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| Fixed Wing Discuss fixed wing aircraft. |
Topic Review (Newest First)
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| 09-12-2011 01:21 PM | ||
| bobmic |
Re: Which prop size to use for windy days Thank you Max, I have a wattmeter and this was the first test I did to make sure I am within the Motor and ESC capability. The data logger is a completely different ball game though but I think it is a bit of an over kill for my plane and flying capability. I got the idea of just trying a couple of props and see which one gives the best performance for me. |
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| 09-12-2011 09:02 AM | ||
| Max |
Re: Which prop size to use for windy days Lots of talk about wind but few responses about props for Bobby: Quote:
Wind or no wind, you need to know HOW your motor/prop combo will perform first. There is no point putting on a different prop that will overload your ESC/Battery. I do however agree with what others have been saying on the forums. You dont have to change your props on account of wind. Find the best prop combo for your power system and it will perform equally well on a calm day as well as a windy day. All you have to do is look at full size fixed pitch aircraft and see they fly just as well on windy days and calm days. While it is true that a coarser prop will pull you faster, and a finer pitch prop will make tak-off's easier and provide braking effect for landing, it is imperative that you match the prop diameter and pitch to the power system. With electrics, Data is everything. You need to be able to asses a motor/prop combo to get the one that works best with your motor/ESC/Battery. To do this, you need something that can measure power while you rev up the motor with different props. The best way to do this is with a datalogger of some sort. The most popular one is the Watts-Up and similar devices that you plug in between your battery and ESC. http://www.hobbyking.com/hobbyking/s...idProduct=6380 Then you run the motor on the ground and read-off what the current draw it. Having this kind of data is a real eye-opener: I personally use the Eagletree Datalogger. http://www.hobbyking.com/hobbyking/s...idProduct=4575 You can use it on the bench like the watts-up, but it can also be left in the plane and flown, allowing you to record and later download and analyze performance while in flight. For example, here is one I did on a friends jet: http://z10.invisionfree.com/AMACforu...showtopic=2695 You can see the data-logger allowed us to correlate throttle positions with in-flight performance allowing us to accurately predict flight time and battery strain. Bench/flight testing different prop combos allow you to find the best prop that is a compromise between load applied to the battery and pulling power. That is really what Bobby was asking, isn't it? Anyway, that's how I would do it. |
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| 09-11-2011 07:51 PM | ||
| OLD PRAIRIE RED NECK |
Re: Which prop size to use for windy days You got it! There has been some work done with in flight variable pitch props (full scale use a version called constant speed props) but they are limited in usefulness due to the fact that you still can't change the diameter. So we are stuck with picking one gear that kind of does the job but not efficiently in the full speed range. You have to sacrifice speed to get short take off/climb & vice versa. In my opinion, the most neglected trim change in setting up a new airplane is trying different prop sizes. Most guys just slap on the prop that someone said was "right" & never try anything else. Some of the most dramatic performance improvements are gained here! It can turn a "dog" plane into "my favorite". |
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| 09-11-2011 04:47 PM | ||
| tskca |
Re: Which prop size to use for windy days Merv, your analogy explains my observations with some of these props. Now, since I fly an aerobatic plane, I prefer the 11 x 7 prop; others transform the same plane to a 3D one and like the 12 x 6. Would this make sense? Ken |
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| 09-10-2011 10:46 PM | ||
| OLD PRAIRIE RED NECK |
Re: Which prop size to use for windy days Maybe this analogy will help: 12 x 4 = 1st gear, 12 x 6 = 2nd gear, 11 x 7 = 3rd gear, 10 x 8 4th gear, 9 x 10 = overdrive. Now 1st gear will get you out of the pits quick & climb hills good (short takeoff run, excellent climb) but don't expect freeway speeds! Overdrive will let you pass everthing on the road like it's going backwards but it will take a long time, a lot of road & a new clutch to get her rolling! And there better not be any hills! Unfortunately we don't have transmittions in our planes so we have to pick a gear that kind of works for our style of driving & that is the best we can do! It's a compromise & not the same for everyone. Hope that helps. Merv |
