kbear

I posted this question on my build thread [link=http://www.rcuniverse.com/forum/m_6707116/mpage_1/key_/tm.htm](3D Hot Hots Build)[/link] as well as in the 3D forum, but I think I will get a better response here, or at least I hope I will. The CG on the plans for this plane range from 21.5-23% MAC. I thought it was generally accepted that 25% was a good nose heavy starting point. Up until now, this has held true for me. If My CG is anywhere between 25 - 27% I usually give it a go, and almost always end up with the CG in the 27 - 30 % range depending on the type of plane. 21.5-23% just seems very nose heavy, to me, for a 3D plane even for a maiden flight. I am balanced at 26.5% right now and was thinking this was good for maiden. I just can't get past the CG on the plans. Can you guys suggest any reasons it would be so far forward. I have check the calculations graphically as well as using some of the online calculators and all come out the same. I would rather not add any more weight to the plane, but I will if you think it really needs to be in the 21.5 - 23% range. Normally I would accept the plans an go with it, but there have been a few discrepancies on the plans already. If you need other info, let me know I will be glad to measure what ever you need. Please help, even if you don't have experience with this model. The more theory, opinion, or experience the better. I know that is might sound like I'm asking you to tell me what I have is ok, but I'm not. I'm trying to understand if there is a good reason for the difference in the plans and my experience. I have been flying for more than 5 years now and have designed many planes (SPADs mostly).

seanreit

My Feedback: (60)

I would not hesitate to fly it at 23% and more forward as necessary, depends on the type of flying you want. In this airplane, the more back you go with the CG the better it will hover, the more forward the more stable. They have a recommended CG set for radical flying. I would dial in some Expo on the tranny and go have a ball.

da Rock

A good reason for a difference in the plan specified CG and what you've worked out? You betcha........... The most probable is that the designer and you fly differently. And next is that the designer was trying to suggest a CG he thought would be safest for the most flyers FOR THEIR FIRST FLIGHTS.

The suggested CGs are really there to cover the FIRST FLIGHTS. It's understood by really experienced flyers that they're nothing more. Newbies lock in on those suggested locations like they were handed down with the 10 commandments to Moses.

If you want to get a really good feel for where the CG can be on any model, go to geistware.com and run their CG application. If you got a yardstick and 10 minutes, you can do it.

And btw, the idea that the CG is simply a relative location on the wing is just simply wrong. The size and location of the horizontal tail has a great deal to do with it. And the formulas for pitch stability used by aeronautical engineers include the tail. The airplane certainly uses the tail in it's activity, and we should be smart enough to include it as well in our computations. But we don't have to do the computations. All we gotta do is measure our model with a yardstick. Eight measurements. They include the tail, btw, as they should. Then plug 'em into geistware. Not even simple arithmetic required.

Run the application once with a 5% SM (you don't even have to understand all the technical definitions for SM) and then 20% SM and read out the CG location for each. Those two give you the SAFE range where the CG can be. If your CG location is in that range, simply adjusting the elevator throw to suit your desired stick response will give you a safe and easy to fly model.

da Rock

One yardstick.
9 measurements.
One choice.
click the button.
read out the CG location.

http://www.geistware.com/rcmodeling/cg_super_calc.htm

da Rock

OK, I've got a Hots I built many, many years ago. Built it from (I think) a Midwest kit in less than a week. I needed something to fly next weekend and the kit was sitting there. Turns out I had time to finish it with a layer of 7/8oz glass cloth and epoxy paint. Turned out very, very light. I made a huge mistake and put a .45 on the front. It was AWESOME fast. I wound up flying it with a .40 and often considered putting a carb on one of my OS.35S's for it, but never got around to doing that.

Anyway, I just now was loading the car to go flying and took a minute to measure the little beast. And I ran the numbers through geistware to see what the CG range is for the little sucker.

It's got a MAC of 9.75" and the CG range is from 2.07" to 2.56". That's a 21% to 26% of MAC range, and since the planform has no sweep, is easy to reference.

Mine is not the 3D layout. Ailerons aren't as large, nor is the tail as large nor the same relative length back. But it takes about 2 minutes to measure a Hots and about 2 minutes to plug your real numbers into geistware.

Mine doesn't have anywhere like the size elevators the 3D has, but I found that mine are very effective. So if I were to build a 3D Hots, the first thing I'd do is run it's numbers. Eventhough following the mfg recommendation almost always is safe enough for a first flight. And there is no law says you can't deviate depending on how the airplane flies for you. If you make your changes reasonable, and remember to expect the elevator sensitivity to change, it's safe as can be to adjust in steps.

