# mustang II vs s-18 thorpe

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#### keefer66

##### Member
just curious both these planes are similar but the take off and landing runs are at opposite ends of the spectrum ? is it the airfoils or just the way that the designers listed or tested the planes .

#### N2T18S

##### Well-Known Member
Keefer,

I don't know where you got your numbers, but the two planes are almost idenitcal. The T18 and the MMII have the same fuse width. The S18 is a couple inches wider and five inches longer. Both designers worked together and apparently shared ideas. I've seen Bernie Fried wheel land his award winning Thorp T18 in 1000 feet.

The biggest difference I know of is in the wings. The MMII has a tapered wing, which is much harder to construct, and the THORP has a Hershey Bar. Everything on the Thorp wings is laid out to use four foot wide sheets of Aluminum. (JT wrote an article on the Sports Aviation Archive on EAA.org like: Birds have tapered wings) The Horizonital Stabilator (like on the Raptor) is unique.

The Folding wings are almost identical. I don't know who copied. But, it is the best system ever designed for one person.

The THORP T18 is usually about 100lbs lighter than an RV7, thus the better performance at half the price. A light Thorp weighs 850lbs. One was built at 701lbs, but it was pretty Spartan. Canvas seats, 1/2 Carbon fiber panel, .025 skins, etc.

As you can see I'm a little perjudice. John Thorp was ahead of his time. Designed the first Homebuilt Aluminum Airplane. Came up with "Matched Hole Construction". All flying Stabilator and a great, timeless Airplane. No jigs, everything is laid out to fit standard size supplies. Wings and controls are removeable for transport. Then you have Lu Sunderland's folding wing!! Still the best after forty years. Classic Sport Aviation has the S18 kits. The T18 plans can still be purchased from Richard Eklund for "plans built". Richard has John Thorps house and shop where he constructs parts on JT's original equipment.

Thanks,
N2t18s

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#### lr27

##### Well-Known Member
I spoke to a guy who'd built a Mustang II. At least I think that's what he said it was. He said it scared him. I don't think because anything was awful about it, just that things happened pretty fast when flying it and by the time he finished it, he was old enough to be slowing down. Kind of sad, really.

#### Wrongway John

##### Well-Known Member
The Mustang wing also has a laminar flow wing, which is supposed to be more difficult to build, and I hear this has some meeting performance specs with those high cruise speeds mentioned, while others are falling somewhat short of what they were expecting, due to not getting this aspect right.

I like the fact I can use smaller engines on the Thorp if one wants to go this route, and JT’s reputation as an aero engineer is legendary, so I feel comfortable with his design. How do others like the Thorp plans? I’ve started researching Thorp’s T-18’again, and it’s difficult to know what I’m getting into just yet. I prefer not to build as a kit, but materials mostly. I understand the plans are $300.00 and are quite comprehensive. I was told there is also a supplemental part for an extra$50.00. Not sure if this is needed or not. Then there was shipping, so I’m guess I’m looking at close to $400.00 for plans. I talked to the guy at Eckland Engineering that says his is the T-18 plans which don’t cover the folding wing, and not sure what else it doesn’t include, so I guess there would be even more additional supplements to buy, I dunno. IIRC, the Thorp S-18 plans that offer the folding wing, wider cockpit, longer fuselage and also supposed to be an improved wing airfoil that lowers stall without affecting cruise and top end, but their plans have to be purchased with a kit. What are most building these days? Does anybody have some used plans for sale? I would like to look things over to see if it is doable, before I took the nearly$400.00 plunge on these plans.

I’ve notice virtually every site I’ve looked at doesn’t mention take off or landing specs on the Thorp; just know it takes up quite a bit of runway. Not sure if the Thorp can actually out perform the RV-7 or not. On Van’s site the RV-7 lists 199 mph at gross weight for their cruise on 180 HP. Van’s forum seems to have many confirming these numbers. What few numbers I seen others share for their Thorps seem to be around 180 mph for the 180 HP, but again, haven't heard from that many Thorp builders. Not much data is given with altitude, temp, etc for me to really know and make a comparison. It seems the Thorp has the potential to make for a faster plane due to generally being 100 lbs lighter, and the wingspan is quite shorter as well, but haven’t read from anybody cruising around 200 mph with the 180 HP engine. Van’s aircraft certainly have way shorter take offs and landings. But gosh, their prices have been through the roof for some time now since their kits have gotten so comprehensive, and is why I think building a Thorp with materials and not a kit form is probably best for me. You’ll have a similar aircraft for about half in it of what the RV’s have in theirs.

#### rv7charlie

##### Well-Known Member
I haven't followed the current M-II kit company in many years, but about 20 years ago when I was getting ready to pull the trigger on a kit, the M-II kit price was quite a bit lower than an RV-7 kit. BUT....the stuff *not included* in the M-II kit drove the price to quite a bit above the price of an RV-7 kit. And the pre-fabrication level was a lot less than the RV kit, as well. I haven't looked at either T/S-18 offering either, but I'd bet that the prefab level is far lower than the RV kits, also. Buyer's choice on the level of work they want to do, but more prefab obviously is worth more in terms of kit cost.

Not sure where you're getting weight figures. At 1300 lbs, less engine, none of the planes we're discussing would be flyable when completed. (Even at 1300 lbs including the engine they'd probably not be flyable; adding fuel would come so close to gross that there's no capacity left for the pilot.

I'm not sure that 'simplistic' accurately describes measuring value in dollars per pound. It's really kinda backward. In a/c, it's typically the case that the *more* it costs per lb, the better it will be. Costs are more frequently judged on the cost to *lose* a pound, rather than the cost to buy a pound. Witness the current fad of replacing a $40 SLA battery with a$300 lithium battery, to save 10 lbs of weight.

Charlie

#### Blueberry

##### Member
Thanks Charlie. I miss typed that. 1300 is after engine weight is removed. Gross weight for MII is recommended at 1600. 1600 - 300 (rough wet weight of a O-360) = 1300. And before everyone jumps on this 300 pounds I know that it all depends.... Lycoming lists dry weight at 254lbs. And I probably should have gone with Empty weight (930 - 1100 depending...) for my comparison.

I am sure you are correct in your assessment of what is and what is not included in the RV kit vs others. I'm not going to get into that debate.

For those that are debating the "Scratch build to save $" vs "Build from Kit", it is a valid view. Strip away the engine, prop and the electronics, most of what's left is aluminum. Take out what it is going to cost you for the aluminum ($9-\$10/lbs) and that is all you are saving. You also need to add in 15%-20% scrap if scratch building into the cost of the aluminum, more if its your first.

#### rv7charlie

##### Well-Known Member
If that's the value yardstick you want to use, you still need to get airframe weight correct. Gross weight is empty weight (including engine) + max fuel + payload (people/cargo).

Empty weight of a typical RV6/7 is between 1000 & 1150 lbs, depending on engine, prop, and builder's attention to weight gain. The FWF propulsion package (not included in the kit) will weigh, real world, between 350 & 450 lbs, depending on engine choice and prop choice. It's unlikely you'll find *any* Lyc 360 at 254 lbs; virtually all will be at least 285-335 lbs without exhaust system or oil cooler/hoses, and often without starter/alternator. You'd also need to subtract the weight of any avionics and interior (seat cushions, insulation, upholstery, etc).

You're still left with the issue that less (weight) is actually worth more money. Driving airframe weight *down* while maintaining strength allows greater useful load, which carries greater value to most of us (more fuel, more people, more baggage). "Everything's a compromise"; we have to pick our tradeoffs, within technically and financially available options.

Charlie