, edited 4 times in total.
80" strand of 0.060" (2.8 grams), 5400 turns/ 0.6 in.oz launched at 5100 turns/0.2 in.oz., centered wings (no offset), CG 23mm, 6mm decalage left, 3mm right. Rudder set for slight bobbing during the climb to avoid the risk of an initial dive. To get past 1'45" on any given launch torque in HVAC air, we need to take risks to get good height. We get either one hit on the banner or else the plane doesn't even climb get past 15 feet, depending on the extent of HVAC interference.
No video, sorry, but here is the next flight, same type of performance at 1'59", also with one hit on the same banner, with some luck after the resulting flight path deviation: https://www.youtube.com/watch?v=5EKOtW7UqQo. That recovery is typical of what we get.
We don't like the risk those hits create so we'll stick with our previous conservative plan(s) for regionals, but it was fun to just see what's possible even in turbulent air.
We've also tried to do our part to get past regionals (which is in 3 weeks), and we have a large garbage bag full of broken boomilevers ($$$) to show for it...
Eric and Luke,
Nice job. Keep up the good work.
I agree that being a little on the conservative side is necessary this year. Want to be sure not to have a bad ceiling hit or wall hit and lose a chunk of one of the flights.
Yea I gotta say, HVAC sucks It's been really hard to test my plane the past few weeks due to the school never managing to turn it off (even though they can and I remind them every week). What are some strategies to test my plane under such a challenging condition? Basically the HVAC makes it hard for my plane to climb in some spots and causes it to fall faster. The difficult part is that it only affects some spots so if I increase torque and it "misses" those spots the plane ends on smacking rafters pretty hard and diving down.
Mr. Chassin, I know you are in a similar situation with HVAC, how do you combat it and get useful data?
I've been stuck at 1:30 ish for like 2 months and the HVAC data I get is misleading sometimes. (For example, one time my plane flew 1:40, but when I flew it again it climbed halfway and stalled out (even after confirmed that all the specs remained the same), so then I decide to decrease decalage slightly but then it only climbs like halfway...)
Sorry if I sound frustrated, it's been a couple of disappointing months...
Yea I gotta say, HVAC sucks It's been really hard to test my plane the past few weeks due to the school never managing to turn it off (even though they can and I remind them every week). What are some strategies to test my plane under such a challenging condition? Basically the HVAC makes it hard for my plane to climb in some spots and causes it to fall faster. The difficult part is that it only affects some spots so if I increase torque and it "misses" those spots the plane ends on smacking rafters pretty hard and diving down.
Mr. Chassin, I know you are in a similar situation with HVAC, how do you combat it and get useful data?
I've been stuck at 1:30 ish for like 2 months and the HVAC data I get is misleading sometimes. (For example, one time my plane flew 1:40, but when I flew it again it climbed halfway and stalled out (even after confirmed that all the specs remained the same), so then I decide to decrease decalage slightly but then it only climbs like halfway...)
Sorry if I sound frustrated, it's been a couple of disappointing months...
As always, any suggestions welcome!
Xiangyu
Xiangyu,
My teams have flown many times over the years with HVAC on and, unfortunately, i'd have to say that there is no consistent strategy that will give you reliable flying data.
As you note, air is usually the worst in certain spots (near vents) but it's also possible for blowers to start and stop periodically giving the results you note above. If we are competing in bad air, we either decide to fly low altitude to just get a score or, if there is a lot of unobstructed floor space, fly with extra power to "punch through" the bad air. We are taking fewer risks this year though as the airplanes are so difficult to trim. We don't want damage that will require more practice sessions to retrim.
Our strategy with our school administration has been to respectfully move our requests up the chain. We have found that with the right teacher standing up for us in a face-to-face meeting with the facilities staff and with upper administration we can win them over and get them to shut off blowers. I like to officially make the facilities staff person an honorary member of the team and remind them whenever I see them that they are an integral part of the student's success and any championship medals are in-part theirs too.
This is quite a bit of work and communication and requires a lot of patience and a lot of positive, enthusiastic communication.
I haven't tried to get the HVAC turned off because I have PTSD from the hassle we went through to get these Sunday morning sessions, but it wouldn't hurt for you to ask. If you can't, it doesn't hurt to gain experience dealing with HVAC. We don't have a specific HVAC data gathering strategy, except to acknowledge that it's one more thing that's impossible to adequately describe in traditional logs, another reason we favor videos we can analyze.
I'll use the video we posted today as an example; here's how we analyzed it, and how we worked around the HVAC disturbances to gain insight anyways:
1) The climb radius was good but the plane was aaaaalmost power-stalling. We prefer that to an initial dive, so no rudder change. We could have added a tiny bit of rudder to force a tad more banking, reduce the stalling tendancy, and maybe even permit higher launch torque (retain more knots).
2) The hit occurred at 27 feet, it took away 5 feet and the plane climbed back 2 feet, so total unobstructed climb would have been about 34 feet which is good for the 35 foot ceiling at regionals. We noted the launch torque on this longer 0.060" motor for use at regionals, but we lowered the launch torque for subsequent flights today to avoid hits, with resulting 1'35" to 1'50" flights, same as with the shorter 0.0625" motor. We take this to mean that at 25 feet, it does not matter which motor size we use, but at higher ceilings, the longer/thinner rubber gets us another 30 seconds flight time (see below).
