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-   -   Reduce speed envelope of Great Planes RV4? (https://www.wattflyer.com/forums/showthread.php?t=59341)

syd123 12-16-2010 07:32 PM

Reduce speed envelope of Great Planes RV4?
 
1 Attachment(s)
First time poster here… Thanks in advance to all responders. Sorry so long :)

Though I've spent many years building glow-powered planes (all quite large) this is my first electric and my first very small plane (41" wingspan). I built this from a Great Planes RV4 kit. Please see picture.

It was designed to be powered by a 280-370 brushed motor, gear-box, and ni-cads. Instead, I decided to use a Rimfire .10 brushless and 3-cell, 35amp ESC, and Lipo's because I anticipated the plane would end up being 10+ oz. heavier because I planned on beefing up the structure quite a bit (I felt the kit, as designed, would yield a very fragile airplane). This isn't what happened. Even after I added stringers b/w the main spar and leading edge, swapped out the wire gear with aluminum gear, sheeted the fore and aft top-decks, added bolt-mounting for wing, mounted servos outside of fuse, etc… The plane finished up just a few ounces more than the plans called for. Apparently, the weight saved by using the lighter brushless/ Lipo setup nearly offset all of the extra weight I added. So now, with the .10 and 11.1v, it seems that I have a plane that is way over powered. It weighs just a tad over 2lbs.

I've flown it four times now and it takes off at 1/8 throttle and climbs with full down trim at anything more than 1/2 throttle. To reduce this tendency I've reduced wing incidence to 1 degree. ..However, with a semi-symmetrical airfoil this will always be a problem if the plane is flying 4 times the speed it was intended (am I correct?).

How can I shrink the speed envelope? .I would much prefer being able to use the entire (or nearly) throw of the throttle stick. I don't have a computer radio, so end-point adjustment of the throttle is out. ..What else can I do?? ..Is there a slower prop than 10x4.5. that would work with this engine/battery combo?? ..Perhaps use just a 2-cell lipo?? I don't want this to fly like a 3D aerobat. ..I want it to fly like a Sportster or Astrohog. ..Basically, just a low-wing sunday flyer that can do loops, rolls, fly inverted, and do an occasional few seconds of knife-edge.

thanks

JetPlaneFlyer 12-16-2010 10:28 PM

Nice looking model!
It's possible that the CG is a bit too far forward. Counter intuitive though it sounds moving the CG back reduces the tendency for a plane to climb at high speed. You have to be careful moving the CG back though because too far produces an unstable plane. Changing the wing incidence without adjusting the CG actually wont help because you will just end up having to add up trim.
If the CG is good then you need to add some downthrust.
A smaller diameter prop will reduce power.. or as you suggest, a 2 cell battery will reduce power (by about half) and also cut out some weight.

syd123 12-16-2010 10:59 PM

thanks JetPlaneFlyer. I very carefully checked and rechecked CG and it is smack dab in the middle of the range indicated on plans. ..If you think it will help, I can add just enough weight to the tail to move it to the very back edge of the range.

You mention a smaller diameter prop. ..What size exactly? All the 9" props I can find have greater pitch than 4.5. ..I've never quite understood whether diameter or pitch has a greater affect on thrust.

Lastly, Rimfire only recommends 3-cell lipos for this motor. ..Is there a risk in using a 2-cell instead?

JetPlaneFlyer 12-17-2010 06:51 AM

Moving the CG to the rear of the range, then re trimming the elevator to suit, will definitely reduce the climbing at speed tendency. But you might still have to add some downthrust to reduce it more. You can normally tweak the CG by moving the battery.

According to the spec here: http://www.electrifly.com/motors/gpmg4505.html your motor is suitable for 2 or 3 cells.. But for best performance 3 cells is recommended. Also note that the motor is recommended for 3lb sports models , at 2lb you are in the '3D' power/weight range which probably explains why she's so fast.

Generally a larger diameter prop pulls more amps so reduce the prop diameter and you reduce the amp draw and so reduce the power consumed. You are already a little smaller than the recommended 10" prop but it would do the motor no harm to go to an 8" prop. Perhaps the best idea if you want significantly less power though is to use the recommended 10" prop and a 2 cell battery, that way you save weight too. Either that or just use throttle management with your thumb!

