Friday, May 26, 2006
"2 blades or 3?"
Lots of friends ask what prop are you going to put on the front, and then quickly ask "2 blades or 3?" This lead me to think about the relative advantages of one over the other. I should say first of all I fly out of a short strip and a constant speed prop has a very significant advantage for landing, so I had already made that decision in favour of constant speed.
I am not an aerodynamicist, but after some thought it appears to me that the 3-blade prop has all the significant disadvantages. They are:
- it costs more.
- it is harder (more expensive) to ship.
- there is more of it to get damaged and repair.
- it is less efficient and therefore very slightly slower, though I suspect this is academic.
- it is heavier, therefore increases the empty weight.
- it makes the cowls significantly harder to get on and off since the lower cowl has to come forward to clear the airbox, (and the training gear leg if you still use one.)
- you can never put it out of the way in a crowded hanger.
The one genuine advantage is it is very slightly smoother, though the 2-blade MT is very smooth.
For an (I)O-320 the radius of the appropriate 3-blade MT prop is 1/2 inch less. If you have the tail that high it seems unlikely that this will save you from a prop strike. This difference in radius is hardly going to make any difference in tip speed so the noise difference is minimal. The most frequently quoted reason for 3-blade is that its "sexier". I guess if you are that is the basis of your decision then there is no point in reading this. Get down to the hairdresser :-)
There is one frequently stated reason which I think is bullshit. If it were true I would be very interested, but I can see no reason why it should be. The stated reason is that it acts as a better air brake. I can see no aerodynamic justification for this statement. If someone knows a good reason why it is true I would love to know.
Postscript dated 12 September 06.
I really wanted to run the air brake or disking issue to ground, so after an abortive attempt to find someone who understood these things on RVSQN, asked the same question on Vansairforce. It is an unusually complex subject. What I concluded from the conversation is that for a given diameter the 2-blade should probably provide better disking, however it is more complicated than that. eg How much of the swept area is solid? In practice, it is pretty clear from users of the 3-blade MT on 200hp engines, that for stopping, the 3-blade IS more effective. They also complain that at the top end it appears to limit the top speed. This conversation was useful to me in that it confirmed to me the intelligent choice for those not in doubt about their manhood was 2-blade! :-)
You can see the discussion thread here and draw your own conclusion.
I am not an aerodynamicist, but after some thought it appears to me that the 3-blade prop has all the significant disadvantages. They are:
- it costs more.
- it is harder (more expensive) to ship.
- there is more of it to get damaged and repair.
- it is less efficient and therefore very slightly slower, though I suspect this is academic.
- it is heavier, therefore increases the empty weight.
- it makes the cowls significantly harder to get on and off since the lower cowl has to come forward to clear the airbox, (and the training gear leg if you still use one.)
- you can never put it out of the way in a crowded hanger.
The one genuine advantage is it is very slightly smoother, though the 2-blade MT is very smooth.
For an (I)O-320 the radius of the appropriate 3-blade MT prop is 1/2 inch less. If you have the tail that high it seems unlikely that this will save you from a prop strike. This difference in radius is hardly going to make any difference in tip speed so the noise difference is minimal. The most frequently quoted reason for 3-blade is that its "sexier". I guess if you are that is the basis of your decision then there is no point in reading this. Get down to the hairdresser :-)
There is one frequently stated reason which I think is bullshit. If it were true I would be very interested, but I can see no reason why it should be. The stated reason is that it acts as a better air brake. I can see no aerodynamic justification for this statement. If someone knows a good reason why it is true I would love to know.
Postscript dated 12 September 06.
I really wanted to run the air brake or disking issue to ground, so after an abortive attempt to find someone who understood these things on RVSQN, asked the same question on Vansairforce. It is an unusually complex subject. What I concluded from the conversation is that for a given diameter the 2-blade should probably provide better disking, however it is more complicated than that. eg How much of the swept area is solid? In practice, it is pretty clear from users of the 3-blade MT on 200hp engines, that for stopping, the 3-blade IS more effective. They also complain that at the top end it appears to limit the top speed. This conversation was useful to me in that it confirmed to me the intelligent choice for those not in doubt about their manhood was 2-blade! :-)
You can see the discussion thread here and draw your own conclusion.
Wednesday, May 24, 2006
If you are going to build a -4 there are a few issues which you need to think about.....
1) The empenage's moving surfaces are made of .016 aluminium. However, there is a standard option you can order from VANS, to upgrade these to .020. It costs only a few dollars. The reason you may want to do this is because many owners have experienced cracks in the lighter weight skins after a few hundred hours of flying. Having said that, many have not. In the 'RVator 24 years' there is a suggestion as to how you bend the trailing edge, and how you put a blob of Proseal to stop this happening. On this issue, I am a belt and braces person, and have the thicker skins and the Proseal. The disadvantage is simple; weight in the tail. If you are going for a large engine, or a 2 blade prop, resulting in large pressure pulses, I would at least spend 5 minutes making a decision deciding what you want..
2) I am told there is also a standard option for thicker skins on the first section of the fuselage - .040 - behind the firewall. Again it is a cracking issue I believe, and possibly again associated with larger engines. I know less about this issue, and am somewhat surprised since the cowl cheeks must provide significant stability for this area of skin.