3) The hit recovery in today's video is what we've come to accept this year. We can do better with a more forward CG and more decalage but the increased drag kills us. It's better to have a somewhat aggressive CG (still relatively conservative because of the tiny stabilizer) and launch at low enough torque to never hit, which seems to be what Brian is doing too.
4) The cruise during those moments in calm air was perfect, no changes. Recovery from HVAC encounters was good, descent is unavoidable, so no changes.
5) The landing was perfect, no changes.
6) An entire row of knots remained. We estimated two HVAC encounters blew us down five feet twice, plus the 5 foot loss from the hit. Therefore we relaunched the plane three times from head height , timed those descents and assessed remaining knots to estimate what the flight would have been from 35 feet and in still air. We liked the few knots remaining and we added 3x 7 seconds to the 1'59"we got, total 2'19". Therfore no motor changes. That time is not real, but it's the type of useful estimate we are not ashamed to use.
In general: this flight is consistent with what we saw on the offset-wings plane on the same motor specs. On a half motor, that plane used 25 feet of height and flew 1'10", so on a full motor that plane will need a 50 foot ceiling to achieve the same 2'20" we got with today's more basic plane at only 35 feet. Therefore, today's plane with the centered wings that banks is actually the better plane since it's simpler to use and achieves the result under a lower ceiling. This same plane will also fly 1'45" at 25 feet with the shorter 0.0625" motor and may well be the setup we use at regionals even if the 35 foot ceiling is for real. Our reasoning is that Luke has used 0.0625" all season and does not have a good feel for the 0.060". He wants to minimize variables under stress and I agree.
It's a lot of conclusions to draw from one HVAC contaminated flight video, but we think it's pretty good analysis. The key IMO is video, especially with HVAC. There's no way we could have gleaned all this information trying to describe it all in logs, trying to remember how many feet it dropped under this or that vent, etc...
I hope you find it useful, and I'm happy to do the same with some of your videos (use a tripod if you're alone).
Thank you both for your tips! I'll try to get a video this weekend. We do have a 4 day break this weekend and generally in the past the fans are off when the school is closed... It'll be a rare golden opportunity to actually get some data...
2 Questions:
What is the difference between a 2 bladed prop and a 3 bladed prop with the same overall surface area (the 2 bladed prop would have larger blades)?
Would a 4 bladed prop work, or are there negatives to it?
vehicleguy wrote: ↑February 11th, 2020, 5:59 am
2 Questions:
What is the difference between a 2 bladed prop and a 3 bladed prop with the same overall surface area (the 2 bladed prop would have larger blades)?
Would a 4 bladed prop work, or are there negatives to it?
Calgoddard addressed this quite nicely:
"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."
There are trade offs; but either one can be successful. Lechassin appears to have found a silver bullet for this year's rules. Only time will tell if more is possible...
MIT '25
MIT Wright Stuff ES '22
BirdSO Wright Stuff ES '22
vehicleguy wrote: ↑February 11th, 2020, 5:59 am
2 Questions:
What is the difference between a 2 bladed prop and a 3 bladed prop with the same overall surface area (the 2 bladed prop would have larger blades)?
Would a 4 bladed prop work, or are there negatives to it?
This forum is still debating what's best and why. We don't claim to have any engineering background to support any claims, so here's our thoughts just based our experience and on precedent in the real world (examples are over-simplified, this is all just for illustrative purposes):
In control line speed competition, they use a one-bladed prop that has all of the desired surface area and is appropriately counterbalanced. That single blade is inherently more efficient than two or more blades because it runs in cleaner air, as opposed to running in the turbulent air of the other blade(s). The blade has a small chord and they spin it FAST (30000rom?) We can't do that.
OK, so at the other extreme there's the inlet fan of a jet engine with numerous blades. The desired surface area is divided among numerous blades, so each one is small and they spin the fan FAST (50000 rpm?). We can't do that...
In the middle would be a propeller-driven WW2 fighter plane. The radial engines turn a huge prop rather slowly (2500 rpm?), but the landing gears could only be so long without being frail, which limited the prop diameter. To us, that looked somewhat like our situation: slow prop with limited diameter. Rather than increase the chord of two shorter blades, the WW2 props use 3 or 4 normally shaped blades. Figuring they knew something we didn't, we took that approach. We just didn't know whether we should use 3 or 4 blades, so we tried them both.
It immediately became clear that 3 blades worked better, so without thinking much about why, we stuck with that even though it's a little harder to lay out the initial jig (3 times 120 degrees vs 4 time 90 degrees).
Last edited by lechassin on February 12th, 2020, 12:54 pm, edited 1 time in total.
I forgot to mention that a good strategy to test in HVAC is to wind the motor using only as much torque as is needed to cruise (in our case 0.015 in.oz). The plane will stay at head level and descend, and the longer you get it to do that, the longer the plane will fly from higher up. As you're doing that, make sure you also test the plane's behavior right after launch, everything else you do affects that. Ultimate height will be random under HVAC, but if it launches well, it will climb well too, then you just need to test best torque when you arrive at any given venue.
It's been really hard to test my plane the past few weeks due to the school never managing to turn it off (even though they can and I remind them every week). What are some strategies to test my plane under such a challenging condition? Basically the HVAC makes it hard for my plane to climb in some spots and causes it to fall faster. The difficult part is that it only affects some spots so if I increase torque and it "misses" those spots the plane ends on smacking rafters pretty hard and diving down. 