Generally prop pitch has less effect on power consumption than diameter does. As a general guide I'd recommend never going for a pitch/diameter ratio less than 0.5.. So for a 9" prop 4.5 is the lowest I'd think about, or 5" pitch for the 10" prop. Optimum all round sport model performance is usually found with a pitch/diameter ratio close to 0.7

Steve

CHELLIE 12-17-2010 06:59 AM

I agree with Jetplaneflyer, use a 2 cell lipo for scale like performance, your over powered with a 3 cell, that will tone it down for you :) Take care and have fun, Chellie

Welcome to Wattflyers syd123 :)

rcers 12-17-2010 01:28 PM

I am not so sure 2s is what you want, but light will help. What won't help is getting that plane too slow. :) Then you will may find it very soft on controls and snap or stall.

It is not really a slow flyer type plane, and needs some speed in my experience.

Mike

Saddlebum 12-17-2010 03:24 PM


Originally Posted by rcers (Post 771384)
I am not so sure 2s is what you want, but light will help. What won't help is getting that plane too slow. :) Then you will may find it very soft on controls and snap or stall.

It is not really a slow flyer type plane, and needs some speed in my experience.

Mike

True but if his combination is getting off the ground on 1/8th throttle, :eek: that's a pretty good indication he has too much power.

...The Bum

rcers 12-17-2010 04:02 PM

Yep good point.

Xptical 12-17-2010 08:08 PM

Sounds like you might need some down-thrust and a smaller pitch prop. Try a 10*3.8 or a 9*3.8.

Dereck 12-17-2010 09:48 PM

Welcome aboard Syd
FIrst off, if you're used to larger IC powered models, you may be fascinated, nay - amazed - by how lightly a small electric model can be built.

First off - visit the master's shopfront. His name is Pat Tritle. Look up any of his designs - they all weigh about half of what mere mortals can built to.

Start with the firewall - in your old life, you were anchoring an IC lump that vibrated, weighed a lot and sent shivers down the airframe every time the fuel in the head went 'BANG!' Now you have a little alloy cylinder spinning around back of your prop, propelled magically around with no vibration whatsoever.

2lb is a little hefty for your model, plus your Rimfire 10 could be a little excessive.

Possible cures, given that taking weight out the airframe now could be tricky. A 1" smaller prop would be the cheapest approach, as you're flying on relatively trivial amounts of available power now. Next up the cost-cutting after trying that is going to 2S.

A balancing act here would be a 2S LiPo - go for the same mA rating you have now, or a little less dependant on what your pack can handle in current draw - and a smaller diameter prop with steeper pitch. You'll be juggling pitch speed (higher) for RPM (lower) and hopefully getting a little freeby with lower weight.

Your next step is a smaller motor. Perhaps not a bad idea, as you certainly seem to have an excess of grunt up front right now. That would require more spending and perhaps some Very Hard Sums though :( Let's try and get your 10 to play with a spherical object first.

And please - put the servos back inside. The last aircraft that looked good with its controls on the outside was DeHavilland's Tiger Moth. Just because the IMAC kiddies put their servos on the outside - to show off how much they spent, one suspects - doesn't mean that the rest of us should demonstrate such appalling taste ;)

Don't expect too much. Regretably, we fly small models in standard sized air and those effectively bigger molecules don't do such a good job of holding up smaller flying machines.

Keep us posted. You've done a good looking little model there

Dereck

Turner 12-17-2010 10:26 PM

You could also get a smaller motor.

syd123 12-18-2010 01:23 AM


Originally Posted by Dereck (Post 771489)
Welcome aboard Syd
FIrst off, if you're used to larger IC powered models, you may be fascinated, nay - amazed - by how lightly a small electric model can be built.
..
2lb is a little hefty for your model, plus your Rimfire 10 could be a little excessive.
...

Your next step is a smaller motor. Perhaps not a bad idea, as you certainly seem to have an excess of grunt up front right now. That would require more spending and perhaps some Very Hard Sums though :( Let's try and get your 10 to play with a spherical object first.

And please - put the servos back inside. ..

Don't expect too much. Regretably, we fly small models in standard sized air and those effectively bigger molecules don't do such a good job of holding up smaller flying machines.

Keep us posted. You've done a good looking little model there

Dereck

Thank you Dereck (and all others as well!) for your well considered input.

