roofus4

Simple question, but I don't know the answer. Would 4 x TT pro .36 engines give a 15 pound plane unlimited vertical.

I know that few will have an exact answer, but from your experiences, what is your opinion. My 13 pound twin could go vertical with 2 .60s on it. But I also understand that 2 .30s don't equal one .60

Rob

William Robison

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[b]Rob:
Assuming the 60 and the two 30 engines have the same total power -

Theoretically, your two 30 engines driving a single prop wont equal one 60 driving the same prop. This is because of mechanical losses with the greater number of parts.

However.

Aerodynamically, two 30s driving their own props will equal or exceed the one 60 engine. It's a matter of propellor disc area - the greater the area generally the more efficient it is. Same effect when you take a 12x8 prop off and install a 14x4. top speed is down, but lugging ability goes way up.

Bill.

roofus4

Thanks for the reply Bill. I had never heard someone say that but, I kind of had an idea that that might be the case. Plus the real comparison here is between an (OS 61) and 2 (TT 36)s. 36 being greater than half of 61. However, there is a 2 lb increase as well. I'm guessing the plane at about 15 lbs but it's not build yet so I don't really know. But it is important to me that the plane have a thrust to weight ratio greater than 1 to 1. Top speed is not my goal. In fact could be structurally harmful to the plane. That being said, should I go with say a 10x4 prop vs something with more pitch?

Thanks again for all your help.

Rob

William Robison

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[b]Rob:

In your first post you mention your 13 pound twin with two 60 engines. Then you ask about a pair of 36 engines. On the same plane?

If a new plane, few 36 twins are around, but the Twin-Air 45 has been shown to fly very well with GMS 32 engines, your 36 engines should do very well also. On the other hand, the Twin Star from Hobbico approaches being a rocket with 36 engines.

My one conversion from a 60 single to a twin is my C-3/10, originally a HobbiStar 60. With the twin Magnum XL 46 engines it's been clocked at 145 mph, with the original 61 engine it strained to reach 75 mph. It's running 10x8 three blade props. Vertical is good, with 12x4 2b props it might keep going forever.

The only thing that can be said with no worry about being wrong is a matter of the weight of your new twin. If it weighs four pounds it will outrun most rockets straight up, if it weighs 15 pounds it will be a dog.

Build it and try it.

Bill.

jrf

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Bill:

145 mph!?! Using your own speadsheet, and 75% efficiency (high for a 3-blade on a draggy airframe) your Magnums are turning 26000 rpm. And the prop tip speed is supersonic.

Must be a typo there somewhere.

Jim

William Robison

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[b]Jim:

Caught me!

The spread sheet is right, the engines were screaming, and the plane was going downhill steeply when measured.

Left some information out to make the point a little stronger.

Actual level flight speed is 105 +/- mph. Still a lot more than the single engined vesion.

Bill.

William Robison

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[b]Jim:

Another point - When gravity is assisting the air speed the effective efficiency of the prop can exceed 100%. At least on paper. As the air speed goes up the engines actually get to the point of running no load, with greater speed the props start driving the engines. When you get fast enough the prop blades are acting as air brakes.

Bill.

jrf

My Feedback: (551)

As every good educator has discovered, the more you teach us, the less hyperbole you can get away with.

Jim

William Robison

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[b]Jim:

By showing the ability to use the word "Hyperbole" without embarrassing yourself you've proved you are already pretty well educated.

Haw.

Bill.

roofus4

I guess I need to be more clear. I didn't mean to confuse you guys. My twin is a 40 sized trainer converted to take twin 61 os engines. Then I put twin 91 os engines on it at no additional weight. It is an absolute rocket!! But I remember that when it had 61s on it, it still had unlimited vertical. It weighs about 13 lbs.

I like the plane, and having multiple engines so much, I have decided to make a sister plane. Only this time it is a 60 size trainer (Hobbistar 60). And the huge difference that I guess I failed to mention is that this one will have 4 engines. So I am comparing a twin 60 to a quad 36. Being that the whole plane is bigger, I am just assuming that the plane will be just a bit heavier. Although I am going to try and save weight wherever I can.

Of course it will not have the performance of my twin 91, I do still want it to go vertical.

