I'm building inside a one car garage, been waiting all winter for a day with little wind so I can do my weight and balance in the driveway. Nature finally cooperated last weekend.
With the airplane fully assembled, no fuel but otherwise ready to fly with me (171 lbs.) sitting in the cockpit she is tail heavy.
The book says 21-30% of chord, as described above she balances at 33% of chord.
I have a BRS installed contributing to the issue, the multi-colored paint scheme is likely a contributing factor too but I think the largest issue is the light Hirth F33 up front. Originally I planned to do part 103 but decided to register EAB so weight of the engine is less of a concern.
Doing the math I need about 30lbs more in the front to make it balance. I'm considering swapping out the F33 for an F23. At least I get some extra power from the extra weight that way.
First. Love the paint job. And i li.e the idea of a bigger engine. Cant go wrong with more power if you are going to add weight might as well be useful weight
That strobe on the tail is not helping matters. Does the f-33 have a electric starter? If itdid it would ad about 5# and then the battery could help also. If it was mine I would use a larger engine especially if going experimental. Great looking airplane.
that thing looks sharp. Did you use the light tail? I see you have a brs, imo I would put that up for sale on ebay. at the altitude your gonna fly at it won't do much good, plus there is a chance of it accidental deploying putting you in more danger.
I would say switch to a lighter tailwheel but you don't have one. when I switched from a hirth f33 to a rotax 277 on my skypup the cg shifted forward quite a bit but the cg was well within the limits. redo your wb checks, your math may have been off.
Sometimes people claim CG out of range that later turned out to be measuring error but in your case your reported CG is right where I would expect it to be.
The original version of MiniMax balances right on with the flat top, nose tank , no chute , and 65 lbs 277 in the nose, The later version with the full turtle deck, wing tanks, parachute and possibly the heavy tail it will balance out with 85+ lbs 447 and such, so yes with the 45lbs F33 you are at least 30 lbs too light in the nose, and remember full fuel will move it further back yet. Also You will still need some kind of protective tail skid because in nose high landing it could fall on the tail and grind the hack out of it.
Other than that I'm in the market for a used cheap F33.
Sometimes people claim CG out of range that later turned out to be measuring error but in your case your reported CG is right where I would expect it to be.
The original version of MiniMax balances right on with the flat top, nose tank , no chute , and 65 lbs 277 in the nose, The later version with the full turtle deck, wing tanks, parachute and possibly the heavy tail it will balance out with 85+ lbs 447 and such, so yes with the 45lbs F33 you are at least 30 lbs too light in the nose, and remember full fuel will move it further back yet. Also You will still need some kind of protective tail skid because in nose high landing it could fall on the tail and grind the hack out of it.
Other than that I'm in the market for a used cheap F33.
Would you like stripped out spark plug holes, cylinderal head bolts, and two forced landings with that birth. They only hirth once and its too late.
Fly it first with 30lbs. in the nose before you do anything else. After it's been flown you will be in a in a better position to evaluate your choices.
George, your analysis makes perfect sense. Also, I think the suggestion of adding a skid plate is a good idea. A small aluminum sheet should be easy to attach under the tail.
My F33 is new, never been started. Anyone needing one send me a private message with an offer.
Switching to the F23 I think I'll need a three blade prop to have enough ground clearance. Anyone have suggestions on what prop might be best?
ULB, If any Max pilots are flying with F-33, I would check with them too. How do they manage to balance out ? Also, how much difference has the change of L/Gear configuration caused to Wt & balance ? I ask this because although the nose leg and wheel are helping, the main wheel's center of mass has moved from about 10 inches ahead of original CG to about 6 or 7 inches behind original CG. Some of the strut weight has also moved to the rear, though only by a few inches.
Now that is the point. The wheels are further back so a different set of w&b criteria apply but I expect the Birth to be too light on nose weight and too light on horsepower, too. A lightly built part 103 might just make it but with heavy paint and a nose wheel, it looks like it needs some revision.
You may still need to swap the engine, but it may be helpful to understand why it happened in the first place. ( I am following this thread, as I also plan to use either a Rotax 277 or Hirth F-33 )
Instead of adding a thirty pound weight, try moving weight forward; strobe under the nose, battery forward of the cockpit, move the engine forward by an inch. Reweigh...