I chuckled reading your suggestions, I appreciate your sense of humor. ..Especially the bit about moving the servos back inside :) ..Though I knew it would spoil the plane's appearance, I did it because I wanted to keep the wing bolted to the fuse and still be able to make changes to the linkages. When I tested the balance of the plane before covering it, it seemed to balance in the range even with the servos all the way aft. ..But you're right, they do look rather clunky. ..And yes, I have a lot to learn about building light.

I flew it today and before doing so I clipped cut the 10x4.5" prop down to 9" (not a great idea, I know, but I was careful and re-balanced afterward). ..It seemed to have helped but it was such a breezy day that i flew only a few circuits before deciding it would be best to land her and wait for a better day.

If I replace the motor, is there one you would recommend? ..And could it run using the 3cell and 35amp ESC (Electrifly) that I currently have?

thanks again to all

Turner 12-18-2010 03:24 AM

I can't make a motor recommendation other than to say look at the next smaller in the Rimfire line which is the 400 and also look at Great Plane's recommendation for that plane. The Rimfire 400 might be a better choice though there is a pretty big gap between that and the .10. Yes any motor a bit smaller than what you have now that is rated for 3 cells will work with your current battery and ESC. You want to keep the kv near but not much over the 1250 of the .10.

I have tried to find some more info on the RV4 but couldn't. Is that a current model offered by Great Planes? I can find the 54.65" RV4 but not one that is 41".

There is a wealth of information here about choosing motors. There are many other members who might be better able to help you choose one. One thing I do is look at the setups offered by many manufacturers of similar sized and type planes and copy their specs.

syd123 12-18-2010 04:13 AM


Originally Posted by Turner (Post 771567)
I have tried to find some more info on the RV4 but couldn't. Is that a current model offered by Great Planes? I can find the 54.65" RV4 but not one that is 41".

Great Planes discontinued it. ..I actually framed it up about 5 years ago but didn't finish it as other priorities (kids) ensued. ..I finally covered a few weeks ago after my 10 year old kept insisting that I finish it so he could watch it fly. ..It's been fun getting acquainted with electric power but it is truly a different animal.

Here's a link to the kit info.

thanks for your response.

http://manuals.hobbico.com/gpm/gpma0010-manual.pdf

syd123 12-18-2010 05:42 PM

1 Attachment(s)
...Guys,

Do you think this motor would be a better match?? I purchased this shortly after buying the kit then quickly misplaced it. I found it last night. ..It suggests using 2-cell lipo which I suppose I could purchase without breaking the bank. ..Would my current Electrifly 35a ESC be suitable? ..I can tell I'll need to buy adapters for the bullet connectors as the ones on this motor are tiny.

Again, my end goal is to wind up with a plane that can do easy loops, rolls, fly inverted, etc.. but also allows me to use more than 1/4 of the throttle range which the current setup doesn't do.

JetPlaneFlyer 12-18-2010 07:08 PM


Originally Posted by syd123 (Post 771661)
...Guys,

Do you think this motor would be a better match??

That would probably work fine, but I'm sure going to a 2 cell battery on the existing motor you have will give you what you are looking for anyway, both motors are in fact similar kv rating. The advantage of using the smaller motor is the approx 20 gram weight saving.

Steve

C140 12-19-2010 02:12 AM

I agree with JetPlane, the weight savings of the smaller motor is a good thing, in addition to using a smaller battery. The real RV-4 is only a 900 pound aircraft empty, which is what makes it so much fun.

If you're on a tight budget, take a look at the Heads Up RC website. Jeff's a great guy and he has clear performance information about all of his products. It really helped me make sense of all the numbers when I was just getting into electrics.

http://www.headsuprc.com/servlet/StoreFront

syd123 12-19-2010 04:12 AM

Thanks folks. ..I may go with a 2-cell lipo as a first step.

I flew the plane again today and I'm glad to report that it flew much better, albeit still at less than 1/2 throttle. The tendency to climb was greatly reduced, I think, by further reduction of wing incidence. Also, I used a different battery pack - a 2100mah, instead of 1800 - which is quite a bit heavier and because of it's girth it needs to be mounted a 1/4" closer to the nose. The additional weight up front actually helped quite a bit. I think the plane was actually quite tail-heavy up till now. My initial flights were made with the balance right on the forward edge of the CG as it appears on the plans. ..i now think that the plans are wrong (and the true CG is actually 1/2" forward), or that other changes I made (to wing incidence, for example) may have affected it. ..In any case, it flew much better today. It's low speed handling was much improved. ..Still, I'm looking to shed some of the power so that I can have finer throttle control. Thanks everyone for your input.