A CG change doesn't suddenly become impossible to fly when you're moving it with some understanding of what it does to the elevator sensitivity.

kbear

Thanks Guys. I ran into that sight while checking other posts and did just as you said. What timing huh. That is a great sight. I plugged in the numbers and feel much better. The static margin for the CG as she sits now is 10.7 and I am quite happy to start there. I was also happy to see that when I tested SM's in the 10 to 15% range, I got a wide CG range. This program will help me allot in my own designs as well. Thanks for the replies. Oh, and I did find the sticky with other calculators on it. Probably should have found that first.

kbear

Da Rock, I just say the photos. Nice plane. You are a bit better at covering than I am, but I'm learning. The flying I have down pretty good. This is only my second build from balsa and the first was almost 10 years ago. My most responsive plane at the moment is My Stick Stick design and I have the CG on it at 30% I think. Most everyone at the field says it looks tail heavy but it is just inside my comfort zone right now. Building SPADs has given me a great chance to experiment and learn.



Thanks again for the help.

da Rock

Interesting looking airplane. It's always nice to have one that IS in your comfort zone, isn't it.

Craig-RCU

In this thread is a discussion of how airfoil choice on an h.stab can affect the C.G. location http://www.rcuniverse.com/forum/m_6866968/tm.htm Nmking09 brings the subject up and I elaborate a bit. Basically the gist is that airfoil and plan-form affect stall characteristics of flying surfaces. The big thick wing and large radius leading "unbalances" the relative stall characteristics of the wing vs. the stab and makes your Hots less tolerant to aft C.G.s. Plus, it has a relatively small h.stab and elevator area compared to the wing which also makes the design less tolerant to an aft C.G.. The wing and h.stab of your Hots also have relatively similar aspect ratios too compared to "more typical" designs. This means that there is little use of a lower aspect ratio tail surface's higher AoA stall resistance to help with pitch stability; again, possibly resulting in your Hots being less tolerant to an aft C.G..

Fuselage lift and the short coupled tail surfaces could be contributing factors to your Hots's possible need for a more forward C.G.. The wing is set relatively far back on the fuselage compared to typical designs and an aft C.G. might cause both yaw and pitch stability issues because the neutral stability point for the fuselage is more forward than is typical. It should be mentioned that fuselage lift has a relatively greater contribution at high AoAs because fuselages are essentially just very low aspect ratio wings and lower aspect ratio wings stall at higher AoAs than do higher aspect ratio wings. So, while no pitch or yaw instability may be experienced under lower AoA "normal" flight conditions, instability could rear it's ugly head at higher AoA "extreme" flight conditions and cause an unrecoverable flat spin or the like. Tossing your Hots around with minimal fuel in the tank would be the acid test to find any undesirable stall characteristics or instability issues.

kbear

Thanks Craig. Very useful info. I'm hoping to get her in the air this weekend.

da Rock

The real beauty of our situation (it's just a model airplane and we ain't in it) concerning the CG is also very simple.

All of the erudite loquacity, all the intricate theory and details that affect all to do with a CG really is fun to talk about.

But we really don't locate anything but a STARTING range with all the theory. And a simple application of the basic formulas gives a very dependably safe CG range. And all we need are a few easy to do measurements.

Modelers don't need anything but a safe starting place. And after the maiden, heck, DURING the maiden we can figure out by observation where the CG can go and how far. I really don't know many modelers who make their final CG location decisions "on paper".

Placing the CG perfectly before hand really isn't needed. In fact, perfect placement according to all the formulas and modifications within is really a waste of time. Because no matter how intricate or extensive the computations, we really don't know many of the inputs with any certainty at all. And the ones we can find out in 5 minutes with a yardstick are absolutely adequate.

BTW, the guy who designed the Hots was from around here. His son still flies at one of the club fields outside of town. I understand that the recommended CG was the result of testing. When I built mine years ago, I did a couple of runs through the calculations to see what basic theory would give for a CG range. His CG was inside the range the paperwork gave. I happened to be working on a flight planning application for major airlines at the time. (years ago) A buddy's team was doing weights and balances as part of the entire package. So I had access to a number of CG location subroutines. I did the figuring at home on paper using the basic tail-area-planform/location/wing-area-planform formulas. Checked the results the next Monday at work on our U-1108s. Wasn't surprised at all when all the answers were pretty much the same.

Plugging in more variables doesn't usually change the results much when the variables are things that only very slightly modify and refine.

The CG on a Hots isn't somehow more critical. Or less. You'll be perfectly safe with it where the plans call for it for the first flights. And your experience and testing will work like gangbusters from there on out.

kbear

Thanks again.