Knife Edge Loop: How pick airframe that can? - RC Groups
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Jan 22, 2017, 01:02 PM
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Knife Edge Loop: How pick airframe that can?


Hi,

I'm looking to pick up an ARF or PNP electric that will do a complete (including the hardest part...the bottom!!) knife edge loop. I realize that lightweight profile planes would be best but I hate the look of those and I feel like the handling will not be smooth in the least. I have an Eflite Carbon-Z Yak and it has a lot of roll & elevator coupling. I can do 3/4 of the knife edge loop, but it rolls so quckly and violently when I apply more rudder to try to get hold the bottom.

I've flown a friend's large gas plane (??Edge 540...3D style) and I was easily able to knife edge loop it. It had no discernible coupling and plenty of thrust-to-weight to help me pull out the bottom of the loop. I've also seen a fellow at our club that is nationally competitive in IMAC do knife edge loops with little apparent effort. So I know these are possible without a little profile airframe.

So what do I look for, aerodynamically and in thrust to weight ratio, to discern if a given platform can really do the Knife edge loop? I'm looking at an FMS Olympus (an entry level pattern plane...on 5 to 6 S); I'm told it knife edges superbly but I don't know if it can pull out the bottom of the loop. The Olympus has good fuse area and thrust-to-weight, but not a huge rudder like the 3D planes do (my Yak is built for 3D but I'm flying it in pattern style).

Thanks,

Andy
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Jan 22, 2017, 01:43 PM
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Andy, unless the design is really bad ( doubtful with a Yak ) you simply have some set up issues. Granted a foam airplane will only get so far on thenpercision scale. Most rudder to pitch coupling is a CG issue. Nose heavy will have you pitching up, tail heavy will have you pitching to the gear. Get the CG right and knife edge gets much easier.
Jan 22, 2017, 08:17 PM
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Quote:
Originally Posted by exf3bguy
Andy, unless the design is really bad ( doubtful with a Yak ) you simply have some set up issues. Granted a foam airplane will only get so far on thenpercision scale. Most rudder to pitch coupling is a CG issue. Nose heavy will have you pitching up, tail heavy will have you pitching to the gear. Get the CG right and knife edge gets much easier.
Hi Guy,

The pitch isn't what's killing me...it's the roll. The Yak needs substantial opposite aileron when the rudder is applied. I have it mixed in and this works great for knife edge flight...at low rudder throws. To complete the knifed edge loop, I need a lot more rudder to pull out the bottom. When I apply this, it rolls violently. Last time out, I tried a bunch of flights with 10-20 loops each and I was timing opposite aileron simultaneous with the bottom of the loop. I think I got 1 decent loop and 40 bad ones. I'm not fast enough.

I have a little Tomahawk Viper Jet that I use and that knife edges much better...no coupling. But the rudder is little...it relies on speed to KE....I never even tried to loop it.
Jan 22, 2017, 08:46 PM
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I have a hunch it is rolling out because of a combination of being nose heavy and you are using excessive rudder throw to compensate.
Jan 24, 2017, 11:32 AM
Xpress..'s Avatar
Quote:
Originally Posted by exf3bguy
I have a hunch it is rolling out because of a combination of being nose heavy and you are using excessive rudder throw to compensate.
I also concur.

My 107" Pilot Yak 54 has nearly zero coupling, even through the last leg of a knife edge loop. Setup is always key.
Feb 03, 2017, 05:32 AM
Learn to build with wood.
Tucci's Avatar
Quote:
Originally Posted by sothpaw
Hi,

I'm looking to pick up an ARF or PNP electric that will do a complete (including the hardest part...the bottom!!) knife edge loop. I realize that lightweight profile planes would be best but I hate the look of those and I feel like the handling will not be smooth in the least. I have an Eflite Carbon-Z Yak and it has a lot of roll & elevator coupling. I can do 3/4 of the knife edge loop, but it rolls so quckly and violently when I apply more rudder to try to get hold the bottom.

I've flown a friend's large gas plane (??Edge 540...3D style) and I was easily able to knife edge loop it. It had no discernible coupling and plenty of thrust-to-weight to help me pull out the bottom of the loop. I've also seen a fellow at our club that is nationally competitive in IMAC do knife edge loops with little apparent effort. So I know these are possible without a little profile airframe.

So what do I look for, aerodynamically and in thrust to weight ratio, to discern if a given platform can really do the Knife edge loop? I'm looking at an FMS Olympus (an entry level pattern plane...on 5 to 6 S); I'm told it knife edges superbly but I don't know if it can pull out the bottom of the loop. The Olympus has good fuse area and thrust-to-weight, but not a huge rudder like the 3D planes do (my Yak is built for 3D but I'm flying it in pattern style).