Dereck 12-23-2010 03:25 PM

Hi Syd
Good to hear you've moved upwards and onwards (in a positive fashion ;) )

The 2S looks to be the way to go. That way, you're not messing with fitting motors into a model that is maybe has a different size of mounting in there.

Next comes sorting out your prop. This gets fun, more so as it involves trying props. Lots of props, drawersful of props. I have about twice as many props as most hobby shops.

After a lot of messing around over a few years, they are all APC, APC-E and APC Slow flier props. That could be a hint

But move carefully. You've dropped to 2S, so whatever prop is going round some slower than before, for a given throttle setting. Accepting that you're going to either spend on another motor - Jeff at Headsup RC is a good go-to source for stuff and info - or tinker lots, the first prop change may involve a shorter diameter at the same pitch you've been flying at - it will be more efficient than a cut-down. Next, oddly enough, comes upping the pitch - though this could mean flying even more at reduced throttle settings. May sound odd, but for this kind of model, a higher pitch at lower RPM can work.

How's about a smaller 2S battery, if you can maintain your CG with less battery weight?

Another great source of e-power info is Ken Myers' internet newsletter 'Ampeer':
http://homepage.mac.com/kmyersefo/

Ken and his buddies have been doing this e-power flying thing for a long while, there's a lot of knowledge by some of the best around that site. Plus its a good read ;)

Changing incidences etc on plans and kits is only for those who do know better ;). The two safe routes are to build kits and plans exactly, and then you can fuss about the designer's incompetency later, if needed, or draw up your own plans.

You then know who is the incompetent to fuss at when things go wrong...

Regartds

Dereck

syd123 12-23-2010 08:12 PM

Thanks again Dereck!

I'm going to take your advice and try a 2-cell Lipo and experiment with different props. ..And for the reasons you mentioned. ..It's much easier than removing the motor, etc.. And I'll peruse the Ken Myer's site that you've recommended.


As for your advice regarding CG and wing-incidence, I agree wholeheartedly. I didn't intend to tamper with the incidence, it just became apparent that I had done something to screw up the 1-degree that was engineered into the model. While preparing the model to be covered I must have over-sanded the front half of the wing-saddle thus increasing incidence to 2 or 3 degrees. Then, to compound matters, I added foam tape to the wing saddle which seemed to cause the trailing edge of the wing to droop a bit thus increasing the incidence even further. So I'm guessing that the incidence was probably a positive 4-5 degrees before I figured out it was contributing to the problem and fixed it. These darn tiny planes!!! ..I'm coming to find that my little screw-ups are twice as bad when the plane is so small.

This little struggle aside, I'm loving the idea of using an electric motor instead of a glow engine. Once I arrive at the field, I'm up and flying in 30 seconds. This is a revelation!! And I truly love the fact that the motor doesn't vibrate the airframe apart, and I don't have an oily residue all over the plane afterwards causing covering to lift up or soaking into and weakening the airframe. ..So, power-wise, I think I'm a convert to electric. However, size-wise, I may be heading back to .60-.90 and up planes after this. For one thing, it's driving me nuts having to wait for calm days. ..My larger models were still fun to fly in anything less than 20mph winds. ..But this little plane becomes "un-fun" with anything more than 8 or 9mph.

Dereck 12-24-2010 09:28 PM

Hi Syd
Right on one part - small models are not for the faint of heart! Also,it is nice to come home not smelling of burnt oil. My S.U., with no aeromodelling background, never cared for the smell of burnt castor.

By way of irony, my e models ended up much bigger than anything I ever flew on glow. 25 was my glow upper end, though my custom design/build service knew how to make them outrun much larger models. Even ended up in IMAA territory with a Sig 1/5th scale Cub.

Have now reverted to smaller - 4S - e-power, on account of being a tightwad who doesn't care to lug big models around.

Trivial holiday point - Ken Myers was the guy who showed me how to get my electrics to where they could exceed my slimers in performance. He's one of the best in our hobby.

D


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