3) Buckling of the firewall and cracking of the steel weldements that join the longerons to the engine mount have been a historic problem with the -4. It is apparently caused by heavy landings and flying off rough strips. The design was considerably beefed up in the mid '90s by VANS. Most of the relevant plan changes are dated 9/94. However, it is not clear that the new design is entirely proof against this problem. There is a rumour that there is an option to order a beefed up lower section of the firewall itself. I have not found any part number for this, though in the plans I think a heavier grade of steel is mentioned. Certainly a few individual builders have added strengthening strips. An example can be seen here.
The left hand picture above is of the top left firewall weldement, delivered 2006, and the right hand picture is the lower right hand weldement.
4) The exhaust system will be another area where you have to make a decision though this can be left until you are making the other firewall forward decisions. The two main options to consider are a crossover system resulting in two exit pipes, or a cheaper and simpler 4 pipe. The down-side of the 4 pipe is noise - though many argue that there is little difference. Talking to Vetterman though, as I understand it, on the -4, he believes the 4 pipe produces a little more power. This comes about because in the crossover system there is not enough room to get the necessary pipe length to produce a tuned system. It's more a question of making a system that fits under the cowl.
5) If you want a 5 point harness for the rear passenger, VANS leave you somewhat on your own. It is not discussed in the plans. Some have put the 5th belt around the flap tube. Alternatively, if you think about it early in your build, you could attach a bracket to the bulkhead under the floor like this . You will have to work around the elevator push rod. There is an argument that a 5 point harness for the passenger is useless, since it is hard to see how you stay within the weight limit for aeros with fuel and a passenger. A 5th point is reassuring though, in heavy turbulence.
6) Another decision to make is footwells for the passenger (comfort), or rudder pedals. They seem to be mutually exclusive. I am building my aeroplane so if you want rudder pedals you have to be me! Lots of people leave the rear pedals out but include the footwells.
7) Heat to the rear seat is another frustration. VANS say you can cut a hole in the spar to put SCAT tube through. However when you ask where to cut it the answer is "It is OK to put the 2" duct through the spar, but if you want to do it that way you will have to cut the hole yourself. We don't have exact dimensions for the appropriate place.... ". My own decision is to give the passenger an electric seat heater and a switch, since if VANS don't know where to cut the hole, I certainly don't! I am also thinking of putting all the heat into the forward compartment between your feet and providing a path from there into the area with the stick. If these two enclosures are sealed the air will have no option but to move aft, beside the elevator pushrod toward the passenger. (The Romans invented this approach though not in an aeronautical context. They called it a hypocaust!)
Postscript Feb 5th '08
There was some interest in the rivet layout etc for the firewall weldements shown above. Not sure where to put them I think I will put them here since there are similar pictures above.
2) I am told there is also a standard option for thicker skins on the first section of the fuselage - .040 - behind the firewall. Again it is a cracking issue I believe, and possibly again associated with larger engines. I know less about this issue, and am somewhat surprised since the cowl cheeks must provide significant stability for this area of skin.
3) Buckling of the firewall and cracking of the steel weldements that join the longerons to the engine mount have been a historic problem with the -4. It is apparently caused by heavy landings and flying off rough strips. The design was considerably beefed up in the mid '90s by VANS. Most of the relevant plan changes are dated 9/94. However, it is not clear that the new design is entirely proof against this problem. There is a rumour that there is an option to order a beefed up lower section of the firewall itself. I have not found any part number for this, though in the plans I think a heavier grade of steel is mentioned. Certainly a few individual builders have added strengthening strips. An example can be seen here.The left hand picture above is of the top left firewall weldement, delivered 2006, and the right hand picture is the lower right hand weldement.
4) The exhaust system will be another area where you have to make a decision though this can be left until you are making the other firewall forward decisions. The two main options to consider are a crossover system resulting in two exit pipes, or a cheaper and simpler 4 pipe. The down-side of the 4 pipe is noise - though many argue that there is little difference. Talking to Vetterman though, as I understand it, on the -4, he believes the 4 pipe produces a little more power. This comes about because in the crossover system there is not enough room to get the necessary pipe length to produce a tuned system. It's more a question of making a system that fits under the cowl.
5) If you want a 5 point harness for the rear passenger, VANS leave you somewhat on your own. It is not discussed in the plans. Some have put the 5th belt around the flap tube. Alternatively, if you think about it early in your build, you could attach a bracket to the bulkhead under the floor like this . You will have to work around the elevator push rod. There is an argument that a 5 point harness for the passenger is useless, since it is hard to see how you stay within the weight limit for aeros with fuel and a passenger. A 5th point is reassuring though, in heavy turbulence.
6) Another decision to make is footwells for the passenger (comfort), or rudder pedals. They seem to be mutually exclusive. I am building my aeroplane so if you want rudder pedals you have to be me! Lots of people leave the rear pedals out but include the footwells.
7) Heat to the rear seat is another frustration. VANS say you can cut a hole in the spar to put SCAT tube through. However when you ask where to cut it the answer is "It is OK to put the 2" duct through the spar, but if you want to do it that way you will have to cut the hole yourself. We don't have exact dimensions for the appropriate place.... ". My own decision is to give the passenger an electric seat heater and a switch, since if VANS don't know where to cut the hole, I certainly don't! I am also thinking of putting all the heat into the forward compartment between your feet and providing a path from there into the area with the stick. If these two enclosures are sealed the air will have no option but to move aft, beside the elevator pushrod toward the passenger. (The Romans invented this approach though not in an aeronautical context. They called it a hypocaust!)
Postscript Feb 5th '08
There was some interest in the rivet layout etc for the firewall weldements shown above. Not sure where to put them I think I will put them here since there are similar pictures above.