Thanks,

Andy
Put your YAK on a diet. I've had two of those, the older red white and blue ones. Best thing I ever did was swap to a 1400 kV motor with a 3s3300 battery and a 10x5 prop (I'm 95% certain on the prop size, but it's been a while). It improves the center of gravity and reduces the wing loading with no noticeable power reduction. After the conversion, there was basically zero roll/pitch coupling in KE and it required a lot less rudder to stay in KE even at lower speeds. Hard on batteries, though.
Feb 07, 2017, 12:37 AM
Registered User
Quote:
Originally Posted by Tucci
Put your YAK on a diet. I've had two of those, the older red white and blue ones. Best thing I ever did was swap to a 1400 kV motor with a 3s3300 battery and a 10x5 prop (I'm 95% certain on the prop size, but it's been a while). It improves the center of gravity and reduces the wing loading with no noticeable power reduction. After the conversion, there was basically zero roll/pitch coupling in KE and it required a lot less rudder to stay in KE even at lower speeds. Hard on batteries, though.
Tucci,

I did try again with a 2200maH pack and I adjusted the cg all the way back to beyond the aft part of the recommended range. I did this with a Life Rx pack.
The CG helped a little but what really helped was to eliminate my mix. Somehow, while the mix is needed for level knife edge flight, it was a big problem at the bottom of the loop.

I still have to pull up hard on the elevator at the bottom but the Yak was starting to KE loop. I make the loop as large as I can but you are right that thrust-to-weight ratio is a little lacking.

I bought an FMS Olympus and I want to see how that does in comparison.

Thanks!

Andy
Feb 07, 2017, 06:21 PM
Ronaldo Nogueira
ronaldopn's Avatar
Sorry to post here as I fly only electrics but I have just started experimenting with this maneuver. Here are the key things I found will make your KE loops easier:
- high thrust to weight ratio (the more, the better);
- good lateral area (SFGs help tremendously here);
- large rudder area and throw;
- properly trimmed airframe with KE couplings correctly mixed (you still need to make small corrections through the loop);

This is also my first experience with SFGs and they were like night and day difference for KE loops. I tried a couple of times without them and it seemed almost impossible to perform the maneuver (too much power required).
Here is a video of my first KE loop attempts (watch in full screen mode as the camera has very wide angle lenses).
Plane is a Skyline Raven 60" 70E, setup and details here.
Tip: at the last quarter lower the throttle to idle and increase rudder. Add throttle again close to the bottom of the loop.
Ronaldo
Skyline Raven 60" 70E 04th/Feb/2017 - Knife edge loop + hover practice (4 min 24 sec)
Feb 08, 2017, 12:53 PM
Registered User
Forgive me...what are SFG's?

I'll work on the thrust-to-weight ratio. Maybe I'll chance it and use the BEC and skip the Rx Life. I can try another prop also.
Feb 08, 2017, 01:17 PM
Ronaldo Nogueira
ronaldopn's Avatar
Quote:
Originally Posted by sothpaw
Forgive me...what are SFG's?
Side Force Generators, you can see them attached to the wing tips on the picture below. SFGs are designed to add lateral area to the plane and improve KE flying (they add lift while in KE attitude). You can KE at lower speeds and angles.
They have a few side effects such as reducing wing rock on upright harriers and slowing some tumbling maneuvers.
This is the first plane I fly with SFGs and I'm hooked
Ronaldo
Last edited by ronaldopn; Feb 08, 2017 at 04:30 PM.
Feb 12, 2017, 11:47 AM
Registered User
Quote:
Originally Posted by ronaldopn
Side Force Generators, you can see them attached to the wing tips on the picture below. SFGs are designed to add lateral area to the plane and improve KE flying (they add lift while in KE attitude). You can KE at lower speeds and angles.
They have a few side effects such as reducing wing rock on upright harriers and slowing some tumbling maneuvers.
This is the first plane I fly with SFGs and I'm hooked
Ronaldo
Ok. I bought the Olympus and it has those. I've assembled the airplane and I need to situate the Rx and do the CG. I have 1 suspect servo...the rudder servo seemed to shake/shimmy under load sometimes. I'll check the wire connection and replace the servo before maiden if I can't find the issue as just a connection issue. It doesn't seem like that...it seems like the servo itself shakes after 25-50% throw.

Do you use an Rx pack? I use one on my 48" carbon yak because the Eflite ESC was causing me to crash (2 crashes before I figured this out...it only happened if I put together consecutive flights late in the evening). I don't see tons of cooling on the Olympus 70A ESC so I'll probably run the Rx pack.

Thanks,

Andy
Feb 12, 2017, 07:33 PM
Ronaldo Nogueira
ronaldopn's Avatar
I don't run an Rx pack in my electrics, it just adds weight IMO. I don't feed the Rx through the ESC either as the built in BECs usually don't supply the current needed.
I use this HK YEP 20A SBEC in all my 60" 3D birds and will be using it on my 30cc electric setup too. Hundreds of flights in different planes without a single issue (and I fly aggressively).
Ronaldo
Feb 21, 2017, 11:59 AM
Registered User
FYI: The FMS Olympus did a great KE loop on its first/maiden flight!! It's foam but it works. I've flown it with and without the top wing (cantilzer) and it flies KE loop much easier with the top wing in place.

Andy


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