Instead of adding a thirty pound weight, try moving weight forward; strobe under the nose, battery forward of the cockpit, move the engine forward by an inch. Reweigh...
This is exactly what I used to do with my R/C models of WWII fighters. I would stretch the nose just a bit.
The paint, strobe and BRS are contributing factors no doubt, but even all of that combined is not the cause of it being tail heavy. George pointed out the real issue and doing some math its simple to see.
Total Weight is 515.45lbs total moment is 26223.70 From my datum the desired CG range is 44.09" - 48.95" Currently the CG is at 50.87"
The strobe is about 8oz, lets pretend it weights 1lb to account for wiring that is distributed further forward and be more optimistic in this change helping. Its arm is 152" Removing the strobe moves the CG to 50.68" a change of less than 1/4"
The BRS, its rocket, launch tube and main bridle weight 15.44lbs, its arm, measured to the rear most part of the pack to make the math most optimistic is 84" Removing the BRS moves the CG to 49.85", a 1" improvement but still too tail heavy.
Removing both strobe and BRS the CG is 49.64", still about 3/4" too far tail heavy.
Pretend I removed the BRS and the strobe and changed to a Rotax 277, the engine this was originally designed for. it weights 20lbs more than my Hirth. The center of the cylinder head on my Hirth has an Arm of -5", so lets pretend the 277 would add 20lbs more weight at an arm of -5" That results in a CG of 47.54" interestingly that happens to be very close to the 'ideal' 28% CG of 47.87" If the only change I made was changing to a 277 the CG is 48.78", still within proper range but only by 1/8"
Changing between tail vs nose wheel does not alter the CG very much at all. The main axle, the bulk of the weight is close to CG in the tail and nose configuration, its impact on CG is minimal. It only moves about 18" back when changing from tail to nose wheel configuration.
Removing the tail wheel from the back and adding a heavier nose wheel in the front makes little difference in CG.
For whatever it might be worth to future readers, I have the 'light' tail too.
Lessons learned for future builders:
If you have a light engine, be sure your max stays on a diet and keeps that booty slim.
A lighter engine can be a bad choice
A Strobe on the tail will push CG back about 1/4"
A BRS mounted behind the rear spar carry through will push CG back about 1"
My final thought/suggestion: Build your bird so it can be fitted with any engine, see Brian's VMAX log around page 13 for example. Buy an engine when your nearly completed.
Also a good opportunity to stock up on instruments. Don't look at this as a issuse but an opportunity to amaze your ga friends.with really cool cockpit instruments.
Have you discussed the situation with Dave at TEAM? I am so very grateful to you because I was considering redoing my bird with an f-33 and now not so much. Thanks for posting about you plight.
If the only change I made was changing to a 277 the CG is 48.78", still within proper range but only by 1/8"
You numbers are good for comparing the effect of weight shift but you are still doing it with empty tanks , for real life scenario you need to redo this with full tanks, even it may not seem far from CG it is 60lbs of it and it will shift CG back , this is to the opposite of the fuselage tank which would work in your favor as with fuel it moves forward,
Just saying . Because if anyone can learn from this when planning on using light engine like that you are better off with the nose tank.
I have heard it said many times over the years that the Mini-Max flies best when it is light (most planes do...). Usually this is in the context of somebody talking about putting a bigger engine and such on. While I may not have first hand experience with light/heavy Mini-Max aircraft... I have experienced both in the model realm.
I had a thing for small WWII fighters, in particular 1/12 th scale Spitfires. This works out to about a 35" wingspan. I had a couple of these and experienced the heavy vs light comparison fairly directly. Wing loading on these smaller wings was rather critical and they would not carry the same kind of loading as a larger wing. The heavy version I referred to as the Little Monster and you flew it to say you flew it. The light version was hand launched using an underhand toss and was much more fun to fly.
I relate this to suggest that adding instruments or extra weight of any kind should be a last choice here due to potentially very detrimental impacts on the flight characteristics. The only place I might bend here is if the engine is sufficiently more powerful, but even then you will suffer a higher wing loading.
You have done some great analysis on alternatives and impact. Now I would recommend running the numbers to see just how far out there you would need to mount that engine to get everything right. Moving stuff is, in my experience, a much better solution than adding weight.
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