RE: Fastest Engine
 

Say another way. I don't understand the question.

RE: Fastest Engine
 

You might go back and review some of your past post regarding the rpm you were getting with the 91 fx and the 12/8 prop. Have a nice holiday.

RE: Fastest Engine
 

What is your density altitude, or elevation where you live?

RE: Fastest Engine
 

300 ft and sinking.

RE: Fastest Engine
 

I'll re-tach and advise. It hasn't been tached in the last few weeks but that's correct according to my memory. Right now it's in the pile of balsa splinters that used to be a Delta Fighter 90 after I lost all control at full speed out of a dive. Pine trees and balsa trees do not mix well.

RE: Fastest Engine
 

This boat is maxed out and has already got the record for a flatbottom, any more and she's airborn. At 32 inch's long and 5.5lbs its just right for the .61, the reason I want a more powerful engine is for a hydro Im going to build which is for another record that is set at 47mph. Thanks guys for the imput it has been very useful.

RE: Fastest Engine
 

The rules state that it has to be air driven,unlimited power and cannot fly, has to stay on the water except for high speed skipping across the surface. Again thanks for all the imput, Im really interested in the Jett, man them are some insane engines. They ought to really make a hydro sceam.

RE: Fastest Engine
 

Can you use a hydrofoil? Would the hydrofoil yield the best control and allow for higher speeds?

RE: Fastest Engine
 

Yes, a hydrofoil is allowed, dont know how stable it would be. A properly setup three point hydro with the right engine and setup can yeild some good speeds and handle very well at high speeds. The trick is adapting airpower to it.

RE: Fastest Engine
 

Very interesting, ill have to look into these. A ducted fan hydro would be awsom.

RE: Fastest Engine
 

I think a ducted fan would definitly qualify as "air powered". But the state of the art of fast model plane power plants is turbine power. These things have exhaust velocities on the order of 600 mph not the measly 150 or even 250 mph of the fastest ducted fans. I don't know how the rules define "air powered" though. If it simply means not water-jet powered or submerged prop, then it might qualify. A turbine will cost you $1500 to $2400 for the least expensive types.

Hydrofoils seem to offer the least drag. I have read that they can be unstable. However, all the pictures of hydrofoil boats that I have seen seem to be aerodynamicly unstable (tail-heavy) and when the whole hull of a boat moving at 70 knots is out of the water there must be a lot of aerodynamic forces acting on it to try to push it off course and make it tumble like a playing card. Based on the hydrofoil area of the two pictures below, I'd estimate the balance point of both of these hydrofoil boats to be well aft of 50% of their lengths. A balance point that far back is nice for keeping the bow above oncoming waves when not hydrofoiling, but if you made a balsa model of either of these boats an tossed it in the air, they would tumble end over end. So, I submit that it is possible to make a stable hydrofoil by making the parts of the boat aerodynamicly stable that do not touch the water when at speed. This can easily be done to any boat by adding large enough horizontal and vertical fins. They can even look cool like the movie "Bat Boats"(see pics below). The movie bat boats seem to have enough vertical fin area but not enough horizontal area. The actual real world bat boat racers seem to be aerodynamicly stable as they are catching air pretty good in the pictures. A hydrofoil bat boat seems to me to be the least drag configuration possible. Be the first on your block to own a turbine-powered, hydrofoil bat boat. :D Google "bat boat" for more examples.

You could make your flat bottom boat aerodynamicly stable by adding a large enough horizontal stabilizer behind your engine. If you built it light, it may only add 4-6oz to the total weight of your boat and allow you to safely return to the surface of the water instead of backflipping after catching air (that flip was cool by the way, but is a speed-run killer). You can figure out how much fin area you need to make your boat stable by making a smaller scale "profile" version of it out of balsa with enough weight in the rear of it to make it balance in the same place. Then add horizontal and vertical fin area behind the engine mount until it doesn't flip around when you give it a toss into the air. Aerodynamicly stable objects need a minimum of 75% percent of their surface area behind their balance points.

RE: Fastest Engine
 

If you do make the smaller-scale model of your flat bottom boat, remember that if you build the full-scale surfaces lightly enough they will have relativly little effect on its ballance point while adding fin area to the small-scale version will change the balance point more significantly. What you'll need to do then, is adjust the balance of the small version so that the added fin area doesn't move the balance point from its relative location on your original. Clear as mud?

RE: Fastest Engine
 

Wow, gettin kind of technical. Thats something else I have to look into and yes the ducted fans are allowed.The thing with the ducted fan though is would it be very effiecint on a boat with the drag factor involved. A jet is pretty much drag free so it cruises where the fan is most effecint, Am I right?

RE: Fastest Engine
 

Pushing the limit and breaking records is technical business as you know. You seemed to accept the technical challenge of testing different props on your flat bottom boat and measuring the results to find out what works best, so I thought that you might be interested in testing other methods to increase speed namely, reducing drag. I just saw a possibility of much higher speeds than 70 mph if you "think outside the box" as it were.

