Squared off wingtip = more drag?

[Joe Ed Pederson]

I have started flying my nearly Brett Buck Skyray with an OS Max FP .20 (Brett Buck tuned) with an APC 9 x 4 at 32 ounces.  Nearly a BB Skyray becauseI am using a wire landing gear from the Sig Banshee instead of an aluminum landing gear and I squared off the wingtips like the Sig Twister.

Brett has written that launching at 11, 300 plus or minus a couple hundred should yield a lap time of 4.8 seconds on 60 foot lines.  When I launch at 11,500 rpm I get 5.1 second laps.   If I launch between 11, 600 and 11,700 rpm I get between 4.9 and 5.0 seconds.

I checked to see if my lines were more than 60'.   They are 60 feet, so the slower lap times are not due to longer than 60 feet lines.

The other thing I could think of was my tach is not accurate -- a real possibility.

If the tach is accurate, the only things I can think of to explain the slower lap times are that wire landing gear may have more drag than aluminum or squared-off wingtips have more drag than the standard Skyray wingtips.

The reasons I would like to get to 4.8 second laps on 60 foot lines is that 4.8 seconds is the lap time Brett recommends for best control response, and  I have read and learned from experience that non-flapped models are sensitive to airspeed in how they handle.  If my tach is accurate it would seem the only way to get to 4.8 seconds is to shorten the lines a couple of feet.

Anyway, if the wire gear and squared wing tips explain the slower lap times, I'll go with an aluminum gear and the standard Skyray wingtips next time.

Thanks for your consideration,
Joe Ed Pedersonn

[Brent Williams]

So did it fly ok?  That's probably the first question to ask, rather than worry about a particular RPM or lap time.

[Pat Chewning]

Yes:  Squared off wing tips are not as efficient as other shapes.  (Modern aircraft have tip winglets)

Yes:  All aircraft are sensitive to airspeed vs control response.

No:  Changing the line length to get shorter lap times will not increase the air speed of the airplane.

Maybe:  There are a lot of other factors that keep you from getting the exact same speed at a given RPM.

Maybe:  The things you can measure (RPM , Speed) are less important than the things you cannot measure (How do you like the way it flys?)

[Brett Buck]

I have started flying my nearly Brett Buck Skyray with an OS Max FP .20 (Brett Buck tuned) with an APC 9 x 4 at 32 ounces.  Nearly a BB Skyray becauseI am using a wire landing gear from the Sig Banshee instead of an aluminum landing gear and I squared off the wingtips like the Sig Twister.

Brett has written that launching at 11, 300 plus or minus a couple hundred should yield a lap time of 4.8 seconds on 60 foot lines.  When I launch at 11,500 rpm I get 5.1 second laps.   If I launch between 11, 600 and 11,700 rpm I get between 4.9 and 5.0 seconds.

I checked to see if my lines were more than 60'.   They are 60 feet, so the slower lap times are not due to longer than 60 feet lines.

   These are ballpark numbers, sounds like it is working fine as-is. If it's too slow, needle it faster, the limit being going "over the top" lean in the maneuvers. If it flies OK at 5.1, just be happy with the slightly reduced speed.

    BTW, the drag from the wire landing gear is probably higher than the entire wing, at least in level flight.

     Brett

[Joe Ed Pederson]

Thanks Brent, Pat, and Brett,

1)  I only have six flights on her and haven't ventured past wingovers, inside loops and inverted.  I'm still learning the beginner's pattern and still have difficulty putting three BIG loops on the same side of the circle without having to bail out, but the lines stay tight on the maneuvers and there's no stalling or mushing even at 5.1 second laps.
I appreciate the advice to fly at the speed that works and that the lap times are just ball park.  I don't know if greater line tension is advantageous, but at 4.9 second laps, the line tension is more than my shoulder cares for.  At 5.1 seconds she still has plenty of line tension, and is much easier on my shoulder.

I do like the way she flies.  Other than turbulence, it is very easy to fly level, she is sensitive without be overly sensitive (handle spacing and controls set up per Brett Buck).  She can draw VERY small loops which gives me confidence I can get out of a jam.  I've only had one previous model that gave me this much confidence. 

2) Thanks Pat for correcting my misconception that shortening the lines, though decreasing the lap times, doesn't increase the speed the model is flying.

3)  Brett, I noticed that when I launch at 11,600 to 11,700 rpm the motor slows significantly on the second half of the wingover and recovers once she's level again.   On the other hand, she runs perfectly through maneuvers if I launch at 11,500 rpm, so I guess 11,500 rpm is the maximum for launch.   

I got the engine new and it only has about 40 minutes of run time so far, but I doubt that (being pretty new) has anything to do with it since it only sags in maneuvers and not at the end of the run.

4) Brett, I knew wire landing gear were draggy, but I didn't know they produced that much drag.   
Does an aluminum landing gear produce significantly less drag?

Thanks again,

Joe Ed Pederson

[john e. holliday]

I know Brett and the other guys are probably more experienced at setting up the SkyRay and the engine, but as I told a local guy that kept trying to beat me, "Put the stop watch in the box until you get the plane to fly the way you want it to fly and are satisfied with it. Then tack the engine and time laps and try for the same settings after that".  I use a tack on the B-25 just to get the engines set the same induvidually before I finally fuel up the engines and start them both for a flight.  Have not got a lap time as the plane feels great to me.   Need to get it out when I get my balance back.

[Brett Buck]

3)  Brett, I noticed that when I launch at 11,600 to 11,700 rpm the motor slows significantly on the second half of the wingover and recovers once she's level again.   On the other hand, she runs perfectly through maneuvers if I launch at 11,500 rpm, so I guess 11,500 rpm is the maximum for launch.   

I got the engine new and it only has about 40 minutes of run time so far, but I doubt that (being pretty new) has anything to do with it since it only sags in maneuvers and not at the end of the run.

4) Brett, I knew wire landing gear were draggy, but I didn't know they produced that much drag.   
Does an aluminum landing gear produce significantly less drag?

   .040 VS .125, roughly proportional.

     If it has enough line tension, and you need to back it off to run right, by all means, back it off. That's why I suggest not using the tach, but instead setting it by ear/clicks. My recommended system is to set it peaked out lean, then back off until you get a distinct drop in RPM, 5-6 clicks. Then fly it, looking for -  exactly what you found! Sounds like from your description that it is going over the top lean in flight, the correct response is to back the needle off a click or two, until it stops. When you get it working in a variety of conditions, then, see what sort of launch RPM you are getting, and then you can at least get a start setting.

     You are doing the right stuff, carry on!

    Brett

[Joe Ed Pederson]

Brett,

Thanks for reminding me on how to set the needle.  My rememberer is often broke or off line.  My forgetter works better and better each year.

Joe Ed

[Steve Helmick]

The squared off tips are draggier than the swept tips of a Flite Streak or SIG  Skyray. BUT, considering that the lines are a HUGE part of the drag of the total, probably not a deal breaker. I'd expect there's probably more wake turbulence created, but again, probably not a deal breaker. As for the wire landing gear....just go flying! You'll discover why aluminum LG works so much better.  Steve

[Air Ministry .]

As theres not supposed to be any YAW & Side Slip . and the airs supposed to be going straight past , theyred not be a lotta differance .
Maybe swept tips hold a little longer , right on ' the Stall . ' , perhaps .

Testing youd need the same overall area , each way . as in Sq ip Extn. & Swept Tip Extn .