Sorry for the confusion.

Rob

(Here are pictures of the twin 91)

William Robison

My Feedback: (3)

[b]Rob:

As I said earlier, the bashed twin was a Hobbistar 60. With its two Magnum XL 46 engines it's a rocket, the same plane with four 36s hung on it should be even more spectacular.

Have fun with it.

Bill.

PS: Picture is the bashed Hobbistar 60, the C-3/10. wr.

roofus4

Bill that is a great bash job. I'm actually more impressed with the conversion to a low wing. It looks great!

Thanks again for all the help. I'm starting to feel better about what engines to select.

Rob

William Robison

My Feedback: (3)

[b]Rob:

Look again. It's a mid wing - plug in with an aluminum joiner tube. Hardest part was boring the ribs for the socket.

Bill.

William Robison

My Feedback: (3)

[b]Might see it better in these two shots.

Bill.

multiflyer

I have been flying a 4 engine patternplane of my own design for many years now. It has 80 inch span and weighs 15 lbs. I have had many different engine combinations on it. 4 40s can maintain about a 45 degree climb angle. The simple way to look at the problem is devide by 4. Would one engine on a 3.75 lb plane give you the verticle you want?

Multiflyer

roofus4

multiflyer

So are you saying that 4 40s cannot pull your 15 lb plane straight up? What kind of 40s are they?

William Robison

My Feedback: (3)

[b]Rob:

Four 30s should pull that 15 pound plane straight up. Either his 40s are very weak, or he's not getting full power.

Bill.

britbrat

The quad set-up should be a good performer, as each of the TT 36's have notably more than 1/2 the power output of the OS .61, & they weigh about 1/2 of its weight. You could use mini servos on the throttles to help keep the weight down.

roofus4

Man I really apreciate the input from you guys.

I am using the lightest servos I can for each function. On the throttles, I am going to use Hitec HS-55s. However, I am still not decided what engines to use. The TT Pro 36s make the most sense, but those OS .32sx's are so appealing. I am a diehard OS man, and everything I have read on those engines has been great! I'm just not sure.

William Robison

My Feedback: (3)

[b]Rob:

Buy the TT Pro 36 engines.

My son has bought nothing but TT engines since he's been back in the hobby, he (and everyone else) says nothing but good about them.

Bill.

britbrat

Will Robinson is dead right. My favorite engines are TT's, although I'm not married to any of them. Having said that, I do not use anything but TT's on my seaplanes --- absolutely rock hard reliable. I think that, all things considered, the very best 2 stroke 46 that you can buy for any price is the TT 46 Pro --- & the little 36 is just like it --- only smaller.

roofus4

Well Guys,

You have me convinced. It sounds like the TT .36s are the right choice. On top of the reliability and how well they run. Which I am taking your word for, the TT's are lighter and MUCH cheaper! Thanks for all the input with this issue. This has been a great help!

Rob

multiflyer

I'm not sure the four TT.36 engines will be enough for vertical at 15 lb. It just so happens that I have a 4 engine sport type pattern plane of my own design that I have been flying for many years now. The 4 .40 engines on will not give vertical unlimited. I can hold about 45 degree climb angle unlimited. Engines turn 10x6 props at about 11,500 rpm on the ground. So not the most powerful 40s around but I would more than just a little power increase to go vertical unlimited. 4 TT .46 just might?

Multiflyer

britbrat

multiflyer, your 40's are very weak (maybe they're LA's), the TT 36 Pro should hit 14,000+ static RPM with a 10-6.

roofus4

If you look at the RCU engine guide under the TT Pro .36, there are some customer reviews of the engine. One guy says that his 5.5 lb rascal has unlimited vertical with that engine. Seems like a lot of weight to me, but if it is true, then I should have no problem getting unlimited vertical. My only concern is that there seems to be conflicting information from comparable examples and I'm not sure what to believe.

Assuming the OS 32sx has similar power as the TT Pro .36, one of the reviews for the 32sx says he can hover a 3.5 lb plane at half throttle! So if the principle that we just multiply by 4 is true, then it would seem reasonable to think that this will work.

Any other suggestions on how to keep the weight down on this bird?

Rob