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| 09-10-2011 09:23 PM | ||
| bobmic |
Re: Which prop size to use for windy days I am aware of the fact the a higher pitch will increase the speed which will make flying in windier condition a bit easier but the question is do you guys think I will notice a difference between 12x6 and 11x7. What happens at lower speeds when things are not as stable? I guess the larger diameter prop should work better for slow speeds - will it have more "toque" if I can relate to cars a bit? And yes when I am talking about wind it definitely includes turbulence and I am looking for the best combination that will reasonable performance at higher altitude but also good control for slow passes, takeoffs and landings. Thx. |
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| 09-09-2011 10:48 PM | ||
| OLD PRAIRIE RED NECK |
Re: Which prop size to use for windy days With the "watch out for the wind at your back", the problem is that we perceive that the plane is going faster when it really is not. So we try to slow it down to the 50 mph that we are used to seeing it fly and now it is actually going 30 mph! Now it's response is sluggish & we are inviting a stall/ spin accident! On the "more lift going into the wind" issue, the problem is still perception related to ground covered! The aircraft is climbing at exactly the same rate it feet per second both ways. It just appears to be climbing faster into the wind due to less ground being covered! Feet of altitude gained compared to time elapsed is identical. Back to the original question: If you want your plane to fly faster for windy day penetration, go down one size on diameter & up one size on pitch. The engine will be loaded roughly the same & the plane will fly faster! (Provided the airframe is not already maxed out drag wise) |
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| 09-09-2011 10:39 PM | ||
| Dave Holmes |
Re: Which prop size to use for windy days Quote:
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| 09-09-2011 09:45 PM | ||
| stardustertoo |
Re: Which prop size to use for windy days Quote:
"An airplane standing perfectly still in a 20mph wind is EXACTLY the same as an airplane moving forward at 20mph on a calm day. The Plane does not know the difference, and therefore will react and fly the same in the air." The only difference, and this is where guys get confused, is the ground speed. This is the speed at which the aircraft is travelling over the ground. Here is how that works in the wind: you are sitting in the airplane, there is no wind, and you are flying at an AIRSPEED (indicated on the airspeed indicator) of 50mph, and your GPS shows you travelling over the ground at 50mph. Now, the wind is blowing 20mph, and you are flying stright into it. You still read 50mph on the airspeed indicator, but now your ground speed in 30mph. Now, you turn around and go the other way. Your airspeed still reads 50mph, but now you are cooking along over the ground at 70mph. Never in there did you change AIRSPEED, and when you are flying, that is the key. Now, how does that related to the original question about prop sizes?? Well, basically, it just means that the airplane is going to fly the same, so there is no real need to change the prop. The only advantage you may have to the 11X7, is the increased speed may give you better "penetration" into the wind. It will simply do the same as it would on a calm day, increase the speed a bit due to the steeper pitch, and you would notice this simply because the ground speed on the upwind legs would be improved. As far as whether it would make it any easier or not, well that comes down to how much turbulence there is. Generally speaking, windy day=turbulence. It all depends on how much you are comfortable with. Anyways, I am blabbing too much...what it comes down to really is your flying preferences. Hope this helps a bit with understanding how wind affects the flight, and then you can experiment on your own and see how you like it. |
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| 09-09-2011 07:35 PM | ||
| Guest |
Re: Which prop size to use for windy days I was once told that the plane doesn't know its windy. The prop provides forward thrust, this in turn causes the plane to move forward. The movement over the airfoil causes lift. Pointing a plane into a 20km/hr breeze will likely give you a lot of lift, at a reduced speed. Running the plane with the breeze in its back causes the plane to fly faster at the same throttle speed as you had going into the wind. I was also told that you should be cautious running with the wind in your back, as the plane gets heavy (less responsive) and bad things happen. I am just passin' along some of the stuff I've heard in the last 10 months. I may be incorrect, but it made sense when it was told to me! 
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