Yes, you are right that a jet is relatively drag free when compared with a flat bottom boat and that ducted fans and turbines are most efficient at "jet" speeds. I thought that it might be possible to make a hydrofoil boat as drag-free as a jet by essentially replacing the main wings on a jet fuselage with hydofoils that are just big enough to lift its fuse out of the water at high speed. Would your friends think its "uncool" to have a 100-200mph model boat or something?:D

RE: Fastest Engine
 

Remove the wings off a Convair XF2Y-1 Seadart and it won't fly but it'll still be really fast. Maybe replace that retractable skid device with a hydrofoil setup.

RE: Fastest Engine
 

[8D]:D

Can you use a movable (aerodynamically ) trimming device, like a wing or wings (front and rear ) for example?
That might help with the pitching problem?

RE: Fastest Engine
 

I'm guessing that it might be better to be able to adjust the pitch of the hydofoils for pitch control rather than have too much surface area above water-level that might get you airborne since the boat is supposed to not fly except for skipping action. So Hooty, when are we going to see the first prototype?:D;)

RE: Fastest Engine
 

Well, what about copying the below shown layout and replace the turbine inlet ducting by the ducted fan unit? The horizontal stab should be placed in the centerline of the „exhaust” outlet for best controllability.

I would add a central operational rudder to the existing fixed twin fins.

To reduce the potential for flipping you could mix the aerodynamically functional “elevator” with the throttle and even with additional horizontally adjustable trim tabs. These tabs usually are mounted on the stern transom of Deep “V”s allowing to adjust the hull’s angle of attack depending on the actual ground speed.

A .90 ducted fan unit should give you more than enough top speed once you get on plane. My personal preference for such a project would be the Dynamax (purely stock, without re-pitched blades) due to better static thrust than Ramtec or VioFan.

Just keep the CG as low as possible.

Go hooty go!

RE: Fastest Engine
 

That's what I was thinking of when I mentioned the movable trimming surfaces. The hydroplane also has front wings on either side of the nose (just ahead of the cockpit ) that are used in conjunction with the throttle.
I believe there are two pedals in a hydro...one for throttle and the other to control trim? The driver works the left trim pedal as much if not more than the throttle pedal.

They used to put the hydro races on ESPN...watched 'em whenever they were on...
When they had a camera shot of them comming at you, you could see the front wings working like crazy as the boat pitched over the choppy water.

RE: Fastest Engine
 

I didnt mean anything by what I said about getting technical, 45mph dosent sound like alot, but its really hualin tators for a flat bottom airboat. It took alot of set up to get it there, mods to the bottom to reduce friction,getting it balanced just right so it dosent ride on the bow but not to much or it flips before reaching max speed, the angle (side to side,up and down)and location of the engine. So getting technical doesent bother me, I like a good challenge and your right, getting technical is what its going to take. I have already heard doubts that 70mph can be had, but I know it can and with the help from everybody on here Im going to prove it........Keep the input coming, Im soaking in every bit of it. Its still on the drawing board but rest assured you will see it. Yes,that is allowed.

RE: Fastest Engine
 

Just had another idea about the wings and pitch control...
How about a Gyro?
Could you use something like a heading hold gyro like they use on the tail rotor of a heli...only mount it horizontally so it controls the wing(s )?

RE: Fastest Engine
 

Sorry, I didn't mean that to come off as an expression of doubt in you. It was a jab at myself for so ungraciously redesigning your boat for you (with an equally ungracious demand that you build my boat) when all you were asking for was advice on more muscle for it.

As far as more grunt for your boat, I would call Jett Engineering and tell them what your application is. They have great knowldge and would be able to design a power system for you. If you tell them what engine and props you have tested they might be able to figure out how much drag you need to overcome and do a custom setup of one of their engines for you if need be.

It would be interesting to hear about the developement that you have already gone through as well as see some higher resolution images of your boat.

Thanks, Craig

RE: Fastest Engine
 

Here’s a nice explanation: What Trim Tabs Do

www.bennetttrimtabs.com/tabsdo.htm

So there certainly will be advantages if using such trim tabs mixed with throttling and the functional „elevator“.

A kind of operative canard wing at the bow could be an additional help too, but would add unwanted drag. That would lead to four electronically mixed devices to keep your boat safe on plane at 70 +++ mph...

RE: Fastest Engine
 

I got an idea! Move this to the boat section... :eek:

RE: Fastest Engine
 

I know this is a airplane forrum but after all we are discussing airpower,lift and other things that have to do with airplanes. Where else could you get all this useful information? These guys have really helped me alot with some good ideas that I could never have got from the boat forrum's. I see that you have some very powerful engines yourself,maybe you could help me out here. I am very sorry I didnt mean